CALIBRATION OF THE ENERGETIC GAMMA-RAY EXPERIMENT TELESCOPE (EGRET) FOR THE COMPTON GAMMA-RAY OBSERVATORY D

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19 93ApJS. . .86. .62 9T

The Astrophysical Journal Supplement Series, 86:629-656, 1993 June

CALIBRATION OF THE ENERGETIC GAMMA-RAY EXPERIMENT TELESCOPE (EGRET) FOR THE COMPTON GAMMA-RAY OBSERVATORY

D. J. Thompson,1 D. L. Bertsch,1 C. E. Fichtel,1 R. C. Hartman,1 R. Hofstadter,2,3 E. B. Hughes,2,3 S. D. Hunter,1

B. W. Hughlock,4 G. Kanbach,5 D. A. Kniffen,6 Y. C. Lin,2 J. R. Mattox,1,7 H. A. Mayer-Hasselwander,5

C. vonMontigny,⁵ P. L. Nolan,² H. I. Nel,^8,9 K. Pinkau,⁵ H. Rothermel,⁵ E. J. Schneid,⁴ M. Sommer,⁵ P. Sreekumar, ⁸ D. Tieger, ¹⁰ and A. H. Walker²

Received 1992 June 29; accepted 1992 October 28

CONTENTS 1. Introduction 629 22-F1

2. The EGRET Instrument and Data Analysis 630 22-F2 2.1. Detector System 630 22-F2 2.1.1. General Description 630 22-F2 2.1.2. EGRET Triggering Modes 630 22-F2 2.1.3. Type and Direction Modes 630 22-F2 2.1.4. Gas Replenishment System 631 22-F3 2.2. Analysis of Individual Events 631 22-F3 2.2.1. Selection and Structuring of the Events 631 22-F3 2.2.2. Direction Measurements 631 22-F3 2.2.3. Energy Measurement 632 22-F4 2.3. Subsystem Calibrations 632 22-F4 2.3.1. Spark-Chamber Performance 632 22-F4 2.3.2. Calibration of TASC with Cosmic-Ray

Muons 633 22-F5 2.3.3. Calibration of TASC with Charged

Particles in Flight 634 22-F6 3. Gamma-Ray Calibrations 634 22-F6

3.1. Stanford Linear Accelerator Center (SLAC)

Calibration 634 22-F6 3.1.1. Calibration Goals 634 22-F6 3.1.2. Calibration Strategy 635 22-F7 3.1.3. The EGRET Cahbration Fixture 636 22-F8 3.1.4. SLAC Calibration Beam 636 22-F8 3.1.5. Calibration Plan at SLAC 638 22-F10 3.2. Bates Linear Accelerator Calibration 639 22-F11 4. Calibration Data Analysis and Results 640 22-F12

4.1. Analysis of Individual SLAC Calibration Runs 640 22-F12 4.1.1. Introduction 640 22-F12 4.1.2. Processing of Individual Calibration

Runs 640 22-F12 4.1.3. Effective Area 641 22-F13 4.2. Calibration File Construction 642 22-F14 4.2.1. Sensitivity 642 22-F14 4.2.2. Angular Dispersion 642 22-F14 4.2.3. Energy Dispersion 642 22-F14 4.3. Calibration Results 643 22-G1

4.3.1. Effective Area 643 22-G1 4.3.2. Angular Dispersion 643 22-G1 4.3.3. Energy Resolution 644 22-G2 Derived Scientific Capabilities 645 22-G3

5.1. Source Detection and Location, Diffuse

Capabilities 645 22-G3 5.2. Spectral Resolution 649 22-G7 5.3. Polarization Sensitivity 650 22-G8 Other Calibrations 652 22-G10 6.1. Brookhaven Proton Background Calibration 652 22-G10 6.1.1. Experimental Method 652 22-G10 6.1.2. Results 653 22-G11 6.2. Calibrations of the TASC Low-Energy (Burst

and Solar) Modes 653 22-G11 6.2.1. Pulse Height Analyzer Calibration 653 22-G11 6.2.2. Sensitivity for Burst and Solar Modes 654 22-G12 Appendix 655 22-G13 ABSTRACT

Calibration of the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory involves simulation, experimental calibration, and verification in flight. The principal properties of the instrument which have been determined as a function of energy and angle are the effective area, the angular resolution (point spread function), and the energy resolution (dispersion).

Subject headings: artificial satellites, space probes — gamma rays: observations — instrumentation: detectors

— telescopes

1 NASA Goddard Space Flight Center, Greenbelt, MD 20771.

2 W. W. Hansen Experimental Physics Laboratory and Department of Physics, Stanford University, Stanford, CA 94305.

3 Deceased.

4 Grumman Aerospace Corporation, Bethpage, NY 11714.

5 Max-Planck-Institut fur Extraterrestrische Physik, D-8046 Garching, Germany.

6 Department of Physics, Hampden-Sydney College, Hampden-Syd- ney, VA 23943.

7 Compton Observatory Science Support Center, Computer Sciences Corporation, NASA Goddard Space Flight Center, Greenbelt, MD 20771.

8 USRA Research Associate.

9 On leave from Department of Physics, Potchefstroom University, Pot- chefstroom 2520, South Africa.

10 MIT Bates Linear Accelerator Center, Middleton, MA 01949.

1. INTRODUCTION

The Energetic Gamma Ray-Experiment Telescope (EGRET), one of the four gamma-ray detectors on NASA’s Compton Gamma-Ray Observatory, is sensitive in the energy range 20 to 30,000 MeV. Its goals are to map the entire sky in this energy range and to investigate all categories of astrophysical sources of high-energy gamma radiation. The Compton Gamma-Ray Observatory was carried into orbit by the Space Shuttle Atlan- tis on 1991 April 5, and was deployed two days later. EGRET was activated on April 15 and began verification tests on April 20. The all-sky survey started on May 16.

Calibration of any astronomical instrument is essential to 629

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19 93ApJS. . .86. .62 630 THOMPSON ET AL. Vol. 86 the interpretation of its results. For a complex detector such as

EGRET, operating in space and studying an incompletely ex- plored part of the electromagnetic spectrum, a knowledge of its operating characteristics is particularly crucial. The present paper describes the pre-launch calibrations of EGRET, the con- tinuing calibration after launch, and the results on instrument performance based on the calibrations and the early post- launch data. The paper is organized as follows: § 2 outlines aspects of the instrument and its data analysis which require calibration and describes subsystem calibrations; § 3 describes the gamma-ray calibrations which have been done; § 4 dis- cusses the analysis system for the calibration data and shows the calibration results; § 5 presents the derived scientific capabilities of EGRET; and § 6 gives results from the calibration of proton-induced background in the EGRET instrument and the calibrations of the low-energy modes of EGRET.

2. THE EGRET INSTRUMENT AND DATA ANALYSIS 2.1. Detector System

2.1.1. General Description

The Gamma-Ray Observatory, the EGRET instrument, and the major scientific goals of the mission have been described by Hughes et al. ( 1980), Bertsch ( 1984), Kniffen ( 1989), Kan- bach et al. ( 1988, 1989), and Hartman et al. (1992). Only those details relevant to the calibration will be repeated.

EGRET (shown schematically in Fig. 1 ) is a multilevel spark chamber, triggered by a scintillator coincidence system, and using a large Nal(Tl) Total Absorption Shower Counter (TASC) as a principal energy-measuring device. As such, it is similar to, but much larger than, the successful SAS-2 and COS B gamma-ray telescopes of the 1970’s. It detects gamma rays above about 20 MeV arriving at angles up to about 40°

from the detector axis.

The basic principle of operation is to detect gamma rays which interact by pair production within the active part of the instrument. An incoming gamma ray gives no signal in the large anticoincidence scintillator surrounding the spark chamber. If pair production of the gamma ray occurs in one of the tantalum foils interleaved with the upper spark-chamber modules, the electron and positron may trigger the coinci-

dence system, consisting of a 4 by 4 array of plastic scintillator tiles in the middle of the chamber and a similar array at the bottom. A coincidence signal in combinations of tiles selected by preset electronics or command, together with a time-of- flight signature indicating downward-moving particles, initi- ates the spark-chamber high voltage pulse and the readout of the spark-chamber and energy data. The calibration of the time-of-flight coincidence system is described by Hunter (1991).

The recorded spark-chamber picture, energy information, gamma-ray arrival time, and auxiliary information are trans- mitted to the ground as one “event.” Because there are more events than useful gamma-ray detections, the data analysis system must select the subset of all events which are unmistakably recognized as gamma-ray produced pairs, and from these ex- tract the arrival direction and energy of each detected gamma ray.

Electromagnetic processes such as pair production, multiple Coulomb scattering, and energy deposit in scintillators are all well understood. Two independent Monte Carlo models of EGRET have been used to estimate its response to incoming gamma radiation. Nevertheless, the instrument is sufficiently complex, operates over such a broad energy range, and has such a large field of view that experimental calibration is important to verify the performance. In addition, the flexibility built into the EGRET instrument in order to allow it to be optimized in space adds additional levels of complexity, some of which are described in succeeding sections.

2.1.2. EGRET Triggering Modes

EGRET has two principal modes of operation under normal conditions:

1. The triggering requirement includes the scintillator tiles in selected combinations, along with a time-of-flight requirement. The time-of-flight threshold is adjustable and is de- signed to accept only particles moving downward in the instrument.

2. The coincidence requirement can be expanded to require a minimum energy deposit in the TASC in addition to the other requirements. The threshold for energy deposit in the TASC is commandable in four steps from 1 to 15 MeV.

The response of the instrument is different in the two cases, because the TASC is about 10% smaller in area than the lower coincidence tile array and is spatially separated from the lower scintillators. The EGRET field of view is, therefore, wider without the TASC in coincidence, but the gamma rays in the outer part of this field are unlikely to have good energy measurements.

2.1.3. Type and Direction Modes

The two 4 by 4 arrays of scintillator tiles allow 256 possible combinations of an upper tile and a lower tile. Of these, only 96 combinations are allowed by the EGRET coincidence electronics, based on minimizing the fraction of the aperture which is blocked by large structures. Two other groups of tile combinations are defined within the coincidence electronics.

These groupings serve different purposes:

1. Type modes are upper/lower tile combinations which all view approximately the same fraction of unobstructed field.

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Seven such groupings are defined. If one type mode is found to produce a high rate of unacceptable events (due to cosmic ray interactions in the walls of the spark chamber, for example), then this mode can be disabled by command. Such a change would alter the EGRET response function, and this possibility must be accounted for in the calibration. To date, no type modes have been excluded from the normal operating configuration.

2. Direction modes are upper/lower tile combinations which all view the same direction in space. They may be thought of as vertical pairs of tiles plus eight combinations which resemble the eight points of a compass. The operation in space uses these direction modes to maximize the EGRET exposure to the sky and avoid the Earth limb, which is a strong local source of gamma radiation. As the Compton Gamma Ray Observatory reaches the point in its orbit where a direction mode is occulted by the Earth, an on-board command disables this mode. Similarly, each direction mode is re-enabled as it emerges from occultation. The field of view and sensitive area of EGRET will, therefore, vary during each 93 minute orbit. Response functions must be constructed for each of 74 possible configurations. Bertsch et al. ( 1989) describe the use of the direction modes in the EGRET data analysis system.

2.1.4. Gas Replenishment System

The gas-filled spark chamber in EGRET is subject to performance variation as the gas ages and eventually deteriorates. An on-board gas replenishment system is capable of evacuating and refilling the spark-chamber volume 5 times during the life of the instrument. The instrument response variation between gas refills must be included in the calibration. The first gas refill took place on 1991 December 2-3, and the second on 1992 December 3-4.

2.2. Analysis of Individual Events 2.2 A. Selection and Structuring of the Events

The correct recognition of a gamma-ray event in a high background of cosmic rays and other radiation is the primary function of a good high-energy gamma-ray telescope. It is not only important that the gamma rays be recognized with a high efficiency, but it is absolutely essential that the other events be rejected with no unacceptable residual, since the astrophysical gamma-ray intensity is so low. It was the development of the automatic, digital space qualifiable picture type device that made high energy gamma-ray astronomy an important part of space astrophysics. A track imaging detector system allows the characteristic wishbone signature of the high-energy gamma- ray interaction to be seen in two orthogonal views, thereby allowing it to be identified with certainty and its basic properties to be measured.

Earlier high energy gamma-ray missions, particularly SAS 2 and COS B, utilized the automatic, digital nature of the magnetic core spark chamber to record and transmit the data to the ground; however, most of the analysis was accomplished by data analysts selecting the tracks to go with each gamma ray on a spark by spark basis. Although there were some mechanisms developed for making the task proceed more quickly, and some automatic analysis was performed, the majority of the

work was done on a tedious event by event interactive basis.

The experience of the research performed with the data from these two satellites clearly showed that an automated analysis would be necessary for the quantity of data from larger high energy gamma-ray telescopes, both for the selection of the desired events and their proper structuring.

Using both simulated and real data from several different magnetic core spark-chamber telescopes, a variety of analysis approaches was tried, including various pattern recognition techniques differing in type and complexity. In the end, tracking procedures were determined to be better both in terms of the quality of the results and the amount of computer time required, especially the latter. An analysis showed that this result arose from the tracking being more suited to simple situa- tions, wherein the “picture” involved only a few tracks and specific additional knowledge could be used to reduce the total effort. A lengthy study of “downward” tracking as opposed to

“upward” tracking led to the conclusion that, at least for this specific application, the downward tracking from the top of the

“wishbone” of the gamma ray led more efficiently to results of at least as good quality and could correctly follow some events which could only be structured in the other approach with added features. The software program called Search and Analy- sis of Gamma-ray Events (SAGE) that was developed for the EGRET gamma-ray data is described in the Appendix.

Studies of the in-flight data through approximately the first 6 months after the launch of GRO have shown that SAGE is able to determine whether the event should be accepted or rejected for 85% to 88% of the events, as expected, and indicate that the others need further study. More importantly, only one track in 1000 events was incorrectly accepted by SAGE; ideally the number would be zero, but nonetheless this is considered to be a very good ratio. About one good gamma ray in 700 is rejected. The remaining 12% to 15% of the events for which SAGE determines that it cannot make a satisfactory decision must be reviewed by a trained data analyst. In this case, both views of individual events are displayed on a graphics unit and examined. Most are simply rejected or accepted by the analyst as structured; only about 0.5% of the events are interactively restructured.

In an independent procedure, a random sample of 1% of the events is chosen from all those in the EGRET data. A data analyst looks at these pictures as a cross check on the consis- tency of performance of the programs and analysts. This check is being made continually over the instrument’s life.

The few events which SAGE incorrectly accepts as gamma rays have little effect on the scientific results for regions of the sky with relatively high gamma-ray intensity, primarily those close to the Galactic plane. At high Galactic latitudes, however, the incorrectly accepted events can amount to 3%-5% of the total number of automatically accepted events. This would be tolerable in a search for discrete sources, but in order to study the diffuse extragalactic radiation an even cleaner data set is desirable. For this reason, analysts review all SAGE-accepted events for high latitude observations, rejecting or re- structuring events as needed.

2.2.2. Direction Measurements

The arrival direction of an individual gamma ray is determined by the following steps:

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19 93ApJS. . .86. .62 632 THOMPSON ET AL. Vol. 86 1. Determine the initial directions of the pair electron and

positron tracks with respect to the detector axes in each projected view.

2. Estimate the gamma-ray arrival direction in each view based on a weighted bisector of the angle between the two particle tracks.

3. Transform the direction cosines of the gamma-ray arrival to the GRO coordinate system to correct for the slight misalign- ment between the EGRET and GRO axes.

4. Use the aspect data of the GRO axes in celestial coordinates given in the EGRET packet header to transform the gamma-ray direction to celestial coordinates (epoch J2000).

5. Calculate the equivalent Galactic coordinates.

6. Use the spacecraft position vector and earth angles together with the celestial coordinates of the gamma ray to determine the arrival direction of the gamma ray in Earth-centered coordinates.

The procedures used for steps 1 and 2 were optimized empirically using the calibration data. Step 1 fits the two longest possible straight lines consistent with the sparks for the two individual tracks, starting with the topmost spark. Step 2 weights the bisector based on the third power of the number of sparks used for each track in step 1.

2.2.3. Energy Measurement

The best information about the energy of a gamma ray comes from the TASC Nal(Tl) calorimeter, which is equipped with two independent pulse-height analyzers (PHAs). How- ever, the geometry of the EGRET trigger system does not require all of the secondary particles in an event to pass through the TASC. Thus it is natural to separate all acceptable events into two classes: those in which both identified charged-particle tracks are aiming for the TASC upon leaving the spark chamber (Class A), and all the rest (Class B). It is expected that the estimated energy will be more accurate for Class A events. The optimum criterion for distinguishing between Classes A and B was determined empirically by studying calibration data. Class A events are those in which the projected path of each identified charged particle passes through at least 10 cm of Nal in the TASC. It was also found that a few Class B events can be promoted to Class A—those with one track that hits the TASC and one that misses, and whose gamma-ray direction vector passes through the TASC.

The TASC energy measurement must be corrected for the energy lost by the electron and positron before reaching the TASC in traversing the Ta conversion plates between the upper spark chambers, the steel scattering plates between the lower spark chambers, the plastic trigger scintillators, the light guides, the pressure vessel, the TASC housing, and some plastic filler material in the TASC housing. This energy loss ( “plate loss”) is estimated from the path length in each material. Typi- cal corrections are about 15-30 MeV.

Class B events require another energy correction to account for the energy carried by particles which miss the TASC. The energy of each track is estimated, based on the amount of scattering caused by the spark-chamber plates. These estimates have a large uncertainty, and they are useful only for energies up to a few hundred MeV, but there is no other information available.

For very low-energy gamma rays, it is necessary to define another type of events; a Class C event is just like a Class A event except that the energy deposit in TASC is too small to trigger one or both of the PHAs, which have energy thresholds of about 20 MeV. The TASC zero-cross discriminator, with a threshold of about 6.5 MeV, provides a two-channel pulse- height analysis for low-energy events. The estimated energy for Class C events is the larger of two values: (a) the one obtained from the scattering of the tracks or (b) the two-channel TASC energy corrected for plate loss. In Figure 2 it can be seen that Class C events predominate at low energies, but are very rare for photon energies greater than 100 MeV in the calibration data from the Stanford Linear Accelerator Center (SLAC).

Not all of the energy in a gamma ray is accounted for by TASC absorption, plate loss, and escaping particles as described above. Low-energy photons from bremsstrahlung and positron annihilation carry energy out the sides of the spark chamber, particularly for low-energy gamma rays. For high- energy gamma rays, the showers are not completely absorbed by the 20 cm of Nal in the TASC. The average energy deposit is a nonlinear function of gamma-ray energy, as shown in Figure 3. Both of these effects are corrected by an empirical maximum-likelihood method for Class A events. The energy spectra observed in the SLAC calibration were fitted with a simple mathematical function, with fit parameters that can be inter- polated to any incident energy and angle. Using these parameters, the probability of any observed energy can be calculated for any incident energy and angle. Given any observed energy in flight, the most likely incident energy can be found by locat- ing the maximum value of this joint probability function.

For some gamma rays there is no reliable TASC energy measurement, either because of a telemetry error or because of pulse pile-up. When this occurs the energy is estimated entirely from the scattering of the tracks, if possible. Such events are classified as Class B.

2.3. Subsystem Calibrations 2.3.1. Spark-Chamber Performance

The performance of the EGRET spark chambers is a function of three commandable instrument parameters (high voltage, sense amplifier threshold, and sense amplifier timing), as well as temperature and gas quality. The performance is measured in terms of the efficiency of an average module (deck) and the average width (or spreading) of the sparks produced, using the straight tracks of charged particles as a reference source. Atmospheric muons were used for this purpose on the ground, and cosmic ray protons are used on orbit. It was found that these charged particle runs are best accomphshed by lower- ing the threshold of the anticoincidence system to about 10%

of its nominal value ( rather than turning it completely off), in order to select high-energy cosmic rays. In general, as the high voltage is increased, the efficiency and spreading both increase.

As the voltage is raised higher, the efficiency reaches a maximum value of ~94% and the spreading continues to increase.

The optimum performance was found to be only slightly de- pendent on the sense amplifier timing value, and hence, has been set to one value for all conditions. The spreading and efficiency are the best measure of the quality of the spark- chamber gas, which gradually degrades through outgassing con-

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Fig. 2.—Distribution of events into Classes A, B, and C as a function of energy. This is based on normal incidence calibration data. For nonzero incidence angle the proportion of Class B events is increased. The few Class C events at high energy are mainly due to imperfections in the SLAC beam.

taminants and breakdown of the organic component ethane (which is used to suppress spurious sparking).

In terms of recognizing and analyzing gamma rays, the EGRET performance shows a gradual decline with running time. When this happens, fewer events can be handled by the automatic analysis software, but data analysts can recover most of the events. Only when the spark-chamber performance becomes seriously degraded, with efficiency below 70%, does the total number of recognizable gamma-ray events show a serious loss. The decrease in efficiency with time can be par- tially compensated by raising the high voltage and adjusting the sense amplifier threshold. Based on an extended period of running in a mode which simulates flight operations, the expected time between refills of the spark-chamber gas was 6 months. The first gas refill took place approximately 7.5 months after activation.

Fig. 3.—Average energy deposit in TASC, as a function of gamma-ray energy, for normally-incident photons. At high energy, the TASC is not thick enough to absorb all of the shower particles. At low energy, much of the total energy is absorbed in the conversion plates or carried by secondary photons that escape out the sides of the spark chamber.

2.3.2. Calibration of TASC with Cosmic-Ray Muons The cosmic-ray muons reaching EGRET on the ground had a mean energy of about 2 GeV and came mostly from near- vertical directions (Particle Data Group 1990). Such minimum-ionizing particles produce an energy-deposit spectrum with a well-defined (“Landau”) peak (Bellamy et al. 1967).

When EGRET was pointed up, muons passing through 20 cm of Nal in the TASC produced a spectrum with a peak at about 105 MeV. Since there is no on-board calibration method, this provided a useful monitor for the gain of the TASC PMT/amplifier/PH A system in the five years between the SLAC calibration and launch.

Figure 4 shows a typical spectrum accumulated for a few hours. No screening was done to distinguish muons from gamma rays or other ambient particles. The energy of the peak can be determined with an accuracy of better than 1%. This can be used to watch for changes in the overall gain of the 16 PMTs as a group.

If the spark-chamber tracks are analyzed, it is possible to clean up the spectrum by excluding everything except muons.

The energy deposit for each event can be corrected for the variation in path length in Nal. These corrections narrow the peak and reduce the continuum level, but the accuracy of the peak energy determination is improved only slightly.

Muons were also used to monitor the gain of individual PMTs. Because the hardware design makes it impossible to locate an identifiable peak in the spectrum of an individual PMT, a statistical method was devised. After analyzing the tracks, each muon was assigned to one of the 36 Nal blocks which make up the TASC. Muons which entered one block and passed to another were excluded. Measurements with ra-

TASC energy (MeV)

Fig. 4.—TASC energy deposit spectrum produced by cosmic-ray muons at sea level. The 105 MeV Landau peak can be used as a calibration for the TASC gain.

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19 93ApJS. . .86. .62 634 THOMPSON ET AL. Vol. 86 dioactive sources and electron beams have shown that the scin-

tillation light output is uniform across each block (Hughes et al. 1986 ). Thus 36 energy spectra can be fitted to determine 36 peak energies. The peaks are used to determine the actual gains by least-squares fitting using single-PMT response functions measured with electron beams. The individual gains can be measured to an accuracy of 2% in a typical 4-hour data run with 20,000 EGRET triggers. By adjusting the PMT voltage settings, the response of the TASC can be made uniform to about 3% (rms) across its entire face.

2.3.3. Calibration of TASC with Charged Particles in Flight In space, cosmic-ray protons produce a minimum-ionizing spectrum in the Nal TASC in the same way as ground-level muons. Relativistic protons passing through the TASC parallel to the Z-axis should deposit about 105 MeV of energy.

The proton spectrum is not as easy to use for calibration purposes as the muon spectrum for these reasons:

1. The incident distribution is approximately isotropic, so there is a broader distribution of path lengths in Nal, and thus a broader energy peak.

2. Many protons have a strong nuclear interaction in the Nal. The energy deposit spectrum for these protons is a broad continuum.

3. There are many energetic electrons and other particles in addition to protons. These particles do not produce a minimum-ionizing peak.

4. Interactions of particles in the GRO structure bombard the TASC with showers of secondary particles, often in coincidence with legitimate proton interactions.

5. The spark-chamber pictures of charged-particle events are very complex. Often it is difficult to recognize the presence of a proton or to locate it accurately. As a result, the rate of useful events is much lower than it is on the ground.

In spite of these problems, protons can still be used in flight to monitor the TASC gain.

Shortly after launch, EGRET operated in charged-particle mode for 18 hours. There were not enough events with clean spark-chamber data to produce 36 useful TASC spectra, as described in § 2.2.2. Using information from the trigger scintillator tiles it is possible to identify a set of events with single particles passing through the TASC parallel to the Z-axis. The spectrum of these events, shown in Figure 5, has a clear minimum-ionizing peak. Using the trigger tile information this spectrum can be divided into 16 spectra, each one associated with one of the 16 TASC PMTs. There is only a small variation in the peak channel, indicating that all of the PMTs survived the launch with little change in gain.

The prospects for more long charged-particle runs are slim.

However, there are regular 30-minute runs every week. Using these it is possible to monitor the system gain for long-term drifts.

There is another TASC diagnostic independent of cosmic rays. The TASC low-energy ( “solar” ) spectra, Figure 6, always contain the gamma-ray lines produced by 40K at 1.46 MeV (from EGRET’s Macor spark chamber modules) and by neu- tron capture on iron (a blend with average energy about 7.64 MeV), which provide a constant standard. Although the PHA and amplifier are different from the ones used in the high-en-

Fig. 5.—Spectrum of energy deposited in TASC by a selected set of cosmic-ray protons. This data comes from an 18-hour charged particle run on May 21 1991. The minimum-ionizing peak is clearly visible.

ergy analysis, these lines are used to track the behavior of the TASC crystal, PMTs, and optics. The low energy spectrum also contains a line-like feature which is an instrumental artifact. Very high energy {E > 200 MeV) interactions in the TASC all appear in the same PHA channel, independent of PMT gain. This channel is ignored in the data analysis.

3. GAMMA-RAY CALIBRATIONS

3.1. Stanford Linear Accelerator Center (SLAC) Calibration 3.1.1. Calibration Goals

The goal of calibrating EGRET is to estimate the instrument response functions by means of exposure to gamma-ray beams. The ideal beam has to provide a flux of photons with properties (energy and direction) that are known to an accuracy much better than the resolving power of the instrument to be calibrated. We denote this flux I{Et, xt)dEtdtit, where Et

and x^t are the true energy and direction of the photons. The instrument, with its axis pointing into direction xa, that is exposed for a time T to this flux records N(Em, xm)dEmdüm

events with parameters E^m, the measured energy, and x^m the measured direction. One writes the measurement process in the form of a convolution integral:

N(Em, xm)dEmdttm

US, I(E^t, x^t)GdE^tdQ^tdt^dE^mdti^m , ( 1 ) where G{Eti Em, xt, xm, xa, t) general instrument function.

Of course it is not possible to calibrate the six-dimensional function G directly and certain reasonable approximations and assumptions have to be made.

It is assumed that G is constant for the duration of the calibration exposures, as well as during corresponding exposures during the mission. This is important to note since EGRET can be operated in different modes. Therefore care has to be

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Fig. 6.—TASC energy deposit spectrum seen in orbit by the low energy pulse height analyzers. Some of the prominent features are labeled. The broad “bump” at high energies is produced by minimum-ionizing cosmic- ray protons, which deposit 100-200 MeV in the TASC. The sharp peak labeled “Instrumental Artifact” results from an electronics design error.

It is important to note that this separation only has meaningful applications in data analysis if the form of the point spread function P does not depend on the measured photon energy Em and likewise the energy dispersion function Q is not depen- dent on the assigned direction of the event xm. This property should be established in the analysis of the calibration data.

The calibration beam should ideally be a delta-function in direction and energy. At SLAC this requirement is sufficiently met for the collimation of the beam which had an angular spread of about 0.'2; the energy spread of the beam however was in the range 5%-20% and must be taken into account. The incident calibration beam intensity can thus be written with normalized distribution functions

I(Et,xt) = Icg(Et,Ec)ô(xt-xc), (5) and the number of incident beam photons is given by = ICT, where T is the duration of the exposure. During the exposure a total of Nevt events are registered in the instrument.

The instrument functions evaluated at the nominal beam energy and direction are then:

taken that exposures are grouped into intervals of constant instrument function and the appropriate calibration data for these intervals are provided.

The physics of the photon detection process in EGRET sug- gests a sequence of separable functions whose product is expected to be a close approximation of the instrument function:

G(Et, Em, xt, xm, xa, t) = TAeR , (2) where:

A((EC, Xc, Xa) ^evt/^gam

P(EC, xc, xm, xa) = j N(Em, xm)dEmliVevt (6)

" Em

g(E„ EC)Q(E„ Em, xc, xa)dE,

= J N(Em,xm)dam/Nevt. (7)

T: effective observation time (deadtime corrected) A(x^t, x^a): geometrical cross section of instrument

e(E^t, x^t, x^a): efficiency averaged over ^4 R(E^t, E^m, x^t, x^m, x^a): general dispersion function . Conventionally one defines:

The required accuracy of the instrument functions is determined in relation to the prospective use of EGRET during the mission: The uncertainty introduced by the calibration (part of the systematic error) should be at least a factor of 2 smaller than the uncertainty resulting from the counting statistics that is expected from a bright celestial source during a typical GRO observation. Therefore we have to expose EGRET in the calibration with about 4 times the number of photons that the instrument will see from such a reference source in orbit.

A⁽(E^t, x^t, x^a) = Ae: Effective area 3.1.2. Calibration Strategy X(Et, xt, xa, T) = AeT: Exposure factor.

The function R is the general dispersion function which describes how photons with “true” parameters Et, xt are dis- persed in measurement space E^m, x^m when the instrument orientation is given by xa. The dispersion function R is normalized ( fx RdEmdüm = 1 ) and is assumed to be separable as R = iSt° the point spread function (PSF) P and the energy dispersion function (EDF) Q:

P{E„ xt, xm, xa) = J RdEm (3) Em

Q(E', E^m, x„ x^a) = f Rdn^m. (4)

J Xm

Following the requirement on the accuracy of the calibration, the necessary number of calibration photons was determined from the photon fluence (i.e., number of photons integrated over time) of a typical celestial reference source, the Crab pulsar. The COS B Crab pulsar spectrum /(>£’) = 4.4 X lO^EXMeV)^-11 cm^-2 s^-1 (Bennett et al. 1977) has a spectral shape that is rather representative for the known gamma-ray sources in the range from about 50 MeV to several GeV. The reference observation time was assumed to be 14 days in orbit with a duty cycle of 45%. The required number of calibration photons for normal incidence (at larger inclinations it is up to 5% larger) was then about 1.2 X 10⁵ around 20 MeV, 2 X 10⁴ at 100 MeV, and 2 X 103 at 1 GeV.

In order to translate this into a calibration exposure time one has to consider the intensity and time signature of the SLAC

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19 93ApJS. . .86. .62 636 THOMPSON ET AL. Vol. 86 photon beam which was used for the calibration. At SLAC the

beam was bunched into pulses with a repetition rate of 15 Hz, pulse width of ~20 ns (<^ EGRET time resolution!), and an average intensity of about 0.1-0.5 photons per pulse. Essen- tially, only pulses containing one photon are useful in the calibration. Assuming Poisson statistics this will be the case in 9%

to 30% respectively of all pulses and the single-photon rate was therefore between about 1 and 5 s_1. A preliminary description of the SLAC calibration of EGRET was given by Thomp- son et al. (1987).

The required calibration exposure time for each combination of direction and energy ranged from as much as 20 hours at the lowest energies to about 2 hours at 100 MeV. Above 100 MeV shorter exposures would be required, but technical aspects of moving EGRET through a regular scan pattern de- mand a minimum run time of about 2 hours.

The calibration photons are delivered at SLAC in a pencil beam of ~ 1 cm diameter. In order to “synthesize” the radiation from a distant source with extended uniform flux the beam has to be scanned uniformly over the sensitive geometric area of EGRET. The sensitive area is given by the cross section of the upper spark-chamber volume with respect to the incoming radiation. Two possible methods of scanning could be en- visaged: a “flying spot” scan with the beam continuously moving or a “raster scan” where the beam remains stationary at scan points and is moved after predetermined Emits on the exposure for a point have been reached. Since the calibration beam was expected to have unpredictable intensity fluctuations a uniform flying spot scan would have required to adjust the scan speed simultaneously, which is technically difficult to accomplish. In a raster scan a beam monitor can be set up with a limit that indicates when the required exposure for one point has been accumulated. The fixture holding the EGRET instrument is then commanded to move to the next scan point.

EGRET was moved with respect to the beam in a scan pattern of exposure points arranged in a regular hexagonal grid, with each scan point receiving a uniform exposure of gamma rays. The spacing of scan points was set to 5 cm, which is small compared to relevant EGRET dimensions ( e.g., the trigger telescope tiles with 20 X 20 cm2 each), while the number of scan points in a pattern needed to cover the sensitive area of EGRET is reasonable (300 to 400 depending on angles). The calibration fixture holding EGRET needs about 8 seconds to move between points. Therefore even in the shortest exposure (2 hours) EGRET was mostly stationary at the scan pattern grid points. The position and the momentary state of the scan (moving or stationary) was recorded with every event in the data.

3.1.3. The EGRET Calibration Fixture

The goal of exposing the EGRET instrument to a quasi-uniform flux of gamma rays was achieved through the use of a custom fixture, shown in Figure 7. This fixture was required to hold the 1800 kg instrument and move it through a large range of positions, tip angles, and azimuth angles, while at the same time maintaining stability.

In operation, the hydraulically driven calibration fixture was able to position EGRET repeatedly with an accuracy of better than 0.2 mm in the horizontal and vertical directions and 0? 1

in tip and azimuth angles. The alignment of the fixture to the beam line was carried out with optical techniques.

The motion of the calibration fixture was computer-controlled. Scan patterns for all chosen combinations of tip angle and azimuth angle were preprogrammed to produce a uniform pattern over the entire active volume of EGRET. Figure 8 shows the number of gamma rays detected at each scan point for the normal-incidence scan pattern at 500 MeV beam energy.

3.1.4. SLAC Calibration Beam

The gamma-ray beam at the Stanford Linear Accelerator Center (Mattox 1987; Mattox et al. 1987) was produced by Compton scattering between electrons from the linear accelerator and photons from a frequency-doubled Nd:YAG laser (wavelength 525 nm). The energy of the gamma rays was determined by the electron energy and the scattering angles. Fig- ure 9 is a schematic diagram of the beam-production system.

The electron beam was focused to converge at the position of EGRET. (The Lorentz factor of the electrons was so high that the backscattered photons followed nearly the same trajectory as the electrons which boosted them to high energy, although coflimation of the beam was still necessary.) The laser beam, directed by mirrors, intercepted the electron beam at an angle of about 0?1. A set of four small dipole magnets gave the EGRET operators fine control over the electron beam position and direction. The two beams interacted in a region 6.4 m long, until the electron beam reached a permanent magnet which directed it into a beam dump. The gamma-ray beam passed through a 1 cm diameter safety collimator, and then traveled 150 m to the EGRET building. At the entrance to the building, the “quadrant detector” doubled as a second collimator and beam-steering detector. The quadrant detector is a thick tungsten block with a hole through it ( 3 mm diameter for 3 and 10 GeV, 1 cm for energies 100 MeV to 1 GeV). Four small plastic scintillators embedded in the tungsten made it possible to tell when the beam was off-center. The quadrant detector was removed at 60 MeV and below, leaving the safety coflimator to define the beam, which attained a diameter of 3.7 cm at EGRET.

Because of the frequent fluctuations in the gamma-ray beam intensity, continuous monitoring of the beam was essential, both to maintain uniform exposure and to allow the operators to adjust the beam when needed. Figure 10 shows schematically the arrangement of EGRET, the beam-monitoring equip- ment, and the connections between them.

The intensity of the gamma-ray beam was monitored continuously with a 15 cm thick plastic scintillator inside the EGRET building ( Lin et al. 1991). About 20%-30% of the gamma rays interacted in this detector. EGRET’s trigger was vetoed whenever such an interaction occurred, so the effective intensity of the beam was reduced accordingly. This sacrifice was considered acceptable, since there was no other way to track the beam fluctuations in real time.

A large Nal scintillator, 50 cm thick, could be rolled into the beam path to provide a good measurement of the beam spectrum and intensity. This was done between cahbration runs and sometimes in the middle of runs. A similar detector was mounted behind EGRET to measure showers which pene- trated through the entire instrument.

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19 93ApJS. . .86. .62 9T

No. 2, 1993 CALIBRATION OF EGRET 637

Fig. 7.—EGRET on the SLAC Calibration fixture

500 I 400 -

>~

300 h 200 1

-400

0 0 0 0 0 0 1 0 0 0

-100 -

20 M 16 15 10 M 24 M M 22 30 19 IS 29 33 6 1 22 20 20 29 27 25 22 IS 29 32 33 19 32 30 10 '

19 24 21 23 23 14 19 22 17 20 14 19 25 25 22 22 10 3 5 36 1 5 32 1 0 2 7 20 3 3 2 6 3 5 34 2 3 2 5 21 i%) ^C 30 26 23 20 20 20 25 24 21 27 27 35 29 19 23 17

24 32 33 22 22 20 24 25 30 21 44 32 25 25 27 17 14 26 32 24 29 15 23 22 27 21 IS 29 25 21 15 I 32 29 40 21 30 35 31 24 24 30 29 23 33 35 34 ^9

31 24 32 39 27 23 31 26 16 28 28 21 20 19 22 20 29 22 27 30 21 9 28 46 24 25 27 31 27 ¿O1 « 21 19 24 16 19 14 26 13 17 21 23 14 17 20 36 10

23 35 21 22 48 21 26 27 42 23 31 21 29 19 20 14 10 21 17 28 22 14 20 15 19 19 19 25 22 25 17

60 41 32 35 65 48 36 36 34 30 IIS 98 46 38 23 37 32 28 31 34 33 48 39 40 34 45 52 35 32 26 12 i 27 26 17 32 32 20 30 26 29 28 22 24 41 31 33

15 17 23 23 23 31 22 26 32 30 26 22 24 29 25 23 0|l 25 52 62 36 26 24 27 30 32 44 34 20 33 22 10 0

22 30 31 40 32 20 46 39 46 42 32 29 34 27 0 1 “I 1 1 0 0 0— 0 0

I I I I I I 1 I I 1 I I

—1 1 0 2 Ö 7—

I I I I I I I I I I I I I I I I I I I I I I I I I I I I

-200 400

X (mm) Fig. 8.—Number of gamma rays detected at each scan point for the normal-incidence scan pattern at 500 MeV beam energy.

The electron beam and laser produced pulses at a rate of 15 Hz. The electron pulses were about 50 ns long, and the laser pulses about 20 ns. The resulting gamma-ray pulse length was much shorter than the resolving time of the EGRET electronics, so when two or more gammas were present in the same pulse, they appeared to arrive simultaneously. Such pulses are more difficult to use for calibration than those with a single gamma ray. Thus the electron beam current was adjusted to maintain an intensity of about 0.1-0.3 gamma-rays per pulse, whenever possible—a compromise to provide a sufficient rate of single gamma rays while keeping the number of multiple gamma-ray pulses low. Figure 11 shows a spectrum obtained with the large Nal detector at 1000 MeV. The single, double, and triple gamma-ray peaks are visible. Table 1 lists the energy dispersions attained at the different beam energies. The shape of the spectrum is due to the Klein-Nishina cross section, the size of the collimator, and the energy, spatial and angular spread of the electron beam.

It was intended that EGRET should operate as nearly as possible as it does in flight. However, some modifications were made in the electronics to improve the quality of the calibration. An extra veto signal was added so EGRET could be shut off between beam pulses and after an interaction in the plastic monitor detector. A signal was brought out to indicate when EGRET was busy processing an event. Signals were brought

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19 93ApJS. . .86. .62 638 THOMPSON ET AL. Vol. 86 Laser beam

Interaction region

Drift length

Fig. 9.—SLAC gamma-ray beam system. The main accelerator beam was focused to converge at the EGRET detector by a series of magnets. The electrons Compton scattered laser photons, boosting them to gamma-ray energy. The electrons were then swept out of the beam by a permanent magnet, but the gamma rays followed the original electron trajectories.

The gamma-ray spectrum was formed by the Klein-Nishina cross section and the geometrical constraints of the collimators.

out from the TASC flight Pulse Height Analyzers (PHA) to allow them to be used as real-time beam monitors.

A dedicated computer monitored and recorded the following beam information on a pulse-by-pulse basis: electron beam current, laser power, pulse height from the four quadrant detector modules, relative timing of the laser and electron beam, energy deposit in the plastic detector, pulse height from the two large Nal detectors, the EGRET busy signal, and the TASC PHA signals. Every 2 seconds a report was sent to the EGRET ground support computer containing an estimate of the number of useful gamma rays to which EGRET had been exposed.

These summaries were used by the computer to calculate when to move the calibration fixture.

3.1.5. Calibration Plan at SLAC

The limited time available at SLAC (about 2 months) required a careful allocation of beam energies and directions for the calibration runs.

From below the EGRET energy threshold at 20 MeV to the maximum SLAC energy at 10 GeV, 10 calibration energies were selected at values of about 15, 20, 35, 60, 100, 200, and

BEAM MONITOR DATA

Fig. 10.—Connections between EGRET and its support electronics in the SLAC calibration setup. EGRET was held by a computer-controlled fixture which moved it through a pattern to ensure uniform exposure over the entire sensitive area. A second computer estimated the gamma-ray beam intensity in real time. The ground support computer commanded the fixture to move and recorded EGRET’s telemetry stream.

Fig. 11.—Spectrum of gamma rays arriving at EGRET. This spectrum was measured using a large Nal detector with very good energy resolution.

The peaks near 2000 and 3000 MeV represent the beam pulses in which two or three gamma rays arrived nearly simultaneously.

500 MeV and 1, 3, and 10 GeV. At each energy 13 different beam directions were measured. These beam directions were located in 1/8 of the field-of-view (tip, or inclination angles 0 = 0° to 40° in 10° intervals, azimuth angles 0 = 0°, 22?5, and 45 ° ). Every recorded event carries the full signature of the time-of-flight telescope tiles that triggered the event. It is therefore possible to reconstruct more restrictive trigger conditions ( “view modes” ) by selecting events from the actual data which were taken with no restrictions imposed. In this way it is possible to derive the sensitivity for the full field of view by exploit- ing the symmetries of EGRET for all expected view modes.

Even the possible loss of symmetry by a technical malfunction (e.g., loss of a trigger tile) can be calibrated by appropriate event selection in the data analysis.

The calibration plan contained 114 regular runs, shown in Table 2. The measurements were performed between 1986 April 26 and June 30 at the Stanford Linear Accelerator Center. Except for a few low energy exposures at inclined orien- tations the scheduled calibration program was completed suc- cessfully. Several special runs were also made at SLAC (e.g., runs with the beam outside of the sensitive area to check for background, runs to check the burst and solar modes of EGRET).

TABLE 1

SLAC Gamma-Ray Beam Parameters 7 Energy e Energy 7 Spectrum

(MeV) (GeV) (FWHM) 15 0.65 20%

20 0.76 19 35 1.0 6.9 60 1.3 7.3 100 1.7 6.1 200 2.5 7.4 500 4.0 9.4 1000 5.8 9.8 3000 10.7 8.9 10000 22.4 14.7

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TABLE 2

EGRET Runs at SLAC; Number of Beam Photons Needed, in Thousands

639

0 = 10° 0 = 20° 0 = 30° 0 = 40°

Energy

(MeV) 0 = 0° </> = 0° 4> = 2T.5 <p = 45° 4> = 0° 0 = 22?5 0 = 45° 0 = 0° 0 = 22?5 0 = 45° 0 = 0° 0 = 22?5 0 = 45°

15 178 193

20 153 167 172 173 166 185 189 193 196

35 71 77 80 80 77 86 87 83 89 92 82 91 94 60 45 49 50 51 49 54 55 53 57 59 52 58 59 100 27 27 27 27 27 27 27 27 27 27 27 27 27 200 27 27 27 27 27 27 27 27 27 27 27 27 27 500 27 27 27 27 27 27 27 27 27 27 27 27 27 1000 27 27 27 27 27 27 27 27 27 27 27 27 27 3000 27 27 27 27 27 27 27 27 27 27 27 27 27 10000 27 27 27 27 27 27 27 27 27 27 27 27

3.2. Bates Linear Accelerator Calibration

Due to the (at that time) imminent conversion of the SLAC facility to the Stanford Linear Collider (SLC) configuration, the primary EGRET calibration had to be carried out earlier than desired, at a time when some of the instrument subsys- tems were not yet in their final configurations. Following the subsequent optimization, it was expected that some of the EGRET performance parameters had changed, and a partial recalibration was carried out in 1988 to characterize those changes before the telescope was integrated with the GRO spacecraft.

There were two primary goals for the recalibration: first, it was expected that the improved spark-chamber performance would provide a significant increase in the telescope effective area (due to increased gamma-ray recognition efficiency) and also that the angular resolution would be improved. Both effects were expected to be most significant at energies below 100 MeV. It was necessary to determine the corrections required to adjust the SLAC data for the improved instrument performance.

The second recalibration goal resulted from analysis of data from the SLAC calibrations for 1, 3, and 10 GeV. In those data, it was found that the telescope effective area decreased more rapidly than expected as the energy increased to 10 GeV.

Prehminary Monte Carlo modeling indicated that the proba- ble cause for this was larger than expected self-vetoing due to back-splash of low-energy photons from the TASC and detection of those photons in the anticoincidence scintillator dome (A-dome). Monte Carlo modeling suggested that raising the detection threshold in the A-dome might reduce the self-veto effect, thereby raising the efficiency at the highest EGRET energies (which are important because of their good angular resolution). However, it was felt to be essential to obtain some experimental verification of this, so as to gain confidence in the ability of the Monte Carlo code to simulate the effect up to the highest energies.

An additional benefit of the EGRET recalibration was a beam calibration of the uniformity of response of the TASC;

following the original SLAC calibration, aU of the high-voltage power supphes for the TASC photomultipliers (PMTs) were rebuilt to improve reliability. Although care was taken to read- just the PMT gains to their original values, it was felt that a beam calibration was highly desirable.

The recalibration goals described above require gamma rays from well below 100 MeV to at least 1 GeV. With the nonavail- ability of the SLAC facility, the MIT Bates Linear Accelerator (linac) was selected as the most likely facility to accomplish all of the goals. It can produce a maximum electron energy of nearly 1 GeV, at which energy the change in the self-veto effect would be small but measurable, and the TASC recalibration could be carried out. The Bates electron beam had also been run at energies below 100 MeV, although not routinely. A tagged bremsstrahlung gamma-ray facility had been used previ- ously at Bates (Booth 1981), but not at energies below 100 MeV or above 400 MeV; several beam development feasibility tests were therefore required. A major advantage of the Bates linac is that it has a duty cycle of about 10-2 ( 15 microsecond pulses at 600 Hz), compared with the duty cycle used at SLAC of 6 X 10 7. Pulse pile-up in the TASC and spark chamber was therefore negligible.

The Bates gamma-ray beam was a tagged bremsstrahlung beam, with a 180° magnetic spectrometer for the electron after the Bremsstrahlung interaction. The beam energies and inten- sities used are shown in Table 3. A beam intensity monitoring scheme very similar to that used in the earlier SLAC calibration was implemented.

Figure 12 shows EGRET in proper relation to the beam tagging and monitoring system. In contrast to the procedure used at SLAC, in which EGRET was moved back and forth across the beam line in such a way that the gamma-ray beam scanned its entire active area, at Bates EGRET was held stationary during each run. By selecting the subset of the SLAC

TABLE 3

Bates Gamma-Ray Beam Parameters 7 e Tag 7 Energy Energy Energy Intensity (MeV) (MeV) (MeV) (s"1) 20 80 60 0.8 35 80 45 1.0 60 80 20 0.1 100 145 45 0.4 200 260 60 0.5 500 560 60 0.2 790 850 60 0.9

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19 93ApJS. . .86. .62 640 THOMPSON ET AL. Vol. 86 data taken while the beam was within the area of EGRET

exposed at Bates, the required normalization is possible. Runs were taken with the EGRET axis parallel to the beam, and with the axis inclined at 20° and 30° to the beam.

The first goal of the Bates recalibration was to compare the performance of EGRET with its performance as measured in the SLAG cahbration. There were three areas of particular in- terest:

First, the angular resolution as determined by the distribution of measured arrival directions compared to the known arrival direction of the beam, the Bates data showed no significant differences from the SLAG results at the same energies.

The SLAG results and the post-launch determination of the angular resolution in operation are discussed in § 4.

Second, because the Bates data were accumulated at discrete points, while the SLAG data mapped the full active area of EGRET, a comparison of the detection efficiency for the two sets of data can only be made for a subset of the SLAG data.

The absolute detection efficiency for the SLAG data was derived for a comparable part of the instrument to that exposed at Bates. Within the uncertainties, the efficiency did not change between the two calibrations. What did change was the fraction of the events which could be recognized by the automatic analysis program. The improved spark-chamber performance at Bates allowed the program to analyze a larger fraction of the total events. At 200 MeV, for example, the fraction of events which the program could interpret increased from 82% at SLAG to 88% at Bates for a similar configuration. The skill of the data analysts (who reviewed the events which the program could not handle) in working with poorer data ac- counts quahtatively for the fact that the net result was similar in the two calibrations. (This reduced efficiency of the automatic analysis program with decrease spark-chamber efficiency has also been seen in the flight data as the spark- chamber gas deteriorates between gas refills. The comparison of the SLAG and Bates results indicates that much of this effect can be removed in the data analysis).

Third, to measure the self-veto from scattering into the anticoincidence system, runs were made at the highest energy available at Bates (790 MeV), with two different threshold settings for the EGRET anticoincidence system, one at the same set- ting used at SLAG (about 20 keV minimum threshold), and one at a higher threshold ( about 100 ke V ). The EGRET detection efficiency increased by (5.7 ± 2.0)% when the threshold was raised to reduce the self-veto effect. Although not highly statistically significant in itself, this measurement is consistent with the Monte Carlo model used to simulate this effect.

4. CALIBRATION DATA ANALYSIS AND RESULTS 4.1. Analysis of Individual SLAC Calibration Runs

4.1.1. Introduction

The data obtained at the SLAC accelerator form the experimental basis of the EGRET gamma-ray cahbration. Results from the measurements at Bates as well as changes introduced by different in-orbit operating parameters are included as corrections to the results derived from the SLAC data. The philo- sophy for the SLAC cahbration was to operate EGRET in its least restricted configuration and to use the instrument’s status

Concrete Block Pb Collimator

. Bean Stop/

\TQQQinQ Systen

EGRET Via stic Monitor

\Sweep Magnet

Fig. 12.—Layout of the complete calibration setup at BATES and event data recorded during the measurements to impose more restricted modes when the data are processed to form cahbration files.

In order to derive the cahbration data sets for the characteristic parameters describing the instrument performance, two basic processing steps are needed:

In the first step the individual “cahbration runs,” for which data were recorded for one beam energy and incidence direction at a time, are processed to a level where the effective areas, angular dispersion and energy dispersion are available for all forseen event selections and trigger modes (CALAN format).

In the second step the CALAN format data for all runs are used to combine, smooth, fit, and interpolate ah data relevant for one event selection into the final cahbration file. The data grid is chosen sufficiently fine that only linear interpolation is needed when scientific analysis programs retrieve parameters from the cahbration file (CALFIL format).

4.1.2. Processing of Individual Calibration Runs Each individual “run” at SLAC consisted of a scan over the active EGRET area at a single combination of energy, tip ( inclination) angle, and azimuth angle. As discussed in § 3.1.2, the scan was performed stepwise through a predefined scan point pattern by moving the instrument across the spatially fixed beam.

The time interval spent on each point was determined by accumulating a predefined number of counts in the “plastic monitor” (Lin et al. 1991 ) which is related to the number of incident photons. One major task of the run analysis is to relate the plastic monitor counts to the number of “useful” photons seen by the EGRET instrument. Another is to exclude dupli- cate scan points or otherwise unusable fractions of the scan sequence from the analysis. Further, the acceptability of the events on the basis of parameters such as the distance of the pair-creation point from the spark-chamber walls was checked. Finally, tables were created and stored for use in the subsequent CALFIL processing step, which provides information on the calibration parameters for all possible operating modes of the instrument.

There are two acceptance criteria to be checked continuously during the processing of a run data stream. First, based on the auxiliary data, which contains primarily the beam- monitoring data, instrument-configuration data and the information on the instrument’s position relative to the beam, the accumulation of useful incident photons and the acceptance of events must be determined. Second, events which are recorded by the instrument during these periods are checked against several parameter limits.

The data checks performed on the auxiliary data include the following:

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19 93ApJS. . .86. .62 9T

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1. The EGRET instrument configuration agrees with the expected configuration;

2. The scan sequence is enabled;

3. The beam position agrees with the currently observed point in the scan pattern;

4. The beam direction is as expected;

5. The beam intensity is above the required minimum.

For individual events it is required that:

1. There is corresponding auxiliary data in the data stream;

2. The spark-chamber technical data processing bits are acceptable;

3. The conversion point in the spark chamber is within a volume defined as acceptable;

4. The event is compatible with an origin from a photon arriving within the expected extent of the photon beam;

5. The event is classified by the event track analysis and structuring program “SAGE” or by the analyst as an acceptable gamma ray.

If the event fails any of these tests, then it will be dropped from further analysis. While this task in principle should be simple, it is rather complicated due to a non-negligible number of bit errors in the data stream. This has made it likely that certain inconsistencies in the derived calibration data are related to unrecoverable errors in the data stream.

4.1.3. Effective Area

The effective area is derived for a calibration run on the basis of the following expression:

^ . number of accepted events , , effective area = ; —;—; (scan area). (8)

number of useful photons

The scan area is defined as the geometric area of a single scan point, which was 21.65 cm2, multiplied by the number of scan points which were covered.

A major complication arises from the non-negligible multi- plicity of photons within the short (20 ns) beam pulse.

EGRET cannot resolve multiple gamma rays arriving within the pulse, and the presence of additional gamma rays compli- cates the analysis. Because the beam intensity had to be relatively high (up to 0.3 photons per beam pulse) to get sufficient statistics within the fixed time frame of the calibration at SLAG, the fraction of multiple-photon pulses cannot be ne- glected. Various interactions of these multiple events within the experimental setup occur and must be accounted for by corrections to the basic equation when the number of valid events and the number of “useful” incident photons are derived. A specific analysis approach, described below, has been developed to minimize the uncertainties in the calibration results.

This method is based on the following approach: If the beam trajectory penetrates the TASC energy calorimeter (8 radiation length thickness), then all photons will convert to electron-positron pairs, either within the spark chamber or in the TASC. All such events will deposit energy in the TASC roughly proportionally to the number of photons in the beam pulse. It is then possible to identify events from beam pulses which

641 contain more then one photon on the basis of the energy deposit in the TASC calorimeter.

One of two alternative algorithms is therefore used for deriving the detection probability at a specific scan point:

1. For the fraction of the scan pattern where the beam hits the TASC, only the calculated number of photons arriving in

“single-photon beam-pulses” are accumulated, and correspondingly only events which have a TASC energy deposit which is acceptable for a single photon are used.

2. For that part of the pattern where the beam misses the TASC, the calculated number of photons in all beam pulses is accumulated and correspondingly all events are accepted, with corrections for the pulses with multiple gamma rays.

At the end of processing of a run, a record is written containing the beam energy, the tip and azimuth angles, and the sensitivity for all 74 possible trigger mode combinations and for the three event energy classes.

Accumulation of accepted events is done by verifying that an individual event is an acceptable photon observed during an acceptable time interval of the run. If the beam path does not penetrate the TASC volume, it is added directly to the event counters. If the beam path does penetrate the TASC volume, the event will be counted only if the energy deposit in the TASC calorimeter is compatible with the deposit expected for a single photon having the known beam energy. Actually an upper threshold is applied to the acceptable energy deposit within the TASC, based on the observed distribution of measured energies. This limit rejects a large fraction of multiple beam photons and also high-energy bremsstrahlung contamination which might disturb the calibration runs below 100 MeV. In addition to the total number of accepted events, the program also accumulates the number of events falling into the various instrument modes and event classes. At the end of the processing of a run, these numbers are used for deriving the effective area.

Since the beam is not a “tagged” beam, where the arrival of each individual photon is signaled, the number of useful incident photons or beam intensity can be obtained only in the form of an average intensity, related to the “plastic monitor”

beam counter, which converts and counts a fraction of the photons in the beam upstream of the EGRET instrument. As the beam intensity can show extreme variations on many time scales through a run, the number of useful incident photons is derived for many sub-intervals of time individually and is then accumulated.

Depending on the beam’s trajectory, hitting TASC or not hitting TASC, either the number of photons arriving in beam pulses containing only one photon, or the total number of photons arriving in single- and multiple-photon pulses is calculated and accumulated. The distribution of the photons in beam pulses is taken to be Poissonian for this purpose. A number of corrections are needed to account for special effects due to the setup of the beam, beam monitors and EGRET instrument:

1. Conversion of photons upstream of EGRET in the beam monitor and air ;

2. Contamination of the gamma-ray beam with bremsstrahlung gamma-photons, especially at low beam energies;

3. Self-veto effects from photons in multiple conversions, especially at high beam energies;

CALIBRATION OF EGRET