RAD: Radiation Assessment Detector

The Radiation Assessment Detector (RAD) is a passive energetic particle detector, which allows to characterize the full spectrum of the radiation environment at the surface of Mars, including Galactic Cosmic Rays²² (GCRs) and Solar Energetic Particles²³ (SEPs), which interact with the atmosphere and penetrate into the soil producing secondary particles, such as γ-rays and neutrons.It was turned on during cruise phase toward Mars to measure radiation in the deep-space environment, while continuing to function on the surface of Mars. These information

22Probably originate in supernovae: 85–90% are protons and the rest are helium nuclei while electrons and heavy nuclei cover only 1%[8].

23During big solar event, solar energetic particles can be found, on the surface of Mars, in greater percentage than galactic cosmic rays, but only for short periods of time. In addition, Mars and the Earth are generally not affected by the same solar particles, because they have different revolution periods and they typically see the Sun from different directions.

Chapter 3 3.1. Instrument investigation

are essential to determine the radiation dose rate in order to plan human exploration and assess the planet’s ability to sustain life. The RAD instrument is mounted just below the top deck

Figure 3.30: The top of the RAD sensor head visible on Curiosity’s body [40].

of the rover with the charged particle telescope pointed in the zenith direction (Figure 3.30) and consists of the RAD Sensor Head (RSH) and the RAD Electronics Box (REB) integrated together in a tiny and compact volume (Figure 3.31).

Figure 3.31: Schematic diagram of the RAD detectors. On the left: cross section view of RAD showing the sensor head channels above and electronic board layouts below. On the right:

possible colored paths show charged and neutral particles that the RAD can detect: green paths are considered valid events, while red paths are rejected [41].

RAD Sensor Head (RSH) The RAD sensor head contains multiple silicon detectors arranged vertically. The view cone is 65° wide, which size is not causal because it is essential to have a broad view of observation and keep the mass and volume of the instrument as small as possible.

As shown in Figure 3.31, it consists of 3 Solid-state Silicon Detectors (SSD A, SSD B, and SSD C) of different widths to make sure they detect the same group of particles, a thick Cesium Iodide Scintillator(CsI - D) and two Plastic Scintillators (one marked with the letter E and an-other with the letter F that encloses D and E). High-energy and low-energy charged particles,

respectively can penetrate the entire instrument or be partially blocked. Charged particles to be detected and analysed must pass through the upper detector and register onto the lower de-tector surface. For example, suppose a charged particle hits only dede-tector A, or any other lower detector without hitting the upper ones. In that case, it will be rejected from the analysis, so for this reason, the role of the anti-coincidence shield (F) and the outer rings of SSD B and C are essential; thus, if the particle triggers detectors D and E without entering the view cone is discarded from analysis.

RAD can determine the mass, energy and charge of the registered particles, but if any of these pass all the layers and deposit most of their energy in the detector, in this case RAD can not determine the mass of these particles; the RAD energy coverage is shown in Figure 3.32. Moreover, neutral particles (γ-rays) and neutrons²⁴, the viewing cone is not an entry re-quirement, because RAD detects them as detected events in D and E, but not any of the others.

Finally, the high atomic mass of cesium iodide, which constitutes the detector D makes it effect-ive at detecting γ-rays, while the plastic in detector E makes it poorly performing at detecting γ-rays, but efficient at detecting neutrons.

Figure 3.32: RAD energy coverage for both charged and neutral particles [41].

RAD Electronics Box The Front-End Electronics (FEE) consists of a charge-sensitive preamp-lifier and a shaping amppreamp-lifier, which changes the shape of the received signal so that it is dis-played correctly. The electronics are designed to have very low noise and good stability through-out the temperature range in which the instrument is exposed. Thus, the RAD Electronics Box (REB) contains three circuit boards:

• RAD Analog Electronics Board (RAE): this board receives the seventeen analogue output signals produced by RSH. In addition, the Voltage-Input Readout Electronics for Nuclear Applications (VIRENA), which is a mixed-signal ASIC, provides, for each input channel, one amplification (with 1x, 2x, 4x, 8x gains), two discrimination levels ("Slow Trigger"

branch and "Fast Trigger" branch referring to the relative timing of the two pulses), and

24Curiosity’s MMRTG generates a lot of γ-rays and neutrons, although most of them are at low energies, so RAD can detect these particles and reject them before they are analyzed.

Chapter 3 3.1. Instrument investigation

two flip-flops (elementary memory devices, used to store status information). Addition-ally, it has a single multiplexed output for analogue signals, which is connected to an analogue-to-digital converter (ADC) that is also located on the RAE board (Figure 3.33).

The signals from the A1, A2, B and C detectors are amplified by two independent shapers in the RSH and each of the two outputs is further divided and amplified in the VIRENA, so that there are four channels for each of these detectors. Fast triggers for individual channels can be enabled or disabled, as can the corresponding slow trigger outputs, but only enabled fast triggers (BU, DH, and EH²⁵) are inspected by Level 1 (L1) and when one or more fast triggers are triggered, Level 2 (L2) trigger processing is launched, which looks for specific (configurable) combinations of slow triggers. The BU fast trigger en-ables L2 triggers for charged particles and silicon dosimetry, while the DH and EH fast triggers enable neutral particle detection and E dosimetry. Events are assigned a "hard-ware priority" by L2, which influences downstream processing [41];

• RAD Digital Electronics Board (RDE): once the analogue signals coming out of VIRENA have been digitised, they are analysed by the L2 firmware in real time. This analysis is referred to as Level 3 (L3) and takes place in a virtual microprocessor instantiated in the RDE FPGA. The L2 trigger assigns a hardware priority value (0 or 1) to each event and only particle event data marked as high hardware priority is sent from L2 to L3 via a FIFO (First Input First Output) buffer, and due to its limited space the same proportions of high and low priority events are not digitised. In any case, the aim is to keep as many rare events (in particular, heavy ions) as possible, even under high-speed conditions. At the end of an acquisition period, all data are packed into an observation package and stored until the rover retrieves them [41];

• RAD Sleep Electronics Board (RSE): the RAD is designed to make observations, with an autonomously managed wake/sleep frequency, without the intervention of the RCE. This function is implemented in the RSE Board. While RAD is sleeping²⁶, the RSE monitors the line receiving RAD’s commands for activity (these can be orders sent by the RCE or noise on the line). RAD wakes up from sleep when two events occur (internal sleep timer expired or receipt of a command) and begins the boot process, only when the RSE applies power to the rest of the instrument. The RSE provides information to the software indic-ating if the wake-up is due to a command or due to the expiration of the sleep timer, then the software through this information decides whether RAD should start an observation, wait for further commands, or go back to sleep, if no other command is received within the next 60 s (thus avoiding confusing a random line noise with a command).

25There are VIRENA outputs for low gain (L), medium, gain (M), high gain (H), and ultra-high gain (U) [41].

26The sleep circuit is always on, drawing a small current of about 2 mA.

Figure 3.33: Block diagram of the RAD Electronics Box (REB) [41].

Electronics and usage time RAD works independently of the rover’s other activities. During the mission RAD takes observations once per hour for 16 min followed by 44 min of sleep time. During its sleep period, RAD gathers and stores all observations in a single packet even if the rover’s main computer is asleep. RAD transfers all the data to the main computer which sends them to Earth, via telemetry, thanks to the UHF communication. Moreover, each time RAD wakes up, it performs a 10-second "pre-observation" measurement and if it detects high particle flux, it changes automatically mode and makes more frequent observation. RAD can accumulate about 400 kB of data in an ordinary sol²⁷and it has 16 MB of a Non-volatile random-access memory (NVRAM), which is a random-random-access memory that retains data without applied power for data storage.

RAD - Power consumption Idling operating mode: 0.1 W;

Normal power operating mode: 4.2 W.