Gas Detectors § § §

Gas Detectors

[the oldest detectors]

§   Ionization chambers

§  Proportional counters

§  Geiger Muller counters

Same basic principle, different intensity of applied electric field

Basic principle:

- gas ionization from radiation interaction;

- electric signal originated by ion-electron pairs collected

through an electric field.

Ionization Process

basic signal

Pair formation:

- direct interaction - secondary process (very energetic electrons can ionize …)

w ∼ 30 eV

average energy per pair production:

weak dependence on gas and particle type

< 1 ! !

# of pairs (e-,ion) depends on particle energy

Example: E

∼ 1 MeV

# pairs ∼ 30000

Ions/electrons suffer of multiple collisions with gas molecules and lose energy

⇒ thermal equilibrium with gas, ricombination, … loss of signal

Thermal motion

produces diffused charge transport from high energy regions

Electric Field is needed to efficiently collect the charges from interaction place Net effect: superposition of thermal velocity (random) and drift velocity (ordered)

v

v

v

diffusion coefficient

v v

= a τ = F/m τ

Drift velocity

p v

∝ µ E

Typical values:

v_drift = 1 m/s

Transit time (1 cm distance) = 1 ms !!!

v

v

def

Gas admixture

90%Ar+10% methane 10%Ar+90% methane

v _drift

v

v

[reduced mass compared to ions]

Region of operation of gas detectors

1 2

3

4

5

Signal proportional to primary+secondary ionization

Same Signal independent of energy Loss of original Signal

Nr. Collected Ions vs. Voltage

1 2 3

4 5

Region of operation depends on ⇒

General constituents

§ 

§ 

§ 

Ionization Chamber: the simplest gas detector

10-100 V

E = constant

E ∼ 1/r

Choice of gas:

- air (γ detection)

- Ar (more dense)

Mode of operation:

- current mode [average rate info]

- pulse mode [info on individual event]

Measured current is true indication of rate of formation of charges

Signal type and size:

w = 30 eV ⇒ n =E_α/w = 10⁵ e-ions

SMALL signals, amplification is needed !

(low density and Z !!!)

Force acting to move charges to electrodes

Energy absorbed by e- and e+

v v

v

Time dependent signals observed in the detector

τ = RC >>t

signal rise time depends charge collection time ton _c

v v

v v

v

(

Δ

The pulse is chopped

at a convenient time

by a shaping circuit

Ionization Chamber pulse mode operation with Frisch grid

The fine mashing grid removes

the pulse-amplitude dependence on position of interaction:

•  The grid is maintained to intermediate potential and is transparent to electrons

•  Incident radiation directed only into active volume Chathode-Grid

d = 1.6 cm d = 16 cm

in PRISMA

Ionization Chamber

@LNL

Frisch

NO resistor

⇒ NO measurable signal

The signal derives from electrons drift only

⇒ NO dependence on slow ion motion !!!

Shield

Application 1

IC for Z identification of reaction products

2 2

mZ E E

E mZ dx

dE × ∝ × =

Bhete-Bloch

Application 2

Ionization chamber

1 µCu (∼ 0.3 µg) of ²⁴¹Am τ = 432 y

- α passing through ionization air-chamber produce costant current

- smoke absorbes α’s ⇒ reduced ionization, lower signal, allarm ..

- α’a have low penetrability: they are stopped by the plastic of detector

NEUTRON detectors

Detection of secondary events produced in

- nuclear reactions: (n.p), (n,α), (n,γ), (n,fission)

- nuclear scattering from light charged particles, than detected For slow and thermal n:

(n,p) and (n,α) mostly …

Best reaction:

γ = 0.48 MeV

•  Exotermic reaction:

it occurs also for very low n energy For E_n = 0.025 eV, σ = 3770 b !!

⇒ ideal for n detection

⇒ useful for measuring the flux of thermal n

∼ 3000 b

∼1/v

•  Natural abundance of ¹⁰B is high: 20%

⇒ material enriched with ¹⁰B increase the efficiency

Application 3

Ionization chamber/proportional counters filled with BF

gas

Paraffine moderator as

Ideal response

Real Response

Wall effect

Due to partial energy deposition of lost ion

Lost

7Li Lost

7Li Lost

α Lost

α

[typical sized tubes: e.g., 2 - 5 cm diameter]

Only determination of interaction, NOT energy Wall effect

prevents

precise energy measurements

Proportional Counters

Basic Principle : signal moltiplication to amplify charge produced by original (e-, ion) pair

⇒ Avalanche Formation:

E

∼ 10

V/m, p ∼ 1 atm

e

n x

n n dx dn

α

α

) 0 ( )

( =

=

density of electrons increases exponentially in the direction of motion of the avalanche

Townsend coefficient

α = 0 if E < E_th

α ∝ E if E > E_th

Townsend coefficients

Ar-CO

mixture

Best Geometry: Cylindrical

Polarity

a = anode radius b = cathod radius

Example:

V = 2000 V

a = 0.008 cm ⇒ E(a) = 10

V/m b = 1 cm

N.B.

Electrons are more mobile than ions

⇒  Avalanche looks like a liquid drop with electrons at the head

Uniform multiplication

is obtained only in a confined region d ∼ 5 a

MonteCarlo simulation

a = anode radius b = cathod radius

Pulse signal from a

cylindrical proportional counter

τ   = RC

The pulse is usually cut by an RC differentiating circuit

Total Charge produced

(E)

Q = n

e M

Gas multiplication Factor

M = 10

- 10

Application

Multi Wire Proportional Counter

[invented by G. Charpak, CERN 1968, Nobel Price in 1992]

Detector sensitive to position of radiation interaction:

- 2 planes of wires (anodes and coathods) - Δx ∼ 2-3 mm

- size ∼ 1 m

Electric field lines and

constant potential energy surface

Wire read-out may be very complicated

⇒ read-out of more than one wire at just one end Position reconstructed by charge division

Read-Out methods 1.   Separate wire read-out

1.   Analog methods:

Center of gravity

Chatode in strips:

Signals from each strip is stored and center of gravity is calculated

More layers: x, y, diagonal …

correction factor for noise effects

Charge Division

Charge collected at the two ends of the resistive anode is divided in proportion to the length of the wire from the point at which the charge is injected

B A

A

Q Q

Q L

y

= +

MWPPAC: the Focal plane detector of the PRISMA spectrometer

at LNL Laboratory (Padova)

- 3 electrode structure:

central cathod 2 anodic wire planes (X and Y) - cathode: 3300 wires of 20µm 0.3 mm spacing negative high voltage: 500-600 V

- X plane: 10 x 100 wires each 1mm spacing

- - Y plane: common to all cathode,

-  130 wires, 1 m long, 1mm steps - spatial resolution:

∆X ~ 1mm,

∆Y ~ 2mm

1 m

13 c m

Focal Plane Detectors of PRISMA:

in-beam tests

X position (channels)

∆t ~ 300 ps

∆X = 1 mm

∆Y = 2 mm

Y position (channels)

MWPPAC (ε ~ 100%)

different shapes due to

PRISMA optics dispersion

in X

(DIPOLE)

focusing in Y

(QUADRUPOLE)

195 MeV ³⁶S +

IC

E (a.u.)

ΔE (a.u.)

Z=16 Z=28

240 MeV ⁵⁶Fe+¹²⁴Sn, Θ_lab = 70°

Z=26

ΔE (a.u.)

∆E/E < 2%

ΔZ/Z ~ 60

Application

Proportional Scintillator Counter

Hybrid detector !!!

UV

High field

With noble gases

75-97 % of energy gained by electrons from electric field is converted into light

Relative gain per collected e

charge

secondary light

light

Energy resolution for low radiation is 2 times better

than with ordinary proportional counters

Low field

X

10

Geiger-Muller Counters

for High Electric Fields:

Secondary avalanches are produced by photons emitted by excited atoms

M ∼ 10

Entire volume gas is involved in the process

⇒  Loss of information on space

⇒  Loss of information on energy (NO spectroscopy)

⇒  identical output

400-1200 V

depending on the gas

Noble gases

To avoid formation of negative ions being based on multiplication process

portable instruments for

radiation monitoring

Dead Time

The Geiger discharge stops when an high ions (+) concentration reduces the field E below the moltiplication threshold

50-100 µs

Second full pulse appears when ALL ions (+) are collected

to the cathode

⇒  G-M tubes are limited to application with low counting rates Method to improve

effective counting range

1. Time-to-first-count-method:

The high voltage is switched between two values:

- Normal operating voltage

- lower value below avalanche threshold

1-2 ms

2. Self-Quencing:

addition of quench gas (5-10%)

with complex molecule absorbing UV photons (ethyl alcohol …)

Quenching gas is consumed (molecule disassociate) à Limit 10⁹ counts