Radiation Detection

• ionization chambers (dosimeters, pulse chambers, particle track chambers)

• scintillation detectors

• semiconductor detectors

• photographic emulsions

interaction with atoms

E = h

12

1. EXCITATION

(Energy of the radiation used for increasing the internal energy of the atom)

2. IONIZATION

(Radiation separates the atom into an electron and an ion.)

ionization level

Particles of different radiation carry different amounts of energy.

typical ionization energy range: 10 - 25 eV

-radiation - 100

number of primary ions:

-, -radiation - 100 000

recombination

+

-

+

-

+-

Ions may collide inelastically to form a neutral atom.

The probability of recombination:

2nnndt

dn

n - number of pairs of ions - recombination coefficient

Excess energy is released in the form of electromagnetic radiation.

ionization chamber dose meters

+ +

+ +

+ +

++

pocket ion chamber electroscope

condenser-R-meter

V

electrometer

V

charge collection

+- -+

+ -

• With no field the ions eventually recombine leaving no record of radiation.

+- -+

+ -

+- -+

+ -

• With a weak field some ions recombine and some are discharged by the electrodes.

• With a sufficiently strong (but limited) field all the ions are discharged by the electrodes (saturation).

• Further increase of the field results in an avalanche discharge.

gas-amplification factor

The collected charge is proportional to the charge of the primary ions. The proportionality coefficient is called the gas-amplification factor.

recombination region: ions remain in the chamber for an extended time; some ions recombine and are not collected

(factor < 1)

saturation region: no time for recombination; no energy for secondary ionization

(factor = 1)

avalanche ionization: ions achieve sufficient energy to ionize neutron atoms

(factor > 1)

Geiger – Müller (G-M) tube

+

ab

concentric electric field:

r

1

ln

VE

b/a

ionization potential:

V2510V

required field:

V

E

where is the mean free path of the ions.

a ~ 0.08 mm

The strongest field is at the anode.

G-M tube operation regimes

0 250 500 7501

102

104

106

108

1010

tube voltage

puls

e si

ze

A

A – recombination region

B

B – saturation region (dependent on energy)

CC – proportional region

D D – limited proportionality

E

E – Geiger region (independent of energy)

F

F – continuous discharge

electric pulse in the Geiger region

qQ

space charge

r0

E1 E2

From Gauss’s law

rl

1

2

qE

0

1

rl

1

2

qQE

0

2

,

With a relatively small signal V const and the charge on the central wire is

tr

bln

2

tQV

ln

2tq

00

0

lb/a

l

Typically the space charge reaches the cathode and gets neutralized in 100 s.

quenching the discharge

Neutralization of ion results in photoelectric effect. The electrons liberated from the cathode often trigger self-perpetuating series of discharge at a frequency determined by the circuit time constant.

Quenching circuits or quenching gases are used to prevent secondary discharge.

Properties of a quenching gas:

• ionization energy lower than that of the main gas and the work function of the cathode material

• absorb UV

• dissociate rather than radiate UV

pulse shape

1.0

0.5

1.0

time (10-4 s)

puls

e vo

ltag

e

0 2 4 6 8

0.89

V

The discharge (slightly) lowers the anode potential.

After the discharge the potential of the anode is restored by the high voltage power supply.

G-M counter

HV- +

counting circuit

amplifier/discriminator

The proportional region:

gas amplification factor ~ 103

moderately strong pulses;dispersive pulse size;

G-M region:

gas amplification factor ~ 106 - 107

very sensitive to radiation;

resolving time

1.0

0.5

1.0

time (10-4 s)

puls

e vo

ltag

e

0 2 4 6 8

0.89

V

0.2

V

The dead time and the recovery time depend on the threshold voltage of the circuit.

counting rate

600 800 1000 12000

100

200

300

tube voltage

coun

ts p

er m

inut

e

A

A – threshold voltage

B

B – fraction of primary events countedC D

C-D – all primary events counted (plateau)

E

E – continuous discharge(a single event results in repetitive spontaneous discharge)