Radiation Detection

1. Radiation Detection • ionization chambers (dosimeters, pulse chambers, particle track chambers) • scintillation detectors • semiconductor detectors • photographic emulsions

2. interaction with atoms 2 1 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.) E = h

3. ionization level Particles of different radiation carry different amounts of energy. typical ionization energy range: 10 - 25 eV number of primary ions: -radiation - 100 -, -radiation - 100 000

4. + - + - recombination Ions may collide inelastically to form a neutral atom. + The probability of recombination: - n - number of pairs of ions  - recombination coefficient Excess energy is released in the form of electromagnetic radiation.

5. + + + + V + + + + V electrometer ionization chamber dose meters pocket ion chamber electroscope condenser-R-meter

6. + + + - - - + + + - - - + + + - - - 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.

7. 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) (factor > 1) avalanche ionization: ions achieve sufficient energy to ionize neutron atoms

8. + a b Geiger – Müller (G-M) tube concentric electric field: ionization potential: required field: a ~ 0.08 mm where  is the mean free path of the ions. The strongest field is at the anode.

9. 1010 F E 108 C pulse size 106 D 104 B 102 A 1 0 500 250 750 tube voltage G-M tube operation regimes A – recombination region B – saturation region (dependent on energy) C – proportional region D – limited proportionality E – Geiger region (independent of energy) F – continuous discharge

10. electric pulse in the Geiger region space charge Q From Gauss’s law q E2 E1 , r0 With a relatively small signal V  const and the charge on the central wire is Typically the space charge reaches the cathode and gets neutralized in 100 s.

11. 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

12. 1.0 pulse voltage 0.5 1.0 8 0 2 6 4 time (10-4 s) pulse shape The discharge (slightly) lowers the anode potential. 0.89 V After the discharge the potential of the anode is restored by the high voltage power supply.

13. amplifier/ discriminator counting circuit HV - + G-M counter 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;

14. 1.0 pulse voltage 0.5 1.0 8 0 2 6 4 time (10-4 s) resolving time The dead time and the recovery time depend on the threshold voltage of the circuit. 0.89 V 0.2 V

15. 300 200 counts per minute 100 0 600 1000 800 1200 tube voltage counting rate A – threshold voltage B – fraction of primary events counted D C E C-D – all primary events counted (plateau) B E – continuous discharge A (a single event results in repetitive spontaneous discharge)