interaction with atoms E = h 1 2 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: 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 + a b concentric electric field: ionization potential: required field: 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 0 250 500 750 1 10 2 10 4 10 6 10 810 tube voltage pulse size A A – recombination region B B – saturation region (dependent on energy) C C – proportional region D D – limited proportionality E E – Geiger region (independent of energy) F F – continuous discharge
electric pulse in the Geiger region q Q space charge r0r0 E1E1 E2E2 From Gausss law, 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.
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) pulse voltage 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 ~ 10 3 moderately strong pulses; dispersive pulse size; G-M region: gas amplification factor ~ 10 6 - 10 7 very sensitive to radiation;
resolving time 1.0 0.5 1.0 time (10 -4 s) pulse voltage 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 1200 0 100 200 300 tube voltage counts per minute A A – threshold voltage B B – fraction of primary events counted C D C-D – all primary events counted (plateau) E E – continuous discharge (a single event results in repetitive spontaneous discharge)