Radiation Detection Instrumentation

Transcription

1 Radiation Detection Instrumentation Principles of Detection and Gas-filled Ionization Chambers Neutron Sensitive Ionization Chambers

2 Detection of radiation is a consequence of radiation interaction with matter. A detector is Some kind of material that interacts with radiation, and the result of such interaction can be Sensed by instrumentation and recorded by a data acquisition system (DAS).

3 Radiation Detector Electronic Instrumentation The Detector: Gas-ionization or Scintillation or Semiconductor detectors Electronic Instrumentation: High voltage supply, signal Conditioning, signal amplifier, scalar and DAS.

4 Gas Counters (Gas Ionization Detectors) A gas chamber filled with a chosen gas. Two electrodes are installed inside of the chamber and connected to a power supply. The electric current measured by the ammeter is a measure of radiation intensity. Gas Chamber Positive Power Supply Radiation Ammeter Negative Resistance R

5 This is an Ionization Chamber Will estimate the voltage pulse height generated by this type of detector. Assuming the incoming radiation interacts with the gas and that the energy required to produce one ion pair is about 20eV. Therefore when the energy of the incoming radiation is 1.2MeV and is completely absorbed in the gas the number of ion pairs n produced is then given by: n 6 1.2MeV ev = = = eV 20eV 4 ion pairs

6 And the electric charge q (Coulombs) produced in the gas is determined by the number of ion pairs n multiplied by the unit charge e 1.602x Coulomb q = ne = = Coulombs The gas-filled chamber at ionization is equivalent to a capacitor, and assuming that the capacitance is 140pF then the amplitude of the generated voltage pulse will be calculated by: q C V = = 6 = Volts = µ V 12 Indeed such voltage needs to be amplified

7 Radiation Source Multi Channel Analyzer Detector Pre-amplifier Amplifier Single Channel Analyzer Scaler HV Power Supply Oscilloscope Timer Instrumentation for Pulse Height Analysis

8 Temperature and pressure corrections for the ionization chamber s measured current Corrections must be made to relate temperature and pressure to the ambient conditions (Ion chambers operate in pressurized mode which varies with ambient conditions of T ambient =20 o C (273 o K), and P ambient =1 atmospheric pressure (760 Torr). I detector = kρv T ambient T P P ambient de dt I detector is the detector current de/dt is the exposure rate K is a conversion factor ρ is the detector s gas density V is the detector s volume K = (2.58 x 10-4 C/kg-R)(1h/3600 sec)(1 A sec/c)=7.17 x 10-8 A-h/R-kg (taken from: Radiation Detection Instrumentation Fundamentals rev0) ebf5d3295fe5eaf9ac7afd3a3/radiation-detectioninstrumentation-fundamentals-rev0

9 This is a current-type detector used for measuring radiation level or dose rates. It can be a pulse-type counter to measure individual ionizing events. It also can be a charge-collection (voltage measurement) to measure radiation dose Ionization Proportional Geiger Mueller Chambers Counters Counters Collected Charge Voltage

10 How it differs from glow discharge? V b Bpd = 1 l n( Apd) l n l n 1+ γ se

11 Glow discharge I-V characteristic Ionization chamber collected charge versus applied voltage Taken from: You are free: to share to copy, distribute and transmit the work

12 Ionization Chambers: are current-type detectors providing integrated measure. Proportional Counters: used for heavy particles that can produce enough ionization. The pulse height is proportional to energy deposition. Distinguishes between alpha and beta particles. Geiger-Mueller Counters: is pulse height and no longer proportional to the amount of ionization. Pulses are quite high and may not be amplified. Gas multiplication is via runaway avalanche. Any further increase in voltage will induce internal discharge (arcing) and damage the tube.

13 Mica window Metal wall Geiger - M ueller Tube Thin wire Insulator Count rate GM plateau Voltage

14 Scintillation Counters The interaction of some particles with materials produces scintillation or flash of light. This scintillation can be amplified in a photomultiplier tube, then be analyzed for the pulse height and counted. Scintillation counter electronics

15 Photo-multiplier tube DA-Type PM with built-in divider circuit and amplifier Photomultiplier tubes basics and applications, 3 rd Edition, Hamamatsu

16 Hamamatsu Learning Center Photomultiplier Tubes

17 Model 44-2 NaI(Tl) 1" x 1" Gamma Scintillator You are free: to share to copy, distribute and transmit the work

18 Solid State Detectors They are semiconductors and compact. The electrical conductivity of the semiconductor is sensitive to presence of impurities. The depletion layer is sensitive to radiation and electric current flow through, thus the measured current is a measure of radiation. + _ n-type depletion layer p-type Radiation or incident particles

19 Identify radionuclides via spectra Determine source intensity Measure linear attenuation coefficient of materials Pulse counting spectroscopy system

20 Measure linear attenuation coefficient of materials ORTEC model 456 HV V Na I ( Ti) detector Lead shield Lead collimators 1.1cm Amplifier Material sample Lead collimators MCA Radiation Source Base Computer

21