Gas-filled Detectors

Transcription

1 Gas-filled Detectors

2 Radiation Gas-filled Detectors In a gas-filled detector, the io9nization provides electrons and positive ions. The acceleration of these charged particles obeys the simple equation of motion of charged particles under the influence of an electric field. Gas Chamber Positive dv F = ma = m = qe = q Φ dt Power Supply m = particle mass a a = dv/dt = acceleration E = electric field Φ = electric potential q = electric charge Negative Resistance R

3 Electric field in various geometry of gas-filled Detectors

4 Φ dv F = ma = m = qe = q Φ = q dt Φ d d Φ d Parallel plates provide uniform electric field and acceleration along the motion between the electrodes which is good for ionization chambers BUT electric field strength is not sufficient for GM or proportional counters Rectangular parallel plates Circular parallel plates E = Φ d

5 Φ Inner electrode of radius (a) Outer electrode of inner radius (b) Inner electrode can be a thin wire of 25μm radius and the inner radius of the outer electrode is typically 25mm Here the acceleration is large near the center Can induce secondary ionization Electric field strength is sufficient for GM and proportional counters Φ 1 Cylindrical geometry E r = nb/ a ( ) r

6 Spherical geometry Inner electrode is a small sphere of a=25μm radius connected to the HV supply via thin wire passing through insulating material Φ The inner radius of the outer spherical electrode is typically b=25mm E r = ab b a ( ) 1 r 2 Φ Here the acceleration is very large near the center Can induce secondary ionization Not quite practical due to manufacturing difficulty

7 More about ionization chambers

8 Californium-252 ionization chamber Cf-252 is a great neutron source It s branch ratio for spontaneous fission is 3% (6 orders of magnitude larger than Pu-240) The other 97% of the time it alpha decays So, the ratio of alpha decay to SF is about 33:1 Cf-252 is electroplated on the cathode of an ionization chamber Alpha particles are discriminated out based on their pulse height relative to fission fragments Each fission fragment pulse marks the time of a Cf-252 spontaneous fission Useful for neutron time-of-flight measurements (slide: Courtesy of Dr. J. Mattingly)

9 Hemispherical plate Cf-252 Alpha/fission fragment discrimination depends on separation between: Maximum alpha energy deposited Minimum fission fragment energy deposited In the parallel plate design: An alpha particle traveling nearly parallel to the electrodes deposits a lot of energy A fission fragment traveling perpendicular to the electrodes deposits a little energy The hemispherical plate design reduces the ratio of the maximum to minimum track length in the chamber ionization chamber (slide: Courtesy of Dr. J. Mattingly)

10 Fission chambers They operate in the ionization region, so they are insensitive to gammas, betas, and alphas They are insensitive to gammas and betas because they don t deposit enough energy to produce a decent-sized pulse in an ionization chamber They insensitive to alphas because alphas deposit a lot less energy than fission fragments

11 A fission chamber is a gas-filled detector lined with fissile material Induced fission in the fissile material releases fission fragments into the gas Useful for reactor flux monitoring Fission chamber (slide: Courtesy of Dr. J. Mattingly)

12 Fission chambers Fission chambers are gas-filled detectors lined with fissile material Special Nuclear Materials (SNM): U-235 (thermal fission) Pu-239 (thermal fission) U-238 (fast fission), technically a U-238 fission chamber is a fast neutron detector Fission chambers measure the energy deposited in the fill gas by the fission fragments They operate in the ionization region They re basically SNMlined ionization chambers (slide: Courtesy of Dr. J. Mattingly)

13 Fission chambers Ref.: Modeling of the saturation current of a fission chamber taking into account the distortion of electric field due to space charge effects, O. Poujadea and A. Lebrunb, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, V.433, issue 3, 1 September 1999, Pages

14 Fission chambers Photonics Nuclear Instrumentation Out-of-Core Fission Chambers Photonics Nuclear Instrumentation In-Core Fission Chambers Photonics model CFUZ53

15 Photonics model CFUZ53 In-core Fission chamber Neutron detector In-core measurements of flux range up to 5x10 14 n.cm -2.s -1 at 350 C

16 Neutron Detectors Neutrons do not directly ionize a gas, and thus neutron reactions that can produce gases can be used, example: 0 n1 + 5 B10 => 2 He Li7 In this reaction the neutron-boron interaction produces helium-4, then He-4 is the acting gas to operate as an ionization chamber. Boron tri-fluoride (BF 3 ) is usually used to induce the neutron-boron reaction. Neutron detectors measure the number of neutron incident on the detector.

17 Neutron Measurements BF 3 proportional counters (slide: Courtesy of Dr. J. Mattingly)

18 Typical gas-filled neutron detector Detecting neutrons fission based fragments on indirect methods. When neutrons interact with various nuclei they initiate the release of one or more charged particles, which can be detected and processed as electrically-measured signal. Ref.: Neutron Detectors, T. W. Crane and M. P. Baker

19 LND, Inc., 3230 LAWSON BLVD., OCEANSIDE, NY Model Neutron Sensitive Ionization Chamber OPERATING CHARACTERISTICS Maximum operating voltage (volts) 2000 Operating temperature range ( C) -50 to +100 Gamma sensitivity (a/r/hr - Co60) 1.2E-9 Neutron sensitivity (amps/nv) 3.5E-14 Resistance - HV electrode to shell (ohms) >1.0E9 Resistance - signal electrode to shell (ohms) >1.0E12 GENERAL SPECIFICATIONS Cathode material Aluminum Outer casing Stainless Steel Fill pressure (Torr) Effective volume 5200 cm 3 Gas filling Hydrogen

20 Reactor core power measurement using Cherenkov radiation Ref.: Reactor core power measurement using Cherenkov radiation.., M. Arkani and M. Gharib, Annals of Nuclear Energy, Vol. 36 Issue 7, July 2009, Pages