Radiation Detection Theory
Semiconductor Radiation Detectors
Centronic semiconductor radiation detectors are based on silicon photodiode technology (refer to electro-optic product information). The diode structure can be used to detect ionising radiation directly as a result of ionisation within the semiconductor material. The design of the detector can be tailored (e.g. depletion width, window material and thickness, operating bias, etc.) to optimise performance characteristics such as energy resolution and efficiency. Neutrons can be detected in the same way through the addition of 10B or 6Li to the photodiode. Interaction of neutrons with these isotopes creates ionising alpha particles which are detected as above. Addition of a scintillator material (e.g. NaI, CsI, CdWO4, GdOS) to the photodiode allows the detection of x-rays through conversion of the x-ray energy into detectable photon energies.
X-Ray Proportional Counters
X-ray proportional counters are filled with a noble gas which can absorb x-rays. When an x-ray photon is absorbed by a gas atom, the excess energy of the system is released via photoelectron emission; generally from the K-energy shell. The energy of the released photoelectron is directly proportional to the incident x-ray photon energy, which means that the 'pulse height' of the signal from the counter will provide information about the energy of the incident radiation.
For low energy (long wavelength) x-ray detection a light gas is required and for high energy (short wavelength) x-rays a heavy gas is needed. Detector efficiency is also governed by window material thickness and type, with thin, low atomic weight windows offering the best sensitivity. In general detectors have an aluminium body and thin beryllium windows to maximize detector efficiency. However, thicker windows are normally needed to contain higher gas pressures when increased sensitivities are required.
Helium 3 Neutron Proportional Counters
The reaction which allows 3He proportional counters to act as neutron detectors is:
3He + 1n (thermal) ® 1H + 3H + energy
The energy released by the reaction between a neutron and the 3He gas filling of the counter is 764 keV. This is transmitted as kinetic energy by the reaction daughter products. 3He neutron detectors provide an output pulse for thermal neutron interactions, which is proportional to 764 keV. The charge produced as a result of the released energy ionising the filling gas is low. Therefore this charge is increased by applying a high voltage to the anode wire of the counter. The ratio of the charge at the anode wire to that initially created by the reaction is called the Gas Gain or Gas Multiplication. Increasing the applied anode voltage increases the gas gain. An operating voltage is chosen to give a sufficient gas gain for counting purposes. Most 3He counters use a quench gas to stabilize high voltage performance and prevent run-away.
BF3 Neutron Proportional Counters
BF3 counters operate in a similar way to 3He proportional counters but rely upon the interaction of thermal neutrons with 10B :
10B + 1n (thermal) ® 7Li + α + energy
In most interactions the resultant energy is 2.31 MeV, which is shared as kinetic energy of the daughter products (0.84 MeV to the 7Li and 1.47 MeV to the alpha particle). 10B has a lower thermal neutron cross-section (3840barns) compared to 3He (5330 barns). This makes BF3 counters less sensitive then 3He counters. The energy released by the reaction is much larger in BF3 counters, however, making it easier to discriminate against a gamma background.
Boron Lined Proportional Counters
Boron lined proportional counters rely on the same 10B + 1n interaction as BF3 counters but in boron lined counters the boron is in a solid coating form rather than a gaseous form. The proportional counter is again filled with a gas which is ionised as a result of the energy released by the boron/neutron interaction, but in boron lined proportional counters this interaction does not take place in the gas itself. The reaction takes place at the wall of the proportional counter, and, since the recoil particles travel in opposite directions, only one energetic particle enters the gas filling and causes ionisation. The depth of the reaction within the boron coating and the angle of ejection of the recoil particle gives a range of expended energy in the filling gas from that of the ? particle down to lower energies. The ionisation charge produced is again multiplied by application of a high voltage to the anode wire of the counter.
REM Counters
REM counters are used in neutron dose rate monitors and comprise of a gas filled neutron detector enclosed in a moderator/attenuator structure. The neutron detector incorporated in REM counters is usually a BF3 counter (cylindrical REMs) or a spherical 3He (spherical REMs). The spherical version of the REM (having a spherical moderator) is used where omnidirectional monitoring is required. The spherical moderator gives a near-isotropic response to neutrons. The moderator of a REM counter is typically polyethylene but may also include heavy metal inserts, e.g. cadmium or lead. The moderator/attenuator structure is designed to give, as nearly as possible, an energy independent neutron sensitivity in the range to be monitored. This range is typically 0.025 eV to 10 MeV.
Neutron Spectrometry Components
By enclosing a neutron counter, e.g. a spherical 3He counter, at the center of a sphere of moderating material, it is possible to measure neutrons of a specific energy range. By varying the thickness of this moderator, i.e. by using spheres of different diameter, or by introducing other materials within the sphere, it is possible to vary the energy range of the measured neutrons (refer to the Bonner Sphere and NEMUS designs of PTB, Germany). The design ensures that, so far as is practically possible, the neutron counter is surrounded by an identical thickness of materials in all directions in order to give omnidirectional performance.
Ionisation Chambers
Incident radiation interacts with the volume of filling gas within an ion chamber to form ion pairs. An electric field applied across the gas attracts the ion pairs to the electrodes, producing a current. The resulting current provides a measure of the ion pairs being formed. As the electric field (applied voltage) is increased, recombination of ion pairs created in the gas is decreased. Eventually the electric field reaches a sufficient magnitude that recombination of the ion pairs becomes negligible. This is known as ion saturation since further increases in applied voltage will not increase the ion current. Ion chambers are intended to operate in the saturated region where output current is proportional to gamma dose rate.
Boron Ion Chambers
The inclusion of a neutron sensitive material in an ion chamber allows the ion chamber to detect neutrons as well as gamma radiation. In some cases a hydrogen filling only is sufficient to give an adequate sensitivity to neutrons. The addition of a 10B coating is more common however (particularly for reactor control detectors) although a fissile material can be used if required. In boron ion chambers the same reaction is used as in BF3 proportional counters. Boron ion chambers are intended for use in much higher neutron fluxes however and generally give a dc output which is proportional to neutron flux when operated at saturation.
The addition of a third concentric electrode to a boron ion chamber allows the technique of gamma compensation to be applied. With three concentric electrodes it is possible to have one inter-electrode spacing sensitive to both neutron and gamma radiation (using a boron coating), while the second inter-electrode spacing can be left sensitive to gamma radiation only (by not boron coating these surfaces of the electrodes). If the output from each detecting volume is combined, e.g. by simple subtraction, then a measure of the neutron flux only can be obtained.
Fission Chambers
Fission chambers are essentially ion chambers coated with a fissile material on the inside of the cathode, giving the detector the ability to detect neutrons. These chambers are normally coated with an oxide of a Uranium isotope i.e. U235 for slow (thermal) neutrons or U238 for neutrons with an energy in excess of 1 MeV.
When the neutron interacts with the fissile coating two heavy, charged daughter products are formed which interact in the gas creating many ion pairs. These heavy particles release all their energy in a short distance causing a localised region of high ion pair density. It is normal to operate these chambers at high voltages to reduce the initial recombination and ensure ion saturation. The amount of charge created in the gas by the heavy daughter products is quite large, giving a very large pulse as a result of each neutron induced reaction. It is therefore not uncommon to operate Fission Chambers in pulse mode. The large pulse allows relatively easy discrimination against non-neutron pulses even at low neutron fluxes.
Fission ion chambers can be operated in dc mode for measurement of higher levels of neutron flux. Fission product activity resulting from the higher coating density generally associated with dc operation limits the dynamic range of fission ion chambers however. For wide range measurement of neutron flux, the output from fission chambers can be monitored using the Campbelling measurement technique.