Most detectors in common use in nuclear power reactors are ionization chambers.5 ‘7 Since the principles of the ionization chamber apply broadly to gas-filled de­tectors, only ionization chambers are considered in this section. Specific characteristics of other types of gas-filled detectors are discussed in Sec. 2-4.

2-2.1 Ionization Chambers

Ionization chambers are used to collect and measure the electric charge of ions and electrons that result from the interaction of incident radiation and secondary radiation from the chamber structure with the fixed, known volume of gas in the chamber. The quantity of collected charge is a measure of the incident radiation.

I he sensitivity of ionization chambers can be increased by incorporating materials8 into their structure which interact with the incident reactor radiation to create energetic ions or electrons These materials can be coated as a thin film on the electrodes of the chamber or they can be included in the chamber as a gas For the detection of thermal neutrons, 2 3 5 U and 10B in various degrees of isotopie enrichment are used Other materials ean be used if it is desired to increase the ehamber sensitivity to fast neutrons Table 2 1 is a partial listing of materials that can

Table 2 1—Threshold Energies of Materials for East-Neutron Detection

Thermal <1 MeV >1 MeV

2 3 3 U 2 3 4 U 2 3 2 rh

23SU 2 3 7 Np 23SU

235Pu

be used in fast neutron detectors Of the isotopes listed, 2 38 U is available commercially in a limited number of chamber configurations 2 3 9 Pu and 2 3 7 Np can be obtained by special order

Many isotopes of the heavy elements are of potential value in fast neutron detection Of these, the most likely candidates are 2 3 6 U, 241 Am, 2 40 Pu, and 24 1 Pu The list is

Basically, an юш/ation chamber consists of at least two insulated electrodes sealed within a metallic case or enclosure Connections are made through electrical seals designed for minimum charge leakage Typical resistance between electrodes and case is high, of the order of 101 ohms or greater A guarded structure can be used for the seals if very low signal levels are anticipated Such a structure consists of insulated conducting guard rings or cylinders surrounding each lead The guard rings are maintained at the potential applied to the corresponding electrode Figure 2 3 shows the structure of a fission chamber

The electrodes must be designed and placed so there is a uniform electric field within the sensitive volume of the chamber Auxiliary electrodes can be used to help attain field uniformity, which is particularly important when the sensitive volume of the chamber must be well defined and when uniform ion collection rates are needed In practical detector designs for reactor service, electrode spacing is small compared to the electrode dimensions As a con sequence, the fringing electric field at the edges of the electrodes does not seriously compromise performance Possible errors are usually small compared to other un­certainties in reactor flux measurements Because of this, auxiliary electrodes for field shaping are not used in normal commercial practice

limited by the availability of accurate fission cross section data, by interfering alpha radiation, and, in some instances, by spontaneous fission

Fast neutron measurements must be interpreted with caution Some of the above isotopes are available onlv bv separation from thermally fissionable isotopes For example, 2 3 6 U and 240Pu may have traces of 2 35 U and 239Pu, respectively These traces may cause errors A second reason for caution is that these isotopes are sensitive to the entire neutron spectrum above the fission threshold Thus, information about the entire neutron spectrum is contained in the signal

High voltage, the magnitude depending on the chamber design and application, is applied to one of the electrodes, and the other electrode is operated close to ground potential This high voltage may be quite low for a chamber of small physical dimensions and low gas pressure (<100 volts) For good performance, the minimum voltage on a large chamber with high gas pressure may be many hundreds of volts Since the gas pressure is fixed to optimize performance (Fig 2 4), the high-voltage limits are also fixed The maximum voltage is limited by the danger of breakdown The applied voltage is usually negative for counters since the collection of electrons is desired at the

low-voltage electrode For other types, the voltage mav be positive or negative The value of the applied voltage depends on the plateau of the chamber

Free electrons and positive ions are produced in the gas fill of the chamber Because ions are much heavier than electrons and not as mobile, electrons are normally col­lected faster 9 However, in some gases there is a tendency for the free electrons to associate with the molecules of the gas, producing negative ions ‘I hese negative ions are also relatively immobile The gas fill of the chamber is therefore commonly selected from one of the electron-free gases, such as the inert gases helium and argon Nitrogen also has good properties and is frequently used A number of gas mixtures promote electron mobilit) to an extent much greater than possible with any pure gas and have found great favor in ionization chambers 10

In general, gases whose chemical activity is enhanced by radiation are avoided since they may be gettered by the metals of the electrodes and the chamber enclosure Gases that are chemical compounds may be dissociated in a high radiation field, recombination is not spontaneous m most of these gases, and their use is avoided A BF^ counter or chamber, for example, ma have a short life 1 1

When a voltage is applied to an ionization chamber and the resulting current is measured in a constant radiation field, the current is found to van with the voltage A graph of pulse amplitude vs voltage (Fig 2 5) shows the pulse amplitude increasing with voltage in a very characteristic

fashion [2] Ac low voltages competing gas processes hold the current down As voltage is increased, the current increases rapidly, and, then, as the voltage is increased further, the current remains nearly constant, increasing only slightly The flat region of nearly constant current is known as the plateau, and the region where the current begins to flatten is called the knee of the plateau The voltage just above the knee is the minimum operating voltage Normally, the operating voltage is selected, somewhat arbitrarily, to be considerably above the minimum

As the chamber voltage is increased along the plateau, assuming that voltage breakdown does not occur, a point is reached where the current again begins to increase Voltages above this point cause gas multiplication This voltage or the breakdown voltage, whichever is smaller, determines the maximum voltage In turn, the maximum rated voltage must normally be considerably below the breakdown voltage for reliable operation, again a somewhat arbitrary choice Since the reactor radiation field tends to promote voltage breakdown, operation at minimum voltage is preferred

If the voltage plateau for a given chamber12 is deter­mined for a number of radiation-field values, it is found that the voltage at the knee increases with radiation intensity, as shown m Fig 2 6 This is caused by recombination and space-charge effects, which increase rapidly with radiation intensity It is important to recognize this fact and to select an operating voltage that is on a plateau at the maximum radiation intensity experienced during operation At this voltage, again somewhat arbitrary, the magnitude of the chamber current is a faithful indicator of radiation in­tensity

Radiation emitted bv neutron-induced radioactivity within the chamber may, under some circumstances, increase the background and limit the range of usefulness of the chamber 1 3 Figure 2 7 shows the residual current due to the induced activity in the fission chamber