2-3.1 Structural Design*

In common with other reactor instruments, nuclear radiation detectors must be rugged This entails con­struction features that should be recognized by the user

The mechanical design is determined by the require­ments of the manufacturing process, by the realities of handling and installation, and by the rigors of the environ­ment in which it is to function During construction, a gas ionization detector must withstand the mechanical stresses when it is evacuated before being filled with gas It must also withstand the high temperature and vacuum used in the outgassing process After it is filled with gas, the detector must be able to withstand the pressure and temperature changes in the reactor environment These requirements in themselves are sufficient to ensure a high degree of rugged ness

The detector must also withstand mishandling and possible abuse during shipping and installation Some applications may require that the detector be moved during operation or between operating periods Unless the accel erations associated with movements are very small, the detector must have electrodes with exceptional mechanical stability to prevent vibration This must be ensured by proper design, e g, including adequate electrode supports

The detector environment is frequently subject to changes m temperature and pressure and to vibration or mechanical noise Differential thermal expansion within the detector is minimized by using materials with similar thermal expansion coefficients, by providing adequate constraints, or by permitting relative motion Any of these methods may make the detector microphomc The high voltage and considerable mterelectrode capacitance of the detector greatly enhance the generation of microphonics This tendency can be minimized by careful design of the electrode assembly

2-3.2 Materials of Construction

The choice of materials of construction for a gas ionization detector is a compromise between radiological, mechanical, and thermal requirements For example, in a light structure made of materials of low atomic number, both the self-absorption and the buildup of detector radioactivity are reduced Reduction of both of these effects is desirable in all nuclear radiation detectors However, such a structure is not optimum for a gamma detector, gamma detection is based on generation of Compton scattered (recoil) electrons, a process that is most efficient in heavier structures made of materials with high

atomic number Moreover, a light structure may not meet the requirements for detector ruggedness

The primary criterion in materials selecting is to maximize the generation of the signal Selecting materials to maximize sensitivity to incident gammas involves dif­ferent considerations from those involved in selecting materials for a neutron detector

In ionization chambers for gamma detection, construction is very important Sensitivity to incident gammas is maximized by increasing the thickness of chamber walls and electrodes, by using structural materials of high atomic number, and by using high density gases of high atomic number There are optimum values for most design features For example, chamber-wall thickness must not be so great that self-shielding causes excessive gamma attenuation In fact, wall thickness need not greatly exceed the range of the most energetic Compton electrons Gas pressure should not be so great as to cause excessive recombination of the ion pairs that are generated by the passage of Compton electrons through the gas The ma terials selected for a gamma chamber should have low cross sections for reaction with any neutrons that may accom pany the incident gammas Moreover, it is preferable that any neutron reactions that do occur should generate a minimum of energetic ionizing radiations Figures 2 15 and 2 16 show the effect of material selection and construction features

On the other hand, if the ionization chamber is designed for maximum neutron sensitivity (with minimum gamma sensitivity), neutron-sensitive materials, і e, ma­terials that create ionizing radiations when exposed to

neutrons must be used The choice is influenced by consideration of the mechanical, thermal and radiation properties of the neutron sensitive material