The graded shield

A more specific problem could be experienced when a detector is installed on a nuclear reactor site, particularly in a reactor hall. There, it may also detect Ar (half-life 1.827h, Ey = 1293.6keV) produced by thermal neutron activation of argon in the air within the voids in the 

Notwithstanding these reservations, it is routine practice in ultra-low background counting rooms to purge, not only the detector space, but also the whole laboratory of radon with filtered clean air. 

The earth s atmosphere receives a cosmic ray flux of about 70 % protons, 20 % alpha particles and 10 % other heavier ionized particles. Their energies are extremely high lO GeV to at least 10 GeV. In the upper atmosphere, at about 25 km above the surface, these particles interact to produce secondary radiations of many sorts, largely pions (ir-mesons). At surface levels, the pions interact further and about 70 % of the total flux generates 

The total cosmic ray flux giving events between 2 and 24MeV is about 0.015 particles per second per square centimetre. The peak at 13MeV corresponds to about 0.08 h keV , which diminishes to about 0.01 h keV at24MeV. 

The steep decline up to 2000 keV, the normal spectrometry range, is due to Compton events, backscatter and bremsstrahlung resulting from the decay of muons into high-energy electrons and positrons. On this are superimposed the 511 keV annihilation radiation and all the gamma-ray peaks from the background nuclides and the peaks from activations described below. 

---

You should be aware that cadmium has a high cross-section for the absorption of thermal neutrons. During this absorption, or thermal neutron capture reaction, gamma radiation is emitted, the most noticeable of which is at an energy of 558 keV. If a detector system is to be used in a neutron field, then cadmium in the graded shield should be avoided. Fortunately, tin has a much lower thermal neutron cross-section and can be used instead. Difficulties due to neutron capture might be expected, and indeed have been observed, when operating a detector close to a nuclear reactor but the problem can also occur in environmental measurements (see Chapter 13, Section 13.3.4.2.)... 

The effective shielding of the detector system from direct and cascade radiation from the Co/Rh source is also very important. A graded shield consisting of concentric tubes of brass, tantalum, and lead was selected. The thickness and the shape of different parts of the shielding were optimized so that nearly zero direct 122 and 136 keV radiation (emitted by the Co source) was in a direct line with the detectors (see Fig. 3.16). 

The estimates of Table 3 are heavily biased towards the upper crust and are systematically higher than estimates for the whole crust (Table 1). For a bulk crustal estimate, samples need to be incorporated in a generic cmstal cross-section. Rocks may be ranked according to metamorphic grade and restored in their original cmstal position as a function of pressure and stmctural level. Data on deep cmstal material have recently been added in the Canadian Shield (Ashwal et al, 1987 Fountain et al, 1987 ... 

We studied possibility of absorbed dose reduction for a cylindrical hollow heat source model of a "Sr radioisotope battery by adding graded structure. The simulated result for addition of three mixed layers conseiwing total materials showed 22 % reduction to the dose of non-FGM structure. For compensation of a dose limitation of unmanned mission in SP-100 project, tungsten shield was needed about 5.7 cm thickness with the graded structure which indicated the minimum dose. 

The nuclear island structures include the containment (the steel containment vessel and the containment internal structure) and the shield and auxiliary buildings. The containment, shield and auxiliary buildings are structurally integrated on a common basemat, which is embedded below the finished plant grade level. 

Select by grade (quality). Look for the USDA shield, which certifies that the poultry has been graded for quality by a technically trained government grader and is the buyer s assurance of quality (see Fig. P-62, right). 

---