5.2.1 Introduction

For a copper capsule, the difference of ε between dry a soil and moderately wet (e.g., less than 15% water) is negligible (less than 1%); therefore, it is not necessary in this case to know exactly the soil humidity to perform the correction, which is

Surrounding media 40K Uranium Thorium

Use of Gamma Radiation Techniques in Peaceful Applications

C soil 0.96 0.95 0.94 0.89 0.95 0.91 C soil + 15% water 0.97 0.95 0.96 0.90 0.96 0.91 S soil 1.03 0.97 1.10 0.95 1.08 0.96 S soil + 15% water 1.05 0.98 1.13 0.96 1.11 0.97 S soil + 30% water 1.06 0.98 1.16 0.97 1.14 0.98 S soil + 50% water 1.07 0.96 1.20 0.95 1.17 0.96 Pure water 1.35 1.01 1.82 1.03 1.70 1.03

Encapsulating Encapsulating Encapsulating Polyethylene Copper Polyethylene Copper Polyethylene Copper

In practice, the limestone soil and the siliceous soil are two extreme cases, which make only a difference of 5% for copper on the value of ε, so only a coarse knowledge of the cashing medium is necessary for the calculation of e with an high accuracy. The variations of ε for 40K with the absorber or medium are less important than for uranium or thorium. This is due to a smaller contribution of low energies to the

An important consequence of the foregoing is that, for the experimenter, copper (or a close material) is well used when the composition (especially moisture) of the soil is not well known. The error made a priori on the value of ε will thus be

On the other hand, in most cases, the dispersion on ε related to the relative intensity of the three gamma radiation sources is insignificant. Indeed, the potassium-40, uranium and thorium series generally intervene for approximately 1/3

In general, it is difficult to determine the respective contribution of the potassium-40, uranium and thorium series to the total dose rate. However, in most cases, the contribution of these three radioelements does not exceed 60% and approxi-

The results obtained with our program under the same conditions regarding the

dosimeter, the surrounding environment, the capsules and the sources of the gamma rays are slightly superior to the theoretical results of G. Valladas [7]. This small difference (in the order of 3%) can be explained by the fact that the nominal values for capsule thickness have been used instead of the mean values which take into account geometry, also to a lesser extent by the fact that self-absorption has not been taken into account in our calculation. The effect of this last correction was

each, and significant deviations from this proportion are rare.

Values of ε for infinite uniform medium with 1 g m<sup>2</sup> capsule wall thickness [5].

• ε = 0.97 0.01 for S soil + 15% water

• ε = 0.92 0.01 for dry C soil.

estimated to be less than 1%.

122

mate values on the whole with acceptable dispersion may be proposed.

an advantage.

Table 2.

reduced.

For example,

40K spectrum (no low energy lines).

The gamma radiation self-absorption coefficient is of great interest in activation analysis. Since it is difficult to measure this coefficient, various calculation methods have been developed.

Measuring the self-absorption coefficient is not a simple thing. The physicists who have faced this problem, for a long time, have always used methods of statistical or non-statistical computation: Parallel beam methods, Monte-Carlo method and many other methods. For our part, we have developed an original technique calculating the self-absorption coefficient of multienergetic γ-radiations [13].

In this chapter, we are presenting a method that we have developed, this which allows us control and calibrate the activation analysis experiments [13]. This method consists of simulating the interaction processes of gamma rays induced by neutron activation of various samples by using the Monte Carlo method adapted to experimental conditions.
