Il Nuovo Cimento (1943-1954)

, Volume 3, Issue 1, pp 131–141

# Sulla misura della densità dei neutroni termici

• E. Corinaldesi
Article

## Riassunto

Si esegue il calcolo della perturbazione che una certa distribuzione di neutroni termici subisce per la presenza di un rivelatore, col quale vengono eseguite misure di densità; vengono inoltre calcolate le correzioni da apportare ai risultati di queste ultime.

## Summary

Density measurements of thermal neutrons are usually performed without taking into account the perturbation induced by the detector in the distribution of neutrons. Such an approximation is permitted as long as the detector is thin; for thick detectors error can be considerable.

In this work we have calculated the perturbation for plain infinite detectors. Successively, considering that the formulae obtained in this way cannot be applied to the case of finite-sized detectors without an appreciable error, we have performed the exact calculation of the perturbation for circular detectors with radiusR=1/2l,l, 3/2l, 2l (l being the diffusion length) on the assumption that the unperturbated densityqτ is a constant. Let δ be the product: absorption coefficient x thickness x √3, λ the free mean path andD the diffusion coefficient for thermal neutrons, Θ the number of neutrons absorbed by the detector per sec. In fig. 2 the ratio between Θ and the unperturbated densityqτ is given as a function of the thickness for different values of β=R/l; the dotted curves show the results obtained by means of the formulae for plain infinite detectors (for convenience ordinates are 1+Ч/2πDlqτ and abscissae are$$1 + \sqrt 3 \frac{l}{\lambda }tgh\frac{\delta }{2}$$.

Figure 3 represents the ratio between the average valuen of the densityn beyond the detector and the unperturbated densityqτ (dotted curve refers to the infinite detector).

## References

1. (1).
E. Fermi: «Ric. Scient.»,7, 13 (1936).Google Scholar
2. (2).
Nielsen:Handbuch der Theorie der Cylinderfunktionen, 1904, p. 198.Google Scholar