Biomedical Engineering Reference
In-Depth Information
Fig. 10.42
(a) Idealized thermal-neutron pulse-height spectrum
from large BF
3
tube in which reaction products are completely
absorbed in the gas. (b) Spectrum from tube showing wall
effects.
in the ratio 96 : 4, as shown in Fig. 10.42(a). With most practical sizes, however,
a significant number of Li nuclei and alpha particles enter the wall of the tube,
and energy lost there is not registered. Since the two reaction products separate
“back-to-back” to conserve momentum, when one strikes the wall the other is di-
rected away from it. This wall effect introduces continua to the left of the peaks.
As sketched in Fig. 10.42(b), one continuum takes off from the peak at 2.31 MeV
and is approximately flat down to 1.47 MeV. (A similar continuum occurs below
