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A certain cloud chamber is filled with helium gas and is operated at 2.0-
atm pressure and 15C. The cloud chamber also contains a small amount of
alcohol vapor and of a volatile compound of lithium. (The effects of the alcohol
and the lithium compound on the stopping power of the gas can be neglected as
simplifying assumptions in the calculations to follow.)
Thermal neutrons produce the reaction Li6(n,a)H3
, for which Q = 4.78 Mev.
The H1 and He4 products are oppositely directed, away from the site of the compound
nucleus, and hence they form one straight track. The variations ofspecific ionization along this track can be measured with a microphotometer and
are indicated schematically in the sketch. Determine and tabulate the followingphysical properties of the a. particle and the II 3
particle which are produced in
this reaction:
(a) Ratio of initial velocities of H3 and He4
.
(b) Ratio of initial specific ionization by H3 and He4
, using the "uncorrected"
mean excitation potential I ~ 35 ev and CK = for helium gas.
(c) Initial kinetic energy of H3 and He4
, in Mev.
(d) Mean range of Ha and He4 and the mean value of the total H8 + He4 track
length in helium gas at 2-atm pressure and 1 5C, if the atomic stopping power of
helium relative to air is 0.36 and is taken as independent of velocity.
(e) Straggling parameters in air and the standard deviation of ranges in
helium gas at 2 atm and 15C for the H3
track, the He4
, and the total H3 + He4
track. Note that the straggling parameter for the total H3 + He4 track is not
the simple arithmetic sum of the parameters for H3 and He4
. For simplicity,
neglect effects of / on a/R.
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