ewan it ort en
-28-
UCRL-5087
As the energy or the bon.barding particles is increased above 30-40
Mev, the mean iree wath oi the projectile in nuclear matter becomes comperable to the diametes oi the target nucleus, rendering the nucleus partially
transpasent to high-energy nucieons, MM The picture of ‘compound nucleus"
formation n.ust pe abandoned, and collisions oi the projectile with individual
nucleons in the target nucleus must be considered. In each oi the collisions,
the energy transtier to the nucleus is a fraction ot the projectile energy. The
total energy transite red to the nucleus then denends on the number of colli-
sions the projectile n.akes within the nucleus and may vary from. the full
energy of the projectile to a small iraction thereot,
Monte Carlo caicutations by Turkevich on the deposition oi energy in
the Biro? nucleus by protons appear in an article by Porile and Sugarman,
The results showthat the mejority o1 the interactions deposit less than hali
the projectile energy at 256 and 458 Mev.
When the transier o: energy to the nucieus is small, comparatively few
riucleons are evaporated either vetove or after iission and the sum of the
complementary-tragment mass numbers 1s close to 238. Similarly, when the
energy transiex is large, the system evaporates many nucleons, predominately
neutrons, and the sum of the complementary-1:ssion-fragment mass numbers
rs much less than 238.
The lower-energy fission events are characterized
by a saddle in the central rezion (A ~ 118)o01 the distribution curve symmetric
about a given A, und by very steep sides in the low- and high-A regions.
As
the bor: barding energy increases, the valley disappears and the low-A wing
has a lower slope than the high-A wing.
From the character of the fission
yield distribution at higher energies, one may infer that for ''mono-energetic"
high-energy tission the valley has all but disappeared and the axis of symmetry
of the distribution has shifted toward lower A.
The measured distribution is,
ot course, a Tr.ixture oi both high- and low-energy events. The fission-yield
distributions in Figs. 1 and 2 display apparent symmetry about a central
mass number A inthe region of Az20 at all energies.
are major yields of both hipk- and low-energy tission.
a RK, Serber,
1§
N,
T.
5001897
Phys. Rev. 72,
These products
Thus, almost any
1il4 (1947).
Poriie and N. Sugarman,
Phys.
Rev. 1Q7%,
1422 (1957).