Fission Product Yield Data for the Transmutation of Minor Actinide ...

where ·P L Ò, ·P H Ò are the mean channel numbers for
the light and heavy fragment groups in a measured
Q distribution from the spontaneous fission of 252 Cf.
Transforming the charge matrix N(Q 1 , Q 2 )
into the matrix of the fragment masses and total
kinetic energies N(M,E k ) without taking into
account the neutron emission effects, N(M,E k ) must
be symmetric with respect to mass M = A/2. Thus
for any values M and E k the condition N(M,E k ) =
N(A – M,E k ) for complementary fragments must be
satisfied within the statistical uncertainties. The
control check has shown that our experimental
matrices meet this requirement.
Due to the low count rate from the 252 Cf
source, the calibration procedure was performed
only at the beginning and end of the experiments.
During the experiments, the stability of the detector
characteristics was monitored, taking the mean
values and variances of the Q 1 and Q 2 spectra from
reactions 235 U + p as reference.
The experimental technique described above
was used to measure the MEDs of fragments in the
proton induced fission of compound nuclei
234,236,237,239 Np, 239,240,241,243 Am, 245 Bk at a proton
energy of E p = 10.3 MeV, 234,236,237,239 Np, 240,241,243 Am
at E p = 22.0 MeV, and 233 Pa, 236 Np at E p = 7.4–30.0
MeV.
4.5.2.2. Corrections and experimental uncertainties
The experimental MEDs measured by the
method of (E 1 –E 2 ) spectrometry are distorted by
several factors such as fragment energy losses in the
target and the detector entrance window, as well as
neutron effects such as neutron emission from a
nucleus before fission (including the preequilibrium
stage of the compound nucleus
formation) and evaporation of prompt neutrons
from fission fragments. These energy losses change
the measured absolute value of the total kinetic
energy E k and the E 1 /E 2 ratio, which influences the
derived mass asymmetry M 2/M 1 of the fragments.
The magnitude of these alterations depends on the
charge, mass and kinetic energy of the fragments, as
well as the charge and mass of the materials of the
target backings, the working layers and the detector
entrance window. Therefore the introduction of
corrections for energy losses necessitates rather
intricate calculations.
One consideration is that the influence of the
energy losses on the experimental results depends
on the location at which the losses take place.
Losses in target working layers and detector
186
entrance windows (if their parameters are similar)
cause similar alterations in the energies of
fragments recorded by both detectors, which leads
to some broadening of the measured MEDs and to
a decrease of the absolute value of E k (but on
average does not violate the condition N(M,E k ) =
N(A-M,E k )). These effects have been considered in
detail in Ref. [4.5.13], where it was shown that they
have a strong influence only in the vicinities of
sharp peaks and narrow valleys, and are observed
only at low energy neutron induced and
spontaneous fission of actinides. The MEDs are
rather smooth in our range of incident particle
energies, and therefore the influence of these effects
can be ignored.
The energy losses in target backings not only
decrease the absolute values of E k but also cause
asymmetric alterations of the MEDs, thus violating
the condition N(M,E k ) = N(A-M,E k ), especially for
large mass asymmetries of fragments. This factor
could be taken into account effectively if during the
experiment all targets are oriented in such a way
that only one of the two detectors registers
fragments passing through the backings, and if
during the calibration a pure backing is placed
between this detector and the calibration source. In
the present work, the corrections for losses in target
backings were introduced in the calibration
procedure.
The experimental matrices N(M,E k) for the
neutron emission effects can be corrected if
information is available on the number of pre- and
post-fission neutrons ν, their energies and emission
direction for every fission event, or matrices
n pre (M,E k ) and n post (M,E k ). As a rule, such
information is not available, and therefore we
carried out the multimodal analysis with
uncorrected experimental MEDs. The consequences
of this limitation on our results could be
estimated from the following.
The emission of neutrons due to direct and
pre-equilibrium reactions of light incident particles
with target nuclei, as well as evaporation of
neutrons from compound nuclei, leads to the
formation of a whole range of fissioning nuclei with
varying excitation energies. Therefore, each
measured MED is composed of MEDs from the
fission of the compound nuclei with masses A CN,
A CN-1 , A CN-2 , etc. The determination of the MED for
the fission of a nucleus with a given mass and
excitation energy is an important but rather
complicated scientific task, which requires detailed
information on the MEDs from fission of at least