Investigations of the (n,gf)-reaction
in neutron resonances of
235U
and 239Pu.
Study of the neutron capture reaction for 238U.
Measurement of the forward-backward asymmetry in slow neutron fission.
Measurement of the neutron total cross sections of 209Bi
and 208Pb:
estimate of the electric polarizability of the neutron.
Estimation of the neutron polarizability from joint analysis of the total
cross-sections of lead-208 and carbon.
Measurements of the neutron total cross sections of lead isotopes.
Study of the fast neutron induced fission of 233U,
238U,
232Th,
237Np
and 239Pu
in the energy range up to 200 MeV.
Study of the fast neutron induced fission of lead and bismuth in the energy
range up to 200 MeV.
Study of the fission neutron multiplicity for spontaneous fission of 244Cm,
248Cm
and 252Cf.
Investigations
of the (n,gf)-reaction in neutron resonances of 235U
and 239Pu.
O.A. Shcherbakov, A.B. Laptev,
G.A. Petrov
Experimental
studies on the two-step (n,gf)-reaction
give unique information not only about fission process itself, but also
about the structure of highly excited states in heavy nuclei, both in 1-st
and 2-nd wells of the fission barrier, and radiative transitions between
them. The fission g-ray
multiplicity has been measured in neutron resonances of 235U
and
239Pu.
The experimental prefission widths Ggf
have been obtained from the observed correlations between the multiplicity
of fission g-rays
and reciprocal fission width Gf-1
of resonances:
The experimental and calculated prefission width Ggf
is shown in Fig. 1 for the 4--resonances
of 235U and
1+-resonances of 239Pu
as function of the ratio of the E1 and M1 components in the
prefission g-ray
spectrum. The comparison of the experimental and calculated Ggf-widths
shows predominance of the M1 radiation in compound nucleus 236U
and that of E1 radiation in the prefission spectra of g-transitions
between the highly excited states in compound nucleus 240Pu.
It was also found that the best agreement between experiment and calculations
is obtained by using the model of intermediate damping of the vibrational
states in the second well and the Giant Dipole Resonance model.
Fig. 1. Experimental
and calculated widths Ggf
for 1+-resonances
of 239Pu
and 4--resonances
of 235U.
Calculation model: I -
single-humped fission barrier,
II, III
-
double-humped barrier (complete and intermediate damping of the vibrational
states in the second well, respectively); a
-
single-particle model (Weisskopf) for probabilities of g-transitions;
b,
c-
GDR model (Axel-Brink, Lorentzian-shaped probability of partial g-transitions
proportional to Eg4
and Eg5,
respectively).
In
another experiment at the GNEIS, the pulse-height spectra of fission gamma-rays
have been measured in isolated resonances of239Pu
in the energy range from 10 eV to 91 eV. The difference pulse-height spectra
for weak (Gf
< 10 meV) and strong (Gf
> 10 meV) 1+-resonances
show a few structures that could be interpreted as prefission gamma-transitions
between the levels at excitation energy 1 - 3 MeV below the neutron binding
energy Bn.
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Measurements
of the capture cross-section of 238U in energy range En<100
keV and gamma-ray spectra from the capture of resonanse neutrons:
study of the nature of the 721.6 eV resonance.
O.A. Shcherbakov, A.B. Laptev
Two
lowest energy resonance clusters in the subthreshold fission cross-section
of 238U
are dominated by the 721.6 eV and 1211.4 eV resonances. Anomalously small
capture width of the 721.6 eV resonance (~ 4.7 meV) is a strong
evidence that this resonance is not usual (class-I, corresponding the first
well of fission barrier) compound state. If the 721.6 eV resonance is predominantly
class-II (corresponding to the second well) in character, then not only
its radiative width Gg
should be small, but the capture g-ray
spectrum of this resonance should be softer than that of other s-wave resonances
(class-I). Prior to present measurements, J.C. Browne (1976) observed a
much softer
g-ray
spectrum for the 721.6 eV resonance than for neighboring resonances, whereas
H.
Weigmann et al. (1975) found no difference. To resolve this contradiction,
the capture g-ray
spectra in isolated neutron resonances of 238U
in the energy range from 400 eV to 1300 eV have been measured at the GNEIS.
The data obtained have been processed after the slightly modified method
of Weigmann et al. The idea was to detect a g-decay
branch within the second well using two different bias values for the g-ray
registration: lower B1 and upper B2. Then, for value of B2
larger than Bn-
EII (2
MeV, height of the second minimum), the ratio of resonance area Ag
measured with two biases B1 and B2:
R = Ag(bias
B2)/Ag(bias
B1)
should be smaller for the
resonance having major class-II fraction than for ordinary class-I resonances
because the softer class-II component will be under the upper bias B2
for this resonance.
The results of the present measurements and those of Weigmann et al (Geel)
are shown in Fig. 2. As it is seen from our data, the capture g-ray
spectrum of the 721.6 eV resonance is much softer than that of the neighboring
s-wave
resonances. Our data enable to make a conclusion that the
721.6 eV
resonance is predominantly class-II by nature. As for the 1211.4 eV resonance,
both our data and the results of Weigmann et al show that there are no
solid arguments to consider this resonance as a class-II state.
Fig. 2. Results
of the capture g-ray
measurements for resonances of
238U.
Measurement
of the forward-backward asymmetry in slow neutron fission.
V.E. Sokolov, A.M. Gagarsky,
A.B. Laptev, A.K. Petukhov, G.A. Petrov, O.A. Shcherbakov
Parameters
and decay properties of low energy p-resonances in heavy fissile
nuclei are practically unknown because of the difficulties existing when
generally accepted method are used. The new method to obtain such information
is the study of the neutron energy dependence of the forward-backward asymmetry
of angular distribution of fission fragments which is the result of s-
and
p-wave interference in neutron capture
W(Q)
= 1 + afb·—(pn·—pf)
where pn
and pf
are the neutron and light fragment momenta. The principal advantage of
this method if compared with the other asymmetry-measurements: a non-polarized
neutron beam can be used. The measurements of the forward-backward asymmetry
coefficient afb
for 235U
and 233U from
1 eV to 136 eV have been performed at the GNEIS. The results obtained for
235U
in
the energy range from 1 eV to 21 eV are shown in Fig. 3.
Fig. 3. Energy
dependence of the asymmetry coefficient afb
and fission yield for 235U.
Several
irregularities caused by p-resonances have been observed in energy
dependence of the coefficient afb.
Estimations of the main p-resonance parameters have been made. Fitting
analysis of the data gives that the average total width of the p-resonances
is some greater than that of s-resonances. For example in case of
235U
<Gfp>
= (200 ± 50) meV and <Gfs>
= (140 ± 10) meV. On the base of analysis of this data, the evaluation
of fast direct fission (without compound nucleus stage) contribution to
the total fission cross section was obtained. It was found to be lower
than 5·10-2
at 95 % - level of reliability.
The information obtained in these measurements is very important for the
fundamental investigations of the P- and T-parity violation
effects that are expected to be resonancely enhanced in a vicinity of p-resonances.
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Measurement
of the neutron total cross sections of 209Bi
and 208Pb: estimate of the electric polarizability
of the neutron.
The
electric polarizability
an
is one of the characteristics of the neutron as an elementary particle
and determines the induced electric dipole moment in an external electric
field: D = anE.
It is known [1] that information about polarizability of the neutron can
be obtained from the neutron total cross section measurements for heavy
nuclei. In Coulomb field, an addendum
V = - 1/2·—DE
= - 1/2·—an
Z2 e2
/r4
caused by polarizability
is added to the neutron-nucleus interaction potential, which in total scattering
cross section leads to several additional terms, main of them sp
linear depended on k, where k is the wave number (or, the
same, on
, here En
is neutron energy). The contribution of sp
to the total cross section of heavy nuclei such as 209Bi
or 208Pb
approximately can be written as
sp»
10-4·—an·—
barn .
Here an
is in 10-3 fm3
units and En
is in eV. So far as with an increase of neutron energy, a contribution
of this term in total cross section increases in proportion to the wave
number k, then, from the principal standpoint, a measurement of
polarizability could be reduced to comparison of total cross sections measured
at different energies. To date, several attempts have been performed to
measure of an
, for references see [2].
The total cross-section of 209Bi
in the energy range from 1 eV to 100 eV and that of 208Pb
from 1eV to 20 keV have been measured at the GNEIS facility for the purpose
of estimating a value of an
. Simultaneously, the total cross section measurements for silicon and
carbon were carried out as a zero-test of the experiment. For silicon and
carbon, the polarizability influence should be negligibly small in comparison
with that of bismuth and 208Pb
because the polarizability contribution to the cross section is proportional
to the square of nuclear charge Z2.
The 40-m flight path was used. The ionization 3He-chamber
was used as neutron detector. Resonance filters of Co(132 eV, 4.3
keV, 5.0 keV), W(18.8 and 4.16 eV), In(1.46 eV) and Al(34
keV) were placed in the beam for permanent background monitoring. In order
to suppress the overlap neutrons, cadmium filter was placed in the beam.
For a distortion of the spectrum shape caused by nonzero dead time to be
equal for bismuth-silicon and 208Pb-carbon
pairs, the samples thicknesses were chosen for their transmissions to be
about equal in energy range of interest.
The measured total cross sections of 209Bi
and Si are shown in Fig. 4 and that of 208Pb
and C in Fig. 5. The given errors are statistical ones and
reflect uncertainties of cross section energy dependence. Only the energy
dependence of cross section is used for an-value
estimation. In order to evaluate a value of polarizability it is necessary
to subtract from total cross section the corrections for Schwinger and
(n,e)-interactions,
solid state correction, absorption cross section and neutron resonances
contribution.
Fig. 4. Measured
total cross sections of 209Bi
and Si.
Fig. 5. Measured
total cross sections of 208Pb
and C.
Bismuth
Analysis of the data for Bi using the method given in [3] with simultaneous
variation of two parameters -
polarizability and resonances contribution (parameter P2
from [3] ) -
leads to an
= (25 ± 11)·10-3 fm3.
One can not obtain better precision of an
for Bi data because of the large contribution of 800 eV resonance.
Lead-208
Applied method of evaluation of an-value
for 208Pb
is described in detail in ref. [4]. In accordance with [4] total cross
section of 208Pb
for investigated energy range can be written under two very common assumptions
-
optical theorem and first Born approximation -
as:
Here (d0
+ h0
+ z0)
is sum phase of s-scattering:
d0
= - k R0'-
describes pure nuclear interaction (R0'-
radius of potential s-scattering),
h0
= k ane
F-
due to (n,e)-interaction (F-
integrated electron form-factor),
z0
= fa
k (6/5 - p/3
·—kR
- 5/7 ·—(kR)2
)-
additional part of phase of s-scattering caused by neutron polarizability,
fa
= man/R
·—(Ze/h)2
, R-
radius of nucleus.
Terms S1,
S2
and S3
are due to resonances. Thus, there are three parameters -R0',
ane
and
an-
for least square fitting procedure in our energy range. Accuracy of
an
estimation is strongly dependent on the number of fitting parameters. And
in case of 208Pb
an estimated value of the neutron electric polarizability is found to be
an
= (2.4 ± 1.1)·10-3
fm3.
In our opinion, the estimated value of an
is in agreement with zero value. A further progress in increasing the precision
of an-measurements
could be related with the substitution of neutron detector for more effective
one and in conjunction with detailed background analysis and therefore
increasing the high-energy boundary of measured cross section. That leads
to an increase of polarizability contribution to the total cross section
because it depends on energy as
.
Especially, we would like to say about a nonzero value obtained at Oak
Ridge [5] for neutron polarizability an
= (1.20±0.15±0.20)·10-3
fm3. We think that
Oak Ridge value of an
does not close the problem of neutron polarizability. Analysis of method
of an-value
evaluation in ref.[5] allows to make several conclusions [4]:
1. Radius of potential s-scattering
R0'
evaluation in ref. [5] was made for total cross section by means of code
REFIT without taking into account contributions due to an
and (n,e)-interactions. But both of them interfere with potential
interaction. Thus, R0'
was in fact struck off varied parameters and that must strongly underestimate
an
error. An analysis of our data showed strong correlation between R0'
and an
[2].
2. The use of cross-section
parametrization [5]
s(k)
= s0
+ s1
k + s2
k2 + O(k4)
is correct if simplification
sin d0»
d0
is right (up to several keV).
But in fact, the data up to 40 keV were involved into analysis.
3. It is necessary to take
into account p-wave component of potential scattering in total cross-section
analysis at neutron energy above 20 keV because at that energy value of
p-wave
contribution becomes comparable with that of an
for expected value of neutron polarizability. For correct analysis of data
[5] at energy above 20 keV, the additional fitting parameter R1'
should be used, but that will increase error of an-estimation.
Thus, we think that error of an-value
estimation in ref. [5] is very underestimated and the problem of neutron
electric polarizability measurement deserves further research.
Final part of this work was supported by RFBR
Grant # 97-02-16184.
References 1. Yu.A. Alexandrov,
Yadern. Fiz. 37, 253 (1983); 38, 1100 (1983).
2. Yu.A. Alexandrov,
I.S. Guseva, A.B. Laptev, V.G. Nikolenko, G.A. Petrov, O.A. Shcherbakov.
In book "V International Seminar on Interaction of Neutrons with Nuclei
"Neutron Spectroscopy, Nuclear Structure, Related Topics", ISINN-5, Dubna,
May 14-17, 1997". Dubna, JINR, E3-97-213, 1997, p. 255-260.
3. Yu.A. Alexandrov,
M. Vrana, J. Manrique Garcia, T.A. Machekhina and L.N. Sedlakova, Yadern.
Fiz. 44, 1384(1986).
4. I.S. Guseva, PNPI
Preprint 1969, Gatchina, 1994.
5. J. Schmiedmayer, P.
Riehs, J.A. Harvey, N.W. Hill, Phys. Rev. Lett. 66, 1015 (1991).
Return
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Estimation
of the neutron polarizability from joint analysis of the total cross-sections
of lead-208 and carbon.
A.B. Laptev, I.S. Guseva,
G.A. Petrov, O.A. Shcherbakov
It
is well known [1] that information about polarizability of the neutron
can be obtained from the neutron total cross section measurements for heavy
nuclei. To date, a lot of the neutron total cross section measurements
for heavy nuclei have been performed to evaluate an
[2]. Investigators approach closely the upper limit of polarizability about
1·10-3 fm3.
The sole exception is the result [3] an
= (1.20±0.15±0.20)·10-3
fm3 obtained in Oak
Ridge. But in spite of a nonzero value of an
, most investigators [4,5,6] think that error of this result [3] is very
underestimated because it takes no account of additional parameters necessary
to fit the total cross section with the aim to evaluate an.
In that situation, when investigations achieved very small error for an-value,
it is clearly to understand the importance of taking into account of all
circumstances that can to lead to distortion of total cross section. In
our previous work [2]
the main problem caused an anxiety was systematical errors of total cross
sections because of errors in knowing of experimental background value
(see chapter "Measurement of the neutron total cross sections of 209Bi
and 208Pb:
estimate of the electric polarizability of the neutron"
in this page). In this work the neutron total cross sections has been measured
with an accuracy Ds/s
~ 10-3
for 208Pb
and C in the energy range from 1 eV to 20 keV using the time-of-flight
facility GNEIS in Gatchina. The results of the
total cross section measurements for 208Pb
and C are shown in Fig. 5.
The method employed for evaluation of an
for 208Pb
is described in ref. [4]. Fitting results for 208Pb
are presented in Fig. 6. Points are experimental total cross section
after the Schwinger and solid-state corrections and contribution of radiative
absorption having been subtracted. The reduced c2
is equal 2.5 for this fitting. The neutron polarizability obtained is an=
(2.4 ± 1.1)·10-3
fm3 and the amplitude
of neutron-electron interaction is ane
= - (1.78 ± 0.25)·10-3
fm .
Fitting results for carbon are presented in Fig. 7, the value of
reduced c2
is equal 1.7 . From Figures 6 and 7 we can see that group
of points in vicinity of energy about 1 keV lies systematically higher
than fitting curve. This energy region corresponds to the case when the
experimental background changes sharply its energy behaviour. Therefore,
the experimental background at this energy range can not be satisfactory
described using conventional resonance filter method.
The total cross section measurements for 208Pb
and C were carried out in similar experimental conditions [2]. At
the same time, the samples thicknesses were chosen for their transmissions
to be about equal in energy range of interest. As the result, the experimental
background were equal in cases of 208Pb
and C in these measurements and, therefore, distortions caused by
uncertainties of experimental background should influence on the measured
total cross sections of 208Pb
and C in the same manner. Thus, to eliminate influence of the distortions
caused by uncertainties of experimental background, the difference
s(208Pb)
- const ·—s(C)
has been used for the neutron
polarizability estimation. Here a value of const = 2.42 was
chosen, so that the average value of the difference to be about equal to
zero in the energy range under investigation. Fitting results for this
difference are shown in Fig. 8. Using this method, the polarizability
was obtained near the same value an
= (2.44 ±
1.32)·10-3
fm3 . The value obtained
for amplitude of neutron-electron interaction is ane
= - (1.75 ±
0.27)·10-3
fm , the value of reduced c2
is equal 0.7 .
Fig. 6. Fitting
results for 208Pb.
Fig. 7. Fitting
results for C.
Fig. 8. Fitting
results for difference s(208Pb)
- 2.42·—s(C).
This method of joint analysis of the experimental data is very useful in
the way of obtained sure result for an
independent on experimental background. That is very important in case
of experimentalists achieve level of precision when systematical errors
of measured cross section due to experimental background are compare with
statistical one. In our opinion, the estimated value of an
is in agreement with zero value at present time and the problem of neutron
electric polarizability measurement deserves further research.
This work was supported by RFBR
Grant # 97-02-16184.
References 1. Yu.A. Alexandrov,
Yadern. Fiz. 37, 253 (1983); 38, 1100 (1983).
2. Yu.A. Alexandrov,
I.S. Guseva, A.B. Laptev, V.G. Nikolenko, G.A. Petrov, O.A. Shcherbakov.
In book "V International Seminar on Interaction of Neutrons with Nuclei
"Neutron Spectroscopy, Nuclear Structure, Related Topics", ISINN-5, Dubna,
May 14-17, 1997". Dubna, JINR, E3-97-213, 1997, p. 255-260.
3. J. Schmiedmayer, P.
Riehs, J.A. Harvey, N.W. Hill, Phys. Rev. Lett. 66, 1015 (1991).
4. I.S. Guseva, PNPI
Preprint 1969, Gatchina, 1994.
5. V.G.Nikolenko, A.B.Popov,
Preprint JINR E3-92-254, Dubna, 1992.
6. T.L.Enik et al.
In book "IV International Seminar on Interaction of Neutrons with Nuclei
"Neutron Spectroscopy, Nuclear Structure, Related Topics", ISINN-4, Dubna,
April 27-30, 1996". Dubna, JINR, E3-96-336, 1996, p. 205.
Return
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Neutron
Total Cross Sections of 204Pb,
206Pb,
207Pb
and 208Pb.
The
neutron total cross sections have been measured for lead isotopes 206Pb,
207Pb
and 208Pb
in the range from 1 eV to 20 keV and for 204Pb
(preliminary results) in the range from 1 eV to 100 eV using the time-of-flight
facility GNEIS. The main experiment devoted to measurement with 208Pb
was carried out with the aim to measure a value of electric polarizability
of the neutron (see previous
experiment). Measurements with 204Pb,
206Pb
and 207Pb
were carried out with the aim to obtain more exact corrections to
be taken into account for other lead isotope admixtures in neutron total
cross section of 208Pb.
Additional motivation of these measurements is that numerical data on neutron
total cross sections of lead isotopes are scarce and at low energies -
practically absent (for example see EXFOR,
IAEA-NDS-CD-05, EXFOR on CD-ROM, Version 21 January 2000 and CINDA
on CD-ROM, Version December 1999, Rev.1, NEA Data Bank, Paris). The following
samples were used for present measurements: a 5.14-g sample of 204Pb
with isotopic composition 204Pb
- 36.6%, 206Pb
- 30.6%, 207Pb
- 13.2%, 208Pb-
19.6%; a 110.2-g sample of 206Pb
with isotope composition 206Pb-
91.2%, 207Pb
- 6.0%, 208Pb
- 2.8%; a 106.7-g sample of 207Pb
with isotope composition: 204Pb
< 0.04%, 206Pb
- 4.04%, 207Pb
- 84.4%, 208Pb
- 11.56%; and 231.6-g sample of 208Pb
with isotope composition: 206Pb
- 1.08%, 207Pb
- 1.79%, 208Pb
- 97.12%.
The total cross section measurements were performed using a conventional
transmission method. Experimental arrangement was placed on a 42-m flight
path. The ionization
3He-chamber
was used as neutron detector and it was housed in a composite shielding.
The "black" resonance filters of Al (34 keV), Co (5.0 keV,
4.3 keV, 132 eV), W (18.8 and 4.16 eV) or Ta (4.28 eV),
In
(1.46 eV) and Cd (0.3 eV) were placed in the beam for permanent
background monitoring. To compensate for neutrons flux fluctuations, the
"sample in beam" and "sample out of beam" measurements were carried out
in short (10 min.) runs in case of 206Pb,
207Pb
and 208Pb.
An accuracy of the measured neutron total cross section is about Ds/s
~ 5·10-2
for 204Pb,
Ds/s
~ 10-2
for 206Pb
and 207Pb,
Ds/s
~ 10-3
for 208Pb.
This work was supported by RFBR Grants # 97-02-16184
and 00-02-17876.
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Neutron
induced fission cross-sections of 233U,
238U,
232Th, 237Np
and 239Pu relative to 235U
from 1 MeV to 200 MeV.
O.A. Shcherbakov, A.B. Laptev,
G.A. Petrov, A.S. Vorobyev, A.Yu. Donets, A.V. Evdokimov, A.V. Fomichev,
T. Fukahori, V.M. Maslov, Yu.V. Tuboltsev
There
is a long standing need in information about fission of heavy nuclei induced
by the particles at intermediate energies. Regular experimental studies
of fission in this energy region started comparatively recently, mainly
due to the increased capabilities of modern neutron sources and experimental
techniques. Among new applications of the fission data above 20 MeV, the
most important are accelerator-driven transmutation of waste reactor materials
and energy production, peaceful use of weapon plutonium, accelerator and
spaceship shielding, radiation therapy.
During the last decade, the measurements of neutron-induced fission cross-sections
for some long-lived actinides in the energy range above 20 MeV with continuous
spectrum neutrons have been systematically performed only at the WNR/LANSCE
facility in Los Alamos and the GNEIS facility in Gatchina. Analysis of
the experimental data available in the energy range 20-200 MeV, as well
as experience in producing evaluated fission cross-sections below 20 MeV,
shows that new independent measurements aimed to improve fission cross-section
data base in the energy range above 20 MeV are necessary. At the same time,
calculation methods for the fission cross-sections of actinide nuclei at
intermediate energies are still under development, except some codes such
as ALICE and HETC. In such situation, the semi-empirical formulae based
on the few known experimental data are used to estimate the fission cross-sections.
These formulae can give not only the systematics for fission cross-section
at a certain energy point but also its energy dependence.
Fission cross-section ratios for 233U,
238U,
232Th,
239Pu
and 237Np
relative to 235U
have been measured using a 50-m flight path. A system of few iron, brass
and lead collimators gives the beam diameter of 18 cm at the fission chamber
location. The last series of the measurements were carried out with the
use of "clearing" magnet placed at 30 m from the source-target. This magnet
removes charged particles produced in the collimators and filters from
the neutron beam.
The fission reaction rate was measured using a fast parallel plate ionization
chamber with electrode spacing 7 mm and filled with methane working gas.
The fission chamber contained 6 foils of oxide fissile material 200 mg/cm2
thick and 18 cm in diameter deposited onto 0.015 cm thick aluminum backings.
Also, a weak 252Cf
deposit was applied on each fissile foil to match the gains of electronics.
The distances between the neutron production target and each fissile foil
were determined using 12C
neutron transmission resonances. For each isotope under investigation,
the time-of-flight and pulse height spectra were accumulated.
The results of present measurements are shown in Fig. 9,10,11,12,13,14.
To obtain fission cross-sections from the measured ratios, the recommended
data for fission cross-section of 235U
(INDC-368, IAEA, 1997) have been used.The error bars represent the statistical
errors only (one standard deviation). The solid lines show JENDL-3.2 data
in the energy range below 20 MeV. Also shown are the data of measurements
carried out at the WNR/LAMPF facility in Los-Alamos by P.W. Lisowski et
al. (1992), P. Staples and K. Morley (1998) and the data of systematics
of T. Fukahori (1999) and V.M. Maslov (2000).
This work was supported by RFBR
Grant # 96-02-18012 and ISTC
Grant # 609.
Fig. 9. Fission
cross-section of 238U
in the energy range up to 200 MeV.
Fig. 10. Fission
cross-section of 237Np
in the energy range up to 200 MeV.
Fig. 11. Fission
cross-section of 233U
in the energy range up to 200 MeV.
Fig. 12. Fission
cross-section of 239Pu
in the energy range up to 200 MeV.
Fig. 13. Fission
cross-section of 232Th
in the energy range up to 200 MeV.
Fig. 14. Low
energy part of fission cross-section of 232Th.
Among
the new applications of the fission data above 20 MeV, the most important
are accelerator-driven transmutation of waste reactor materials and energy
production, accelerator-based conversion of weapon plutonium and production
of tritium, accelerator and spaceship shielding, radiation therapy, etc.
During the last years, a need in information about neutron and proton-induced
fission of lead and bismuth at intermediate energies increased significantly,
mainly due to probable use of these metals as structural materials in the
neutron-producing targets of high-current proton accelerators of new generation.
Besides, the neutron-induced fission cross-sections of 208Pb
and 209Bi
are very convenient as standards in the intermediate energy range because
they have thresholds at 25-40 MeV, which eliminates the influence of low
energy neutrons.
The fission cross-section ratios for Pb(nat) and 209Bi
relative to 235U
have been measured using the neutron time-of-flight spectrometer GNEIS.
The fission reaction rate was measured using a fast parallel plate ionization
chamber. The chamber contained a total of 6 sections, every one containing
two pairs of cathode and anode plates spaced by 5 mm. The lead and bismuth
targets have been produced from high purity metal using evaporation in
vacuum, while for 235U
targets the painting technique has been used. The Pb/Bi - fission
chamber contained 10 foils of material 260-520 mg/cm2
thick and 1 or 2 foils of 99,992 % enriched 235U
150-260 mg/cm2
thick and 180 mm in diameter, deposited on one side of 0.05-mm-thick aluminum
backings. The foils were oriented perpendicular to the neutron beam and
all target deposits facing away from neutron source. The distances between
the neutron production target and each target foil were determined using
carbon (in separate run) and lead (permanently) filters with well-known
energies of neutron transmission resonances.
An absolute normalization of the measured cross-section ratios has been
done using the thickness values of the targets and detection efficiencies.
Finally, the cross-section ratios have been converted to cross-sections
using the recommended reference fission cross-section of 235U
of A. Carlson et al. (INDC-368, IAEA, 1997). The results of present measurements
are shown in Fig. 15,16 as fission cross-sections of
Pb(nat)
and 209Bi
for the neutron energies from threshold up to 200 MeV together with the
experimental data of other authors and parameterizations. The displayed
uncertainties of the present data are statistical ones being as much as
4-7 %, 2-3 % and 1.2-1.6 % at 60 MeV, 100 MeV and 200 MeV, respectively.
Fig. 15. Fission
cross-section of Pb(nat) in the energy range up to 200 MeV.
Fig. 16. Fission
cross-section of 209Bi
in the energy range up to 200 MeV.
The
comparison shows that above 45 MeV the present data for Pb(nat)
are in a good agreement with the white neutron source measurements with
Pb(nat) of P. Staples et al. (1995) at Los Alamos, as well as with
the parameterization of A.V. Prokofiev et al. (1998) based on the measurements
with Pb(nat) and 208Pb,
including the data of P. Staples and other known old and recent experimental
data carried out mainly in separate energy points. It can be also seen
that the ENDF/HE-VI data based on the on the evaluation of T. Fukahori
and S. Perlstein (1991) for 208Pb
lies much higher than all experimental data.
In the case of 209Bi,
an agreement of the present data with that of other authors is not so good
as for Pb(nat), but also within the stated uncertainties. There
is a noticeable discrepancy between our data, as well as the data of P.
Staples et al. (1995), and the parameterization of A.N. Smirnov (1997)
at neutron energies below ~60 MeV and above ~120 MeV. The ENDF/HE-VI data
lies up to 2 times higher than all experimental data below ~150 MeV, but
above this energy the tendency to agreement can be seen.
The data on neutron-induced fission cross-sections of natural Pb
and 209Bi
presented here were obtained from the measurements carried out jointly
by PNPI and KRI in the course of ISTC
Project 609-97 in collaboration with the Nuclear Data Center /Japan
Atomic Energy Research Institute.
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Multiplicity
distributions for fission neutrons emitted from complementary fragments
in spontaneous fission of
244Cm, 248Cm
and 252Cf.
The
most direct information about the shape of nascent fission fragments at
scission point can be obtained from the number of prompt neutrons emitted
from each of the complementary fission fragments. The first neutron multiplicity
measurement of this type combined with the direction-sensitive spectroscopy
of fission fragments for spontaneous fission of 252Cf
has been done at KRI (Russia) [1] more than decade ago, followed by the
analogous measurements carried out by the TUD-HMI collaboration (Germany)
[2]. These experiments revealed some unusual effects, such as shape-asymmetric
cold fission. Recently, a new series of such experiments was started by
the PNPI-KRI-IRMM collaboration within the framework of the ISTC
Project 554. In the present report the first results of the measurements
carried out for spontaneous fission of 244Cm,
248Cm
and 252Cf
are presented.
The fission source was placed between two large 200 litre Gd-loaded
liquid scintillator tanks which were used for neutron detection in a 2´2p-geometry
-
to separate contributions from complementary fragments. An efficiency of
the neutron registration was about 55% for each detector. A 16-cm thick
iron shielding inserted between the tanks was used to decrease both fission
neutrons and capture gamma's scattering from one tank into another (cross
talks). The fission fragments were collimated toward neutron detectors
by means of a pin-hole collimator combined with a common cathode of the
twin parallel plate flow-gas ionization chamber with Frish grids. At the
counting rate of "useful" events (coincidence of collimated fission fragments)
1-3 s-1,
in total about 3·106
of such events for every isotope have been accumulated.
To deduce mass and kinetic energy distributions of fission fragments, the
pulse height data have been corrected for grid inefficiency, pulse height
defect, energy losses in the sample, backing and pin-holes. On the basis
of reference values of most probable mass and energies of the fission fragments,
the provisional distributions have been constructed which then were corrected
for mean neutron multiplicities. Neutron multiplicity measurements have
been corrected for pile-up effects, background and detector efficiency,
including cross talk effects. Detector efficiency have been calculated
using Monte-Carlo method and known moments of the prompt neutron multiplicity
distribution [3].
The average neutron multiplicity <n>,
total neutron multiplicity nt
, variance of n,
covariance cov(nl,nh)
as a function of total kinetic energy (TKE) and energy distribution
are shown in Fig.17(left). The same values except covariance
as a function of fragment mass are shown in Fig.17(right).
The minimum in n(m),
as well as in variance is observed around m=128-129 for all nuclides
due to presence of a Z, N magic shells. The average level of variance
for 252Cf
is twice less then in the data of Signarbieux et al. [4] but in a good
agreement with results of Alkhazov et al. [1]. For total neutron multiplicity
the increased neutron yield is observed in symmetric fission region for
248Cm
and 252Cf
unlike to 244Cm.
The average value of the cov(nl,nh)
for 244Cm
is approximately twice less then for 248Cm
and 252Cf.
Variances show rather flat energy dependence except the weak decreasing
at lowest and highest values of TKE, that is in a reasonable agreement
with results of Alkhazov et al. [1] for 252Cf.
Fig. 17. Energy
and mass dependence of average neutron multiplicity <n>,
variance s2
and covariance cov(nl,nh)
together with energy and mass distribution.
References 1.I.D.Alkhazov,
A.V.Kuznetsov, S.S.Kovalenko, B.F.Petrov, V.I.Shpakov. Proc. of Int.
Conf. “Nuclear Data for Science and Technology” (30 May–2 June 1988, Mito,
Japan). Ed. by S.Igarasi, 1988, p.991-994.
2.I.During, M.Adler,
A.Marten, B.Cramer, U.Jahnke. Proc. of Int. Workshop “High-Resolution
Spectroscopy of Fission Fragments, Neutrons and Gamma-Rays” (1-2 February
1993, Dresden, Germany). Ed. by H.Marten and K.-D.Schilling, 1993, p.104-113.
3.N.E.Holden,
M. S.Zuker. Proc. of Int. Conf. "Nuclear Data for Basic and Applied
Science" (13-17 May 1985, Santa Fe, New Mexico, USA). Ed. by P.G.Young,
1985, p.1631-1634.
4.C.Signarbieux,
J.Poitou, M.Ribrag, J.Matusze. Phys. Lett. 39B, 503 (1972).
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Investigations of the (n,gf)-reaction
in neutron resonances of
235U
and 239Pu.
, here En
is neutron energy). The contribution of sp
to the total cross section of heavy nuclei such as 209Bi
or 208Pb
approximately can be written as
