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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963
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maddi doon.com
ORNO--1343
MASTER P
- 70ے کا یوم
"JUN 24 1965
.
. Fast Neutron Capture Gamma-Ray Spectrum from 2380(n,r) 239v.*
.!
I. Bergqvistt
Oak Ridge National Laboratory
· Oak Ridge, Tennessee
and . .
Research Institute of National Defense
• Stockholm, Sweden
.-.-..-
....
.: PATENT CLEARANCE OBIAIXED. RELEASE. TO
• THE PUBLIC IS APPROVED. PROCEDURES
ARE ON FILE IT! Rani NG SECTION
Introduction: Much attention has recently been paid to the level structu- ;
re of 2590. Experimental studies using the (d,p)'V) and' (n,r) reactions
1 5 have given an accurate value of the neutron binding energy, Eg :
= 4.80 $ 0.01 Mev, and rather detailed information about level positions
below an excitation energy of 1 Mev. Spins and parities have been de-
termined for some levels!). In particular, an identification of the
163173* band with a sequence of levels between 0.13 and 0.19 Mev has
been proposed 12/1914). Transitions to these states have been observed
in thermal(2) and in ev-resonance capture15), i.e. in 8-wave neutron cap. ;
ture from capturing states 3* . Thus, the transitions should be
M1 (E2). In p-wave capture, " > 2, the same transitions should be
of E1 type. The gamma-ray spectrum obtained from the capture of 30 kev.
neutrons, at which energy p-wave capture is important, is expected to
contain information about the states at 0.13 - 0.19 Mev as well as about
average E1 and M1 widths.
Experiments and calculations: The experimental system consists of the
ORNL 3 Mv terminal pulsed van de Graaff generator, a heavily shielded.
9" x 12" NaI(Ti) spectrometer and a two-parameter analysis system. Neu-
trons were produced by the 'Li(pon) 'Be reaction near threshold. The
flight path was 40 am. The samples, placed in the forward neutron cone, i
were in the form of 3" diameter, 1/4" thick disks of depleted uranium.
The background corrected pulse distribution from the capture of neutrone
:ith energies in the range 27-40 kev 18 shown in Fig. 1.
Since the average observed spacing of levels with j* . ** is 18 ev,
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canti
resulting spectrum represents the average of the spectra from some 10°
capturing states. The situation can then be regarded as "statistical".
A spectrum calculated according to the statistical modellº is compared
with the experimental results. The transition probability was assumed
to be proportional to D' and Newton's level density formula was used to
describe the energy dependence of the level density above an energy gap,
of 0.8 Mev. Below the energy gap only a sequence of levels, all at an
excitation energy of 0.13 Mev, is taken into the calculation. The se-
quence fulfils the requirement that transitions from a state J can occur
to levels in this sequence (in general this means three levels with spins
J, J $ 1, respectively). To be able to make comparison between calcula-
tion and experiment the calculated :
spectrum is convoluted with the res-
ponse function of the spectrometer.
The result is shown in Fig. 1 (dashed
curve). However, corrections must.
be applied to account for the gamma- .
-ray attenuation in the uranium samp-
le and in the LiH-shield in front of
'the crystal. A rough calculation was
made and the corrected puise distri-
bution is also shown in Fig. 1 (solid
line).
The calculated pulse distribu-
Fig. ?. Experimental pulse
tion was used as a basis for deter-
distribution of 238U(n,r) 239v.
mining the absolute intensities of
The curve shows the spectrum
gamma-ray lines in the peak at 4.7
calculated according to the
Mev and the strength of the bumps at :
statistical model,
3.6 and 4.0 Mev. The 4.7 Mev peak
was found to be asymmetric on the
high-energy side. This asymmetry could not be accounted for unless an-:
other line at about 4.84 Mev was assumed. Thus, two gamma-ray lines at
4.71 0.02 'Mev and 4.84 + 0.04 Mev with an intensity ratio of about 4:1
were obtained from the unfolding excercises. The bumps at 3.6 and 4.0
Mev cannot be resolved into separate lines in the present work. We have
only tried to determine that distribution of intensities in intervals of 1
0.1 Mev which fits the experimental results.
Results_and_discussion: The calculated and experimental spectra (Fig. 1) 1
agree within an estimated uncertainty of about 10 % over the whole range

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of pulse heights except for the bumps at 3.6 Mev and 4.0 Mev ir the
measured spectrum. The experimental peak at 4.7 Mev is excellently re-
produced in the calculated spectrum.
The pulse distributions from ey-resonance capture, which were mea-
8Vred by Jackson using an 8" x 6" NaI(TI) scintillator, show on the
average the same features as those from. 30 kev capture. In Fig. 2 the
pulse distribution from the 37 ev
neutron resonance is given, togethes
with the distribution from fast neu.
tron capture (solid line). The in-
tensities of the 4.7 and 4.0 Mev ra.
diation from this resonance are 1.01
$ 0.12 and 0.84 + 0.09, respectivel;
relative to the average intensities
from the twelve neutron resonanoos
included in Jackson's investigation
! On the average, the peak at 3.6 Mev
is much less pronounced than in the
pulse distribution from the 37 ev
Fig. 2. Experimental pulse
resonance. It is found that, aroun
distribution for the 37 ev
3.6 Mev, the average puise distri-
neutron resonance in 490U
bution is also close to the solid
from the work of Jackson!).
line in Fig. 2. From the compari-
For pulse heights below 3 Mev
son of the pulse distributions from
only every fourth event was re-
average ev resonance (s-wave) captu
corded. The solid line re-
re and from the capture of 33 = 7 k
presents the experimental
neutrons (s- and p-wave capture) we
pulse distribution from the
conclude: The relative intensities
capture of fast.' neutrons.
of the bumps at 3.6 and 4.0 Mev are
about the same. The 4.7 Mev peak i
about four times stronger in fast neutron capture. The larger part of
this factor can be attributed to the effect of p-wave capture.
The rotational band based on the [622* ground state has been suun
died. in detail in the (d,p) work . Since the capturing states in
$-wave neutron capture have j" **, and in p-wave capture j* .no
primary transitions to this band have multipole order higher than dipole
except for those transitions from the states to the ground state. We
expect, therefore, that the average intensity of the primary transitions
from so and p-wave neutron capture to any member of this band is relati-

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vely weak. We may in the present investigation disrecard the possibili."
ty of primary transitions to the members of the ** .** band. It remains
to study the transitions to the levels at 0.134, 0.14 and 0.192 Hev, which
are presumably members of the same rotational band.
The gamma-ray peak at 4.71 + 0.02 Mev (Fig. 1) 18 identified as ari-
sing from transitions to the states at 0.13 Mev and the asymmetry, pro-
ducing a gamma-ray line at 4.84 +0.04 Mer, is due to transitions to the
cround state from capturing states at ER + 0.03 Mev.
Since the peak at 4.7 Mev is four times stronger in f'ast neutron
capture than in average ev-resorarce capture (presumably pure 8-wave cap-
ture) it seems resonable to attribute positive parity to the band with the
base state at 0.13 Mev. In s-wave neutron capture primary transitions
to members of this band are then of M1 (E2) type. In fast neutron capture
with Em = 33 + 7 kev, the observed intensity of 4.7 Mev. gamma rays, I .
1.4 photonshoo captures, should then be due to E1 transitions, from p-wa-
ve neutron capture, assumed to be about 60% of the total number of cap-
tures at this neutron energy, and to M1 (E2) transitions from s- and d-wa-
ve capture. Combining these results, one finds that the average intensi-
ty of M1 transitions is 0.4 photons/100 captures and of E1 transitions
2.0 photons/100 captures. From the statistical calculation, in which in-
herently is assumed that the sequence has K-5, we obtain I. - 1.6 pho-
tor.s/100 captures. Furthermore, the average intensities of ground state
transitions relative to 4.7 Mev transitions to the band is very sensitive
to the Kt value. The ratio is 2:9 if * **, about 1:2 if k* . ** and
1:1 if x* . 3*. The experimental ratio is 0.25 + 0.10. Thus, we find
experimental justification for the assignment K* - * to the band with
the base state at 0.13 Mev. It is perhaps possible to combine level bands
of various intrinsic spins and parities to fit the experimental results
from (n,r) reactions. However, the (d, 1)) results give no evidence for
rotational bands at low excitation energies other than those discussed
above.
The identification of the 0.134 Mev state as the base of the [
band fits into the level systematics of neighbouring nuclei, i.e.990,
2350 and 239pu.
We wish to compare the average reduced widths of E1 and M1 transi-
tions to the states at 0.13 Mev in the same manner as adopted by Bartho-
lomew!!!: The seduced widths are given by
Jobs (ev)
ch .com
2
*31
E, (Mev)]? D(Nov) 4275,"21",, (Nev)]3 D(Mev).
Assuming the total radiation width, T: - 25 mev, to be the same in 8-wave
and p-wave capture and .D VD(ER) - 18 ev to be independent of the pari-
ty of the capturing states, we obtain
. Ker 24.10°3 and Kuya ~ 3.10-2. .
The averace value of kg, agrees with the average value, kes ~ 3.10°, of
Bartholomew's survey. However, the average value of kx is almost an or-
der of magnitude larger than the value quoted there, Kysy ~ 4.10 .
The level diagram (Fig 3) is based on the --- - ---
results from (d,p) (1) and (n,r)(2)(3experi. .
iments. Observed levels are shown as solid li-
nes. The calculated level positions of vari-
ous members of rotational bands are marked by
hatched lines. The number of levels below
about 1.1 Mev is sufficient to explain the
experimental gamma-ray spectrum (Fig. 1). No .
levels with J aro observes between the
6311* band and the 0.688 Hev level. The un-
resolved bump at 4.0 Mev, corresponding to
transitions to states with 0.65 S E S 1.1 .
cv, would require a total of about 14 levels
with J SI, approximately half with positive :
anc half with negative parity. This agrees
with the results of Jackson 5 in his study of
the fluctuations in the .partial radiation
widths of transitions to this region. About
14 levels with 1.1 SE S 1.3.Mev and with in
the same specifications as above would. explain
Fig. 3. bevei
the bump at 3.6 Mev.
diagram of 2390.
IIIIIIIII
References.
Research sponsored by the U. S. Atomic Energy Commission under contract with the
Union Carbide Corporation
Visiting scientist from Research Institute of National Defense, Stockholm, Sweden.
B. E. F. Macefield and R. Middleton, Nuclear Phys. 59, 561 (1965).
2. H. T. Motz, E. T. Jurney and W. T. Ford, Bull. Am. Phys. Soc. II, 9, 664 (1964).
3. B. P. Maier, to be published.
4. N. F. Fiebiger, Institut fur Kernphysik, frankfurt/M, IKF-8 (1963).
5. H. E. Jackson, to be published.
16. E. S. Troubetzkoy, Phys. Rev. 122, 212 (1961).
G. A. Bartholomew, Annual Review of Nuclear Science, 11, 259 (1961).
.
.5.
ADDENDUM
This paper will be presented with the following minor revisions:
(a)
F4.8. 2 will have a new caption to clarify that the average ev
resonance gamma-spectrum follows the solid curve to 4 MeV and the
dotted curve (37 eV resonance) beyond.
Page 4 ------ p-wave neutron capture assumed to be 60% ----- will
be changed to ------ p-wave neutron capture is iselo, 60% ----.
(c) Page 5. Assuming the total ----- will be changed to the total
radiation width can be taken to be F = 25 MeV for both s-wave
and p-wave capture since
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END
DATE FILMED
18/ 27/65