G: .
.
vit
-
.
I
.
.
b
h
.
.
*
I OF |
ORNL P
2493

.
eta
st
L
:
:
-
•
1
+
.
:
::-.
1
S
.
:
2.
.
MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
.
ANTZI2
TY,
"
U
Til
WY,
2

CORNUL-P- 2493
coon'6608253
aLi.
RELEASED VOR ANNOUNCEMENT
CFSTI PRICES
II BUCLEAR SCIENCE ABSTRACTS
NEUTRON CROSS SECTIONS AND RELATED MEASUREMENTS" Has 100. mn.50
SEP 2 6 1966
J. A. Harvey, Oak Ridge National Laboratory
Oak Ridge, Tennessee, U.S.A.
MASTER
*
*
ABSTRACT: Except for the nuclear explosion with its extremely-high instantaneouf
intensity, fission sources are inferior to pulsed accelerator sources for high-
resolution neutron cross-section measurements. A fission booster combined with a
pulsed accelerator is a usefui combination for cross-section measurements with low
energy neutrons where moderate energy resolution is sufficient. Most of the
neutron cross-section measurements requested for reactor design are possible with
existing neutron sources. The greatest need for very intense neutron sources is
for detailed partial cross-section measurements of the multiparameter type.
Recent theoretical investigations suggest that highly-excited compound-nucleus
states may have relatively simple configurations and are not "featureless" com-
pound states beyond theoretical description.
1. INTRODUCTION
The interest in neutror. cross sections comes from two groups: nuclear
engineers who need neutron cross section data in the design and understanding of
reactors, and nuclear physicists who need neutron data in the study of nuclear
structure. : If one surveys the latest "Compilations of EANDC Requests"\t) one sees
that most of the requests are for measurements with resonance and fast neutrons
and relatively few are for measurements with thermal energy neutrons. In fact most
of the requests for thermal energy measurements are for condensed-matter inelastic-
scattering measurerents which have been discussed in the first paper of this
session. Also thermal-neutron cross-section measurements are of little value to
nuclear physics unless details of the reaction are also studied, such as the gamma-
ray spectra from thermal-neutron capture or the mass distribution, etc. from ther-
mal neutron fission. Since the other paper in this session and the discussion
panels will consider these two fields, I will restrict my review to measurements
made with resonance-energy and fast neutrons and consider only the need for higher
fluxes for measurements with neutrons of these energies.
A. Requested Neutron Cross Sections
Most of the requested cross sections (1) are of the "one-parameter" type,
OMCE), ry(E), O(E), CHE), etc. For example, only the cross section for a
NIT
I
.
.
.
...
.
.
.LUI
!
.
NEL
1
,
7m
There are
particular process such as neutron capture as a function of neutros energy is
required and not the spectra of the gamma rays for each neutron energy. The ex-
ceptions to this statement are (1) elastic scattering crose sections where the
angular distribution of the elastically scattered fast neutrons is required and
(2) inelastic scattering cross sections where the energy distribution of the out-
going neutrons (or the gamma rays) is required and often the angular distribution
of these inelastically scattered fast neutrons (or the gamma rays) is requested
in addition. Since reactor calculations are made with a finite number of neutron
energy groups, the cross sectiuns in general are not needed with extremely good
neutron energy resolution. However, Monte Carlo methods using detailed cross-
section data will assume increasing importance in future reactor calculations.
Most of the cross sections which are required are for isotopes which are available
in reasonable quantities or for the natural element. In general, if a cross sec-
tion is small, It's value is not needed to high accuracy. Most of the desired
cross-section measurements for the fissile and fertile materials as well as for
many or the measurements on the moderator, structural and shielding material.s
are possible with existing“ neutron sources. However, with the limited man power
and existing facilities, many, many years will be needed to satisfy all the re-
quests. Although an increase in the strengths of neutron sources would increase
the production rate some, other factors are much more important.
B. Neutron Cross Sections for Nuclear Structure
Althougii a great deal of information has been obtained concerning nuclear
structure at high excitation from high-resolution "one-parameter" type experiments
such as measurements of neutron total cross sections, it is now apparent that
additional information from multi-parameter experinents is required. For example, .
the energy spectrum of the gamma rays from neutron captiere in individual reso-
nances, or the fission-product mass distribution from individual resonances will
help determine the character of the capturing states. These two experiments can
readily be expanded into multi-paraneter experiments where coincidence and angular-
correlation measurements of the capture gamma rays are made or the alpha, neutron
or gamma-ray emission from fission is correlated to the mass distributions. These
multi-parameter experiments will require very intense sources as will be discussed
later.
2. DESIRABLE CHARACTERISTICS OF NEUTRON SOURCES FOR CROSS SECTION MEASUREMENTS
Neutron sources can be divided into two classes: monoenergetic neutron
sources with a small energy spread (5.1%) and polyergic sources with a large
These include nuclear explosions and the ORNL electron linac under construction.
energy spread often extending over many decades of energy. The monoenergetic
sources may be either steady-state or pulsed sources and much cross-section data
have been obtained below 10 eV using a crystal spectrometer at a rcactor and in
the keV and MeV-energy regions using electrostatic accelerators. To use polyergic
sources for high-resolution cross-section measurements, the neutrons must be pro-
duced in short time bursis and the tine-of-flight technique must be used to de-
termine their energi.es. Cross-section measurements of many different types have
been made with pulsed neutron sources from thermal energies to ~ 10 MeV and for
most measurements pulsed sources are superior to steady-state monoenergetic
sources.
There are many factors which must be considered in evaluating the advantages
of a particular neutron source for a specific cross-section neasurement, such as
intensity, neutron energy distribution in the source, energy resolution desired,
background, instantaneous intensity, repetition rate for pulsed sources, etc. . For
any particular experiment there will be a compromise between intensity and resc-
lution. The figure of merit of a pulsed source for high-resolution resonance-
energy cross-section measurement: (with energy resolution AE/E) is approximately
where P is production rate during the burst of the useful neutrons within the
energy range to be investigated
w is the duration on the burst including moderation time if required
(Pw) is the number of useful neutrons per turst
. f is repetition rate of the bursts, and must be sufficiently low to prevent
overlap
(PW)f is the average production rate of useful neutrons
and is the time jiiter of the detection equipment.
The magnitude of 7 depends on the experiment but may be as large as a microsecond
in the eV-energy region and as small as a nanosecond in the MeV-energy region.
Pulsed neutron sources have been made in a variety of ways: a fast chopper
on a steady-state reactor, a pulsed reactor, a photoneutron target on an electron
linac, a booster on a pulsed electron accelerator, a spallation target on a high-
energy proton cyclotron, and a nuclear explosion. Excluding the nuclear explosiou,
existing pulsed accelerators are capable of producing approximately the same number
cf useful neutrons in considerably shorter pulses than can be ohtained from exist-
ing fission neutron sources. Hence, considering the figure of merit only, pulsed
accelerator sources are superior to puised fission sources (excluding a nuclear ex-
plosion) for high-resolution measurements in the keV and MeV-energy regions becausa
of the short (w menoseconds) pulses which can be obtained from the accelerator.
Comparisons of the pulsed neutron sources will be made in much greater detail later
3.
on during this seminar. However, I want to emphasize this conclusion now since
most of the high-resolution measurements that I will discius in my talk were made
with high-intensity, short-pulse accelerators, ani &t::11 grater intensity is re-
quired for many desirable experiments.
In considering the usefulness of a pulsed neutron source, there are severa).
other factors in addition to the figure of merit defined earlier. The background,
resulting from gamma rays and from undesirable energy neutrons produced in the
source, can often render a particular experiment impractical even though there is
a sufficient intensity of good neutrons. For some experiments it may be desirable
to produce neutron bursts with an energy spread of a factor of only 10 om even
less in order to reduce backgrounds from unwanted neutrons. This technique is
often used with pulsed electrostatic accelerators; in addition the neutrons from
the source are sometimes restricted in angle due to the kinematics of the reaction.
If the background not associated with the source is large (such as in the measure-
ment of the Pission cross section of a highly alpha-active sample) it is desirable
to have an extremely high production rate during the burst such as is available
with a nuclear explosion. For coincidence measurements the duty cycle should be
high; hence, a source with a high repetition rate is desirable.
Some sources such as a fast chopper, crystal spectrometer and Van de Gruaff
require much smaller samples for transmission measurements than the moderated
pulsed accelerator sources. Activation measurements have been made at low energies
using a nuclear explosion; however, for higher energies (210 keV) monoenergetic
sources are superior. It will also be difficult for pulsed sources with broad
energy spreads to compete against monoenergetic (also pulsed) sources for high-
resolution inelastic-scattering measurements. These are only a few of the more
obvious factors which can determine the value of a neutron source for a particular
cross-section experiment.
3. NEUTRON CROSS SECTIONS REQUESTED FOR REACTORS
The requests listed in the latest compilation are approximately equally
divided between the requests for the fissile and fertile materials and the re-
quests for moderator, structural and shielding materiils. The problem for the
fissile nuclides is that many of the cross sections are needed to high accuracy
over a wide energy range such as the fiesion cross section of u requested to an
accuracy of 1 - 2% over the energy range from thermal to 8 MeV. At the present
time after many, many man years of effort there are several + 3% measurements
which disagree with each other by as much as 77. At a recent conference on
Neutron Cross Section Technology, it was estimated that to measure the (n,a)
cross section of 'Li to a few percent accuracy over a wide energy range has re-
quired ~30 scientific man years of effort at an estimated cost of a million
dollars. For most of the measurements on the fissile and fertile materials,
.
existing high-intensity sources are suitable, with one notable exception. In
order to determine the Doppler reactivity coefficient for i fast reactor the de-
teiled resonance structure or <5°u and 5pu must be abu: cd with extremely good
energy resolution (AE/B ~ 10°*) from w10 tu w100 keV. This resolution 18
nearly an order of magnitude better than can be obtained at present and, hence,
requires a much more intense source.
For the moderator, structural and shielding materials, the two most re-
quested types of cross sections are the capture cross sections up to a few hundred
keV and the inelastic-scattering cross sections up to ~ 10 MeV. Capture cross
sections for materials like Na, Cr, Fe, Ni, Mo, etc. are requested to an accuracy
of ~ 10% or no 5 mb. Capture measurements have been made for years with various
kinds of detectors with both monoenergetic and pulsed neutron sources. Slide 1
shows some recent results on Na obtained in a 6-hour run by Block and collabora-
tors (57 using the RPI linac and a large scintillation tank for detecting the gamma-
rays. A few of the observed resonances such as those at 53.4, 240, and 309 kev
vere known from transmission measurements and have neutron widths of several keV.
These data have not yet been analyzed but assuming that the radiation widths of
the large resonances are probably comparable to the value of 0.6 eV obtained for
the 2.85-keV resonance, the ratio of capture to scattering in these large reso-
nances is n 1:1000. The efficiency for detecting scattered neutrons with the
scintillation tank has been reduced to less than 10that for detecting neutron
capture; hence, very small capture cross sectious (w mos) can be easily measured.
Capture results have also been obtained by other techniques, 1.e., using Moxon-Rae
detectors at the Harvell linac, with a pulsed Van de Graaft at ORNL, and by groups
at LiSL using a nuclear explosion, etc. I conclude that existing neutron sources
are sufficiently intense for the capture cross-section measurements needed for
reactors.
Inelastic cross sections for materials like Na, Si, Ca, Fe, Pb, etc. are
requested to an accuracy of ~ 10% up to ~ 10 MeV. The most suitable source for
these measurements is a pulsed (v one-nanosecond) monoenergetic source from an
electrostatic generator. Slide 2 shows some recent data on Fel") obtained in 80
minutes with 5-MeV neutrons from a (a,d) sourc: using a pulsed Van de Graaff at
ORNL. The energy resolution of the elastically scattered neutrons is determined
principally from the time-of-flight resolution (2-nanosecond bursts at 4 meters)
and that of the low-energy inelastic groups are determined principally from the
energy spread of the incident neutrons (~60 keV). . Since measurements must be made
for many incident neutron energies (and often different angles of scattering, for
which multiple detectors can be used), a complete set of measurements can be very
time consuming. An increase in the source strength would be very desirable for
VI
inelastic scattering measurements above a few Mey. Below ~ 2 MeV the present
sources are surficient for inelastic scattering as demunstrated by the vast amount
of data from A. B. Smith and collaborators at ANL.
An alternate approach to obtain the integrated (over angle) inelastic-
Ecattering cross section is to measure the energy spectrum of the emitted gamma
rays as a function of neutron energy. Most of the measurements of this type have
been made using monoenergetic sources, but a polyergic source and the time-of-
flight technique have also been used. Dat have been obtained with both NaI and
Li-drifted Ge detectors. For poor-resolution measurements (which would probably
satisfy many of the reactor requests) it is possible that a proton recoll detector
could be used to measure the spectra of the scattered neutrons using a pulsed
polyergic source; hence, all inelastic spectra data for all incident neutron
energies could be obtained simultaneously. A high-intensity, short-puise source
such as the ORNL electron linac should have sufficient intensity for this proposed
technique. It seems very unlikely that double time-of-flight measurements would
be possible.
4. NUCLEAR STRUCTURE OF HIGHLY EXCITED STATES
A. Nuclear Models for Highly Excited States
Soon after several very narrow resonances (~ 0.1 eV) were observed with lcw
energy neutrons a few decades ago, they were explained on the basis of the compound
nucleus model. In the pompound nucleus model the excitation energy is shared
amongst all the nucleons in the nucleus. Since the structure is so complicated
it takes a great many oscillations for the nucleus to approach a configuration
where it can emit a neutron, or a gamma, or undergo fission, etc., which accounts
for the long lifetimes and, hence, the narrow width: observed for low-energy reso-
nances. Some 15 years ago a gross structure (width of several MeV) was observed
in neutron cross •sections, and these were soon explained from an optical nuclear
model as resonances in a complex nuclear potential well. The incoming neutron
acts as a single par cicle in the field created by the ouer nucleons in the nucleus.
This optical model is a natural continuation of the shell model of the nucleus :
(which has been so successful in understanding the low-lying bound excited siates)
into the unbound region where neutron emission is possible. Hundreds of theoreti-
cal papers have been published on the compound and optical nuclear models and
there is no question of the usefulness and accuracy of these models, in spite of
their apparent contradiction. In the past few years experimental data have sug-
gested cross section variations which are more rapid than those explained from
the optical model arıd yet much wider than compound-nucleux resonances. The exper-
imental data to determine this intermediate structure needs to be very compreher-
sive and accurate. The greatest need for more intense fluxes is for these neas-
urements.
..
.
1
.
*
.
:
2.
.
:
-
.
..
-
With the present day high-resolution neutron spectrometers, often over a
hundred "compound-nucleus" resonances can be observed in ú single isotope. The
parameters of the resonances have been analyzed to inrestigate distributions of
the parameters, such as the spacing distributions and the width distributions, and
there is good agreement between experiment and theory. 'Fossibly more experimental
data would produce discrepancies which could be significant. By avereying over
these "fine-structure" resonances or by appropriate poor-resolution measurements
the quantity (r/D), known as the strength function for s-wave or p-wave neutrons,
can be obtained. The experimental data are in fair agreement with an appropriate
spheroidal optical model which has a diffuse surface, surface absorption, spin-
orbit coupling, etc. However, the experimental points do scatter more than would
he expected from their quoted errors. Also some experimental data seem to indi-
cate there are local variations of the strength function, 1.e., that the s-wave
strength function varies over an energy region of ~ 50 kevl») or even w hundreds
of ev.'°) More extensive data are needed to investigate this effect for many
nuclides. Also in the study of the spacings of nuclear levels by the method due
to Dyson, there has been the hint that there might be long-range correlations
among "line-structure" resonances which would be very surprising. However, the
fact that very small resonances might be missed and that small resonances which
may be p-wave resonances are included in the consideration make the present evi-
dence rather untrustworthy. High-resolution capture measurements to detect the
very small resonances and measurements to determine their l-valves are required.
These measurements will probably require more intense sources than we have today.
B. Intermediate Structure from Doorway States
. In order to understand the intermediate structura observed recently in
neutron cross section data, Feshbachy and co-workers have suggested a mechanism
called a "doorway" state. A doorway state is simply the state that a reaction
must go t'ırough as the first stage towards forming the compound nucleus. Doorway
states involve comparatively-simple excitations and their nature depends on the
nuclear model. If one considers the shell model, the doorway state for an even-
even target nucleus can consist of only two-particle one-hole states since this
is the only kind of state which a two-body residual-interaction operating on the
target nucleus p!us incident particle will generate. In this model the doorway
states are called 3-quasi-particle states (2 particles + 1 hole). These three-
quasi-particle states can change into more complex states such as :5-quasi-particle
states, then 7, etc. via successive two-body interactions until a "true" compound
nucleus is formed. If one considers the collective model, the doorway state might
consist of the target nucleus excited to a collective state (such as the dipole or
quadripod.e vibrational state) with the incident particle dropping into a single-
7.
-
-:
.
.
4
R
:.
.
.
.
particle level. Other possibilities have also been suggested. In order to obtain
experimental data concerning these doorway states one can either make poor-1880-
lution measurements and average over the compound-cucleus resonances experimentally
or make high-resolution measurements over a broad energy range (hundreds of keV)
and then average the neutron strengths over the resonance numerically. If the
experimental resonances are sufficiently widely spaced as in the case of cºpb,
one can coinpare the actual number of predicted 3-quasi-particie states and their
neutron strengths to the experimental data.
C. Three-Quasi-Particle "Doorway" States
Using this "doorway" state approach, Shakin'°' has calculated the 2-particle
l-hole states with spin 1/2 and even parity which might be produced by neutrons
upon the even isсtopes of Po and Sn. Theory predicts seven s-wave resonances in
20° Po with nevtrons up to 2 MeV and the sum of their neutron widtks was predicted
to be ~ 50 keV. Experimentally only 2 or possibly ö resonances have been observed
and their neutron widths add up to u 120 keV. Figure 3 shows a few angular distri-
butions"> (obtained with a neutron energy resolution of a few keV) which are
necessary in order to interpret the total cross section shows in the figure. The
s-wave resonance in this energy region occurs at 1.735 MeV and has a width of 505
keV. There are many odd-parity resonances, including four f-wave resonances
which have rather large neutron widths (5 few percent of the Wigner limit, which
is the limit for a single-particle resonance) which are possible candidates for
doorway states. In addition to 2-particle l-hole doorway states, odd-parity
states could also be produced by coupling the positive-parity neutron states to
the 3° collective state of UºPb. More-intense neutron sources would lower the
limit for the size of the small resonances which might be missed as well as make
it possible to extend the measurements to higher eaergies.
Shakin's calculations for the even isotopes of tin predictei many 2-
particle l-hole states with a few hundred keV spacing and an s-wave strength
function which decreased from (0.7 to ~ 0.1) x 10°* for the even tin isotopes.
The experimental s-wave strength function determined from transmission measure-
ments 12o) up to a few keV is in good agreement with this theory; 'aowever, more
· experimental data are desired from higher energy measurements. The experimental
level spacings are a few lundred eV indicating that the strengths of the doorway
states have been spread over ~ 1000 compound nucleus states. Also several strong
P-wave resonances were observed which might be due to the eifects of doorway
states.
There are also other measurements of partial cross sections such as (n,12)
and (n,p) which indicate the existence of intermediate structure; however, more and
higher statistical accuracy data are needed. Feshbach!") has emphasized that it
. 18 essential to minimize the number of channels for the nuclear reaction in order
to enhance the fluctuations. One should therefore examine partial cross-sections
Euch as one on a (and also the spectra of the en étted particles) in order to
specify as many quantun numbers of the emergent particles as possible. To detect
the internediate structure, these cross sections must be measured over a wide energy
range and if possible with good energy resolution to see how the doorway states
are distributed to the compound- nucleus states.
1). Analogue States
Perhaps the clearest example of a doorway state 1: the analogue state pro-
duced from an isobaric analogue reaction. Figure 4 shows some data on the
apparent resonances are observed at 1.87 and 2.45 MeV whose spins and parities
were determined to be 3/2° and 1/2*. These two resonances corresyond to the ana-
logue states of the 4th and 6th excited states of Ar whose spins and parities
are 3/2° and 1/2*. 'The middle curve of this figure shows. recent high-resolution
datalt.) frora Duke University for the first third of the upper curve. The neutron
energy spread in this experiment was only 200 eV; hence, AE/E is only 10~4.
There are many resonances (one as narrow as 120 eV) in the region near 1.87 MeV.
The bottom curve covers a 60 keV interval and all of these resonances correspond to
3/2 states. Some twenty strong 3/2° resonances were found spread over an energy
region with a width of ~ 100 keV, whereas the 3/2° resonances outside this region
were much Weaker. This is a beautiful example of a doorway state some 60 keV wide
whose strength gets distributed amongst the fine-structure states of the conipound
nucleus.
Figure 5 shows high-resolution data in the region of 2.45 MeV, the region
LSVEIEN-
*
around 2.45 MeV have spin 1/2*. Most of the proton strergth of the 1/2* resonances
is located in a u 100 keV interval. This is another example of a coorway state.
Two other proton-induced reactions were investigated in this work, namely the (2,6)
and the (p,n) reactions. The (p,q) reaction does show some effect of the 1.87-MeV
analogue, resonance, but none in the 2.45-MeV resonance. The (p,n) reaction which
This can readily be understood since the spin of *°K is 4, which would require f-
wave emitted neutrons from these 1/2* states whose penetrability rould be small.
Hence, if one were to measure the *{n,p) reaction, this analogue: state would not
enhance the cross section. It is expected that (n,p) reactions will not produce
analogue states due to the excess of neutrons in all but the lightest nuclides;
however, it is possible that intermediate structure would appear from other door-
way states.
.
í
-
..
.
-
24
.
***
wa
E. Doorway States from Neutron Capture
It has also been suggested that doorway states can lo determined from the
Capture of either low-energy or fast neutrons. In comparing thermal neutron cap-
ture gamma-rays to (d,p) reaction yields for "Fe and "Fe, Ikegami and Emeryta)
observe an anti-correlation between the (n,r) and (a,0) reaction, 1..e., the gamma-
ray transitions go to regions of excitation of the compound nucleus where the
single-particle stripping strength is small. This is shown for "°Fe in Figure 6
where the spectroscopic factor for the (d,p) reaction is compared to the dipole
strength for the gamma-ray transition. Comparisons between the high-energy garma-
rays from neutron capture and the (d,p) process have been made throughout the
periodic table by Groshev." The strengths of the strong al garma rays to low-
lying p-states are in agreement with the large spectroscopic factors for ļ = 1
groups from the (d,p) reaction. This agreement has been taken as evidence for the
direct capture process. This process follows from the optical model, since the
neutron can radiate when it is in a region of changing nuclear potential. However,
in the case of 54 Fe and 5°re, Ikegami and Every suggest that the anti-correlation
arises from 3-quasi-particle (seniority 3) states which radiate to low-lying states
which are not simple seniority-one states fed in the (a,p) reactiot:. For "Fe, for
example, the 2-proton 1-neutron state [(faali? (15/2) (05/2)]19+ is expected
near the neutron binding energy; this state has allowed El transitions to the states
[friplat (15/2). (P3/8/p_7742-.3/2- which are expected at no 4 MeV. Similar con-
figurations involving two neutrcns and a neutron hole are also available. Ikegami
has also pointed out that some of the seniority-three excitations near the neutron
binding energy have allowed El transitions to seniority-one (singl.e-neutron) con-
figurations. For example, the three-neutron configurations
Afalo) (Eg/s) (pzio or p, 12)] 118+ have allowed El transitions to the single-
neutron Pzla and Pylo configurations, respectively. This could be the explanation
for the strong excitation of both the ground state and the 14-ke V excited state in
'Fe by the (n,r) reaction. The ground state is known to be predominantly
seniority 3, [fiel+ (P3/2/n), while the 14-keV excited state is of seniority
1, (P3/2).
The existence of three-quasi-particle doorway states has been introduced to
explain strong high-energy M transitions\1) from neutron capture in a few s-wave
resonances in the even-isotopes of tin. In Shakin's calculations for the peutron
widths of the 3-quasi-particle neutron states of the isotopes of tin, two states
of the following configurations were quite strong:
[89/2)-4 (872) (81/2 or d3/2)/11/2+.. These 2 states could decay by M transi-
tions to the low-lying sy/2 and dzia single-neutron states respectively. However,
for lack of intensity from a fast chopper, these NaI gamma-ray measurements were
only possible for neutron resonances below 400 eV; with more intense neutron
2.0
.
sources it would be very desirable to extend the measurements to higher energies
and use Li-drifted Ge detectors.
In a recent paper>) on the energy levels of trinn from (a,p) and thermal-
neutron capture gamma-ray measurements, an interesting compurison has been made of
the relative Intensities of the 2 processes (Figure 7). The high-energy ganima rays
populate the same states as the (d,p) process, which yields only neutron states.
Since these high-energy gamma rays do not populate excited-proton states, the
authors suggest that these high-energy gamme-rays arise from direct capture which
utilizes only a small part of the thermal-capture cross section of 127 barns.
However, the predicted direct capture cross section for this mass region 18 only
0.5 barn, which 18 quite small. Lanello) has computed the contribution from col-
lective capture; however, it is only the same order as the direct capture. The
Tm(n,r) data showed a very weak El transition to the 1° ground state in contrast
to the strong ground-state intensity from the (d,p) reaction. The authors 163)
suggest "this unusual forbiddenness is probably to be understood in terms of the
specific character of the capturing state." However, I think that this emphasizes
the need for gamma-ray spectra measurements in many resonances to allow for the
Forter-Thomas variation in partial radiation widths since the thermal cross section
of Im 18 dominated by the strong 1* resonance at 3.92 eV.
F. Capture Game-Ray Svectra Measurements in Individual Resonances
To emphasize the value of gamma-ray spectra measurements from individual
resonances, I would like to present some thermal-neutron capture gamna-ray measure-
ments upon LowW, 10*w, 100w. In all 3 isotopes strong El transitions were observed
to the [51071/2" and [510.73/2° rotational states and weak transitions to the
51273/2° state. It was considered that collective capture would be needed to
explain the anomaly. However, capture spectra measurements from resonance capture
have shown that although the transition to this 51273/2® is weak in the 4.16-eV
resonance (which dominat '3 the thermal capture) this transition is strong in two
other resonances. These capture spectra measurementslt upon W were made with a
20-cm? Ge detector using the RPI linac. Figure 8 shows preliminary data for 3
resonances in Low. The 2 high-energy gamma-ray lines from the 4.1-eV resonance
are el transitions to the 51071/2" and [51073/2° rotational states and no transi-
tion to the [51273/2° state at 209 keV is observed. However, in the other 2 reso-
nances the transitions to this state are strong.
Figure 9 shows datalo) from Saclay for 4 resonances in 199pt obtained with
a 6 cm Li-drifted-Ge detector 30 meters from the neutron source. The strong
fluctuations in the intensities of the 3 high-energy gamma rays for these 4 reso-
nances is striking. From spectra measurements from 22 resonances with spin and
parity 1", the authors conclude that the distribution of partial radiation widths is
li
waar jy
-
in good agreement with a Porter-Thomas distribution.
Capture spectra measurements +9) have been made in the keV energy region
from resonances in 20° po using a large NaI crystal anu a prūsed van de Graaff at
ORML (Figure 10). For intensity reasons the neutron energy resolution was poor
but suficient to permit the measurement of the gamma-ray spectra from 5 reso-
nunces up to 75 keV. The resolution of the NaI crystal was sufficient to resolve
transitions to the ground state and first 2 excited states of colpo, wüich all
have negative parity. Since 3 of the resonances are p-wave resonances, these high
energy transitions must be M. The authors(19) bave suggested that two-particle
one-hole states based on the 1,910 1, excitation for neutrons and h holos
for protons are possibilities to explain these high-energy Ma transitions. To im-
prove both the neutron energy resolution and the gamma-ray energy resolution by
using Ge-detectors would require a considerable increase in the intensity of the
neutron source which 18 expected from the ORNL linac.
Sometimes in thermal-neutron capture gamma-ray measurements there 18 an
ambiguity whether a gamma ray is from the capturing state or a cascading gamma ray.
From capture spectra measurements in the keV energy region this ambiguity can ...
often be resolved since the gamma ray energy will be increased by the energy of the
neutron if the gamma-ray is a primary transition.
G. Other Neutron Cross Section Measurements Requiring Intense Neutron Scurces
There are many other high-resolution, multiparameter experiments of interest
which require intense neutron sources; some will probably require more intense
sources than are presently available.
(1) Garama-ray spectra measurements from neutron capture between compound-
nucleus resonances to investigate direct and serai-direct capture.
(2) Gamma-ray spectra from neutron capture in resonances of the fissile
auclides.
(3) Coincidence and angular-correlation ineasurements between cascading gamma
rays (or between gamma-rays and conversion electrons) from neutron capture in in- .
dividual resonances.
(4) A study of the gamma rays in coincidence with fiosion to determine the
importance of the (n,xf) process.
(5) A survey of subthreshold fission for many resonances in the even-even
heavy nuclides.
(6) Measurements of the energy spectra of alpha particles and protons from
individual resonances and over a wide neutron energy range.
(7) Transmission measurements of very small samples, for example, using
polarized targets and polarized neutrons to determine the spin state of individual
resonances.
(8) Detailed studies of the fission process in individual resonances.
..
12
REFERENCES
Research sponsored by the U. S. Atomic Energy Corsai,spirn under contract with
the Union Carbide Corporation.
(1). "Compilation of EANDC Requests", EANIC 55 "U", March 1965.
(2). J. P. Barry, Bull. Ans. Phys. Soc. 11, 655 (1965); Proceedings of Conference
on Neutron Cross-Section Technology, March 1966, to be published as AEC
Report, Conf. 660303.
(3). R. C. Block, R. W. Hockenbury, z. Bartalome, R. R. Fullwood, Annual Tech-
nical Report, RPI Linear Accelerator Project FY 1966 (to be published).
J. A. Biggerstaff, Y. Cassagnou, W. E. Kinney, M. V. Harlow, J. M. McConnell,
F. G. Perey, P. H. Stelson, Phys. Div. Ann. Progr. Rept., Dec. 31, 1965,
ORNL-3924, p. 54.
(5). J. A. Biggerstaff, W. M. Good, and H. Kim, Int. Conf. on the Study of Nuclear
Structure with Neutrons, Antwerp, July 19-23, 1965, EANDC-45-5 (paper No.
67).
(6). T. Fuketa, Bull. Am. Phys. Soc., 2, 20 (1964) and private communication.
(7). F. J. Dyson and M. L. Mehta, Jour. Math. Phys. 4, 701 (1963);
M. L. Mehta and F. J. Dyson, Jour. Math. Phys. 4, 713 (1963).
H. Feshbach, Proc. of Int. Conf. on the Study of Nuciear Structure with
Neutrons, Antwerp, July 19-23, 1965, p. 257;
C. Shakin, Ann. Phys. (New York) 22, 373 (1963).
(9). J. L. Fowler, Phys. Rev. 147, 870 (1966).
(10). T. Fuketa, F. A. Khan, J. A. Harvey, Phys. Div. Ann. Progr. Rept., Jan. 31,
1963, ORNL-3425, p. 36.
(11). G. A. Keyworth, G. C. Kyker, E. G. Bilpuch; 7. W. Newson, submitted to
Nuclear Physics.
(12). H. Ikegami and G. Emery, Phys. Rev. Letter 13, 26 (1964); -
8. Ikegami, Japan Atomic Energy Research Institute Report, JAERI-1102, 55
(1965).
(13). L. V. Gros.hev, A. M. Demidov, V. N. Lutsenko, and, V. I. Polekhov, Proc. Sec.
Int. Conf. on Peaceful Uses of Atomic Energy, Geneva, 1958, Vol. 15, p. 138;
L. V. Gros bev and A. M. Demidov, IAE-1037 (1965), ORNL translation 1259.
(14). J. A. Harvey, G. G. Slaughter, J. R. Bird, and G. T. Chapman, Phys. Div. Ann
Progr. Rept., Dec. 31, 1963, ORNL-3582, p. 62.
(15). R. K. Sheline, C. E. Watson, B. P. Maier, U. Gruber, R. H. Koch, 0. W. B.
Schult, A. T. Motz, E. T. Jurney, G. L. Struble, T. v. Egidy, Th. Elza,
and E. Bieber, Phys. Rev. 143, 857 (1966).
(16). A. M. Lane, Proc. Int. Conf. on Study of Nuclear Structure with Neutrons,
Antwerp, July 19-23, 1965, p. 343.
13
c
./
.
bie
=
*
=
-
.
-
(17). E. R. Rae, W. Moyer, R. R. Fullwood, J. L. Andrews, Annual Technical
Report, RPI Linear Accelerator Project FY 1966 (to be putlished).
(18). H. E. Jackson, J. Julien, C. Samour, P. L. Che villori, J. Morgenstern,
and F. Netter, to be published.
(19). J. A. Biggerstaff, J. R. Bird, J. H. Gibbons, and W. M. Good, to be
published in Phys. Rev.
5
STRE
LEGAL NOTICE
This report mas prepared as an account of Government apocsered works, Maither the United .
mate, nor the Commodem, mas muy varmi notte d elle at the contes
A. Makes my marriety or wantedom, pred or implied, mida me n e
mary, pletenews, sweetness of the blutnymi owned who marare low
of my wormation, www , wenn es mirador dested to report na med
patrately owned notes of
M. Asomo y Habilities will run the mooh w how m
frmite
wowotorunun, arts, mother, or med och north
no mond on the above, persus wortung und Seball of the newestentes no maladies and more
Warns a watercter om Camtasia,
w here we continua , bo
moet ongles o contracter of the Communication, as employee a wote water prepara
denominatus, wenn mand to, w
twor
ent
With the cominciun, or MS phone wall courtoi.
208
FIGURE CAPTIONS
Hig. 1. Capture cross section of Na in the ke v energy region with an energy reso-
lution of 2.5 nsec/m.
Fig. 2. Inelastic scattering cross section of fe at 5 MeV. The numbers above
each peak are Q values in MeV.
Fig. 3. Differential elastic scattering or cºpt with an energy resolution of a
few keV.
Fig. 4. Proton yield from "Ar(p,p) in the region of the analogue state of the
4th excited state of Ar.
Pg. 5. Proton elastic scattering cross section of Alp,p) in the region of the
analogue state of the Eth excited state of Par.
Pg. 6. Comparison of the (n,r) and (d,p) strengths for final states in "fe.
Fig. 7. Histogram showing the comparison of the energies and intensities of the
(å,p) and (n,r) spectra.
Fig. 8. Gamma ray spectra from 3 resonances in 102w using a 20 cm Ge detector.
The 2 strong gamma rays at 5.3 MeV in the middle curve are due to capture
in the 18.8-eV resonance in toqW.
Fig. 9. Gamma ray spectra from " resonances in Pt. The positions of the five
high energy transitions are indicated by arrows.
Fig. 10. Yield of gamma rays as a function of neutron energy for “cºpo target.
**
*
E.
SYY.

SO-35.5 kev
RPI CAPTURE DATA
8001
--- 117.8 K&V
0.64 in SODIUM
2.5 nsec/meter RESOLUTION
670 Kev
- 300 KeV
- 240 KeV
-144 KeV
como 53.4 KeV
COUNTS
-- TIME ZERO
my
Econo
g
.
1100
150
2004
-
250-
300
7.6 KOV
• 7.6Kev
-
COUNTS
2013 mai marihua
i t 350L
400
a na
450 500
CHANNEL NUMBER
550
- 699
Figure 1.
.
-
YO
.:
-
-
-
-
*

..
ORNL-DWG 66-2237
a
- Fe -
- ANGLE = 55.0°
no
ou
--0.84
o
--3.39 —
---3.12 -2.66
3.37 -2.941-1-
--3.44
-3.60
counts
-
1-2.08
.
-3.84
o .
i .
40
60
80
100
120 140 160
CHANNEL NUMBER
180
200
220
240
Figure 2
*
*
Om
66-218R

En • 1.7611 0.0020 MeV
boyz + 150
8p%20-43°: 89425-34°
dy * *+90 ; 84-45°
&quogo
15-0.0022
87 76 POT --240
1.7600
X-0.5
BottITTUUD LAU
LULU
En - 1.749 + 0.0020 MeV
85% +790
8p%-35°8042--14*
884*-17°, 2.--45°
8p9 -90
(%-0.0053
8p72 {POT = +10°
15.-1.7499
X=1.6
oip) (borns/sterodion)
Eng (MOV
وه لللللللللللا
- 12
10
8
4
2
0
6
Olberns)
LUD battIDUUU
E, - 1.701 0.0020 MeV
884,- +710
&py = +50 i 8p8*. 33º
864--63°; 80% 2-120
(1/-0.0030
88562 {POT - Oº
Eo 1.7010
rns)
.
Op py*+80°
X=4.7
DUYULUU
0.8 0.4 0 -0.4 -0.8
. cos $ (C.M.)
Figure 3
'lf:
".
W
.
ON
14 )

མ་ པ ལ ་ བ མཝ ཝ་ཨབ་ ༡ ཝ, ཨ་ ་ ་ པ ཨམ་
ས་ ་་་ ་ ་མ་ ་ ་ ་ ་ ་
་ ་….
- ་ ཁ་ ་ ་ ཨ་ བ ，
་
་
，
vist
O / ORUTH
་ ་ ཨ་ te）
ས
s
•louse
4 & • 1estowa）
་
་
་ ་
སྨཁས་བ་
དང་
་
་ ་ Ese/
་
Ha
《ནག་ཡོང་
a
A" (RpA40
.
.
.
· Figure 4.
I OLAE = 135°
Aro Pop)
·
·
AVE P.P)
...'
.
.
..
ith
THEORETICAL FIT
Ep (MeV
ORNL - AEC - OFFICIAL
Figure 5
ORNI - AEC - OFFICIAL

*

BEI) /[ BEII] F
F857
(2J+IIS /[ (2J+IJSIAN F
DF
டபபடடடடடடடடடட
-
TTTTTTTTTT
T
00/-
EXCITATION ENERGY OF FINAL STATE (MEV)
Figure 6 .
(d,p)

4
EXCITATION (KeV)
RELATIVE INTENSITIES
Histogram showing the comparison of the energies and
intensities of the (d,p) and (1,7) spectra.
: Figure 7 .
CAN
162 w(n,y) 's w

Ensi14 eV
(Jollei
En=21eV
(J='12)
• Ens 4.1eV
(1:12)
400+||
/
suo
-
100
7.0
7.5
8.0
L_II
5.0 5.5 .6.0 6.5
GAMMA RAY ENERGY - MeV
Figure 8
1
.!
529 eV
.
Wel
1.
100
-
41000 "
nimise
997 eV
COUNTS
'
2000
'
'
-
118 eV
195 Pleo
1920-
1404
1143
689
056
:

200
6
1000
600 tot
PULSE HEIGHT CHANNEL
Figure 9
ORAL-DWG 64-5065

POTPO 41 kov
35-46 KOV -
25 ker
12
16 KOV
TIME-OF-FLIGHT CHANNEL NUMBER
FLIGHT TIME DECREASANG
to
TIME OF FLIGHT
t
ARBITRARY UMTS
é
"O
IIIIIIIIII
20 40 60 80 100 120°
GAMMA RAY CHANNEL, NUMBER
Yield of Gamma Rays as a Function of Neutron Energy for 206Pb Target (Neutrons in 20 Channel, Gamma Fays in 128 Channel).
Figure 10
ri
WAX
*
.:,:...:

LAYAH
•
.
.
END
-
-
-
.
DATE FILMED
10/25 /66
..

.



WXT
:
;
=
. .
t
+.-
.,
int
.
-