“Forced-Gamma Emission” Studies Involving Nuclear
Isomers Using Fast Neutrons and Bremsstrahlung X Rays
N. A. Guardala+, J. L. Price+, J. H. Barkyoumb+, R. J. Abbundi+, G. Merkel* and
J.J. Carroll#
+
NSWC/Carderock Division, 9500 MacArthur Blvd, W. Bethesda, MD 20817-5700
* Army Reserch Lab/Adelphi, 2800 Powder Mill Road, Adelphi MD
#
Dept. Of Physics and Astronomy, Youngstown State Univ., Youngstow, Ohio
Abstract. We propose to perform a series of experiments involving nuclear isomers which will investigate the
probabilities and mechanisms for de-exciting the isomeric level down to the ground state upon exposure to external
radiation in the form of fast neutrons and bremsstrhalung x rays. The isomers have half-lives on the order of 1 hr to 10
days which is a convenient time scale to measure statistically meaningful changes in the specific activities of the
isomeric state. Furthermore, the selected isomers are relatively easy to produce in our laboratory in sufficient quantities
so that they can be made in a reasonable time frame and without recourse to any exotic means of production, handling or
preparation and without the need for high-purity separated isotopes as the feedstock. We believe that studies undertaken
in this fashion will produce fundamentally valuable information on the factors which govern and influence“forcedgamma emission” in nuclear isomers This type of information will potentially be very useful in similar studies
involving longer-lived isomers such as:178m2Hf, 242mAm and 108mAg which have the potential to be used in various
emerging new technologies in the later part of the 21st Century.
and that is the term by which this process has become
known widely. While there are many known isomers
which have the potential to serve as excellent
candidates to observe this phenomena of triggering ,
in only case, 180mTa (1) has this been observed
unequivocally. Another isomer in this mass region,
178m2
Hf (2) has drawn a significant amount of attention
in terms of experimental efforts to determine the
probability to trigger forced-gamma emission in it. In
particular, the efforts have been directed to determine
the probability for its triggering using x rays in the 20130 kev region, with reports that the decay of 178m2Hf
can be produced by using x rays as low as 20 keV in
energy. Studies using synchrotron radiation have
attempted to measure the cross section for triggering
using radiation in this energy range.
INTRODUCTION
Recently, there has developed serious interest in
the nuclear physics community to study nuclear
isomers from not only the point of view of nuclear
structure phenomena but also as a means of producing
technologically valuable devices mostly for use as
compact power sources that require a minimum
amount of auxiliary and supporting machinery and
equipment. The isomers which would be used in these
type of power devices would all have half-lives in the
tens to hundreds of years regime and therefore would
allow them to be stored “on-the-shelf” for long
periods of time. When it was desired to release the
stored energy of the isomeric levels in brief time
interval it has proposed that exposing the specific
isomer to an external radiation of the right energy will
result in transitions from the isomeric state to nuclear
levels that are no longer forbidden to cascade via γ-ray
emission to the ground state.
This process of applying suitable external radiation
to an isomer and then observing the “normal” γ-ray
decay to the ground state has been termed triggering,
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
2003 American Institute of Physics 0-7354-0149-7/03/$20.00
279
ISOMERS SELECTED FOR
“TRIGGERING” STUDIES
This method is applicable only in the case of photon
triggering and not for triggering brought about through
the n,n’γ process. The reason is that the amount of
background γ-rays produced from inelastic scattering
from other levels which are not related to the isomer is
much too large and thereby overwhelms the
probabilities of observing γ-ray cascades which would
be indicative of triggering events taking place. The
photon method is much more “gentle” in terms of the
number of states which can be populated both in the
isomer and from ground state levels. Therefore
coincident measurements of γ-rays will be attempted at
NSWC using the suitable isomers which seem to have
the possibilities for x-ray triggering based on the
knowledge of the appropriate values ∆E, the energy
difference between the isomeric level and a level
which is strongly connected to a cascade to the ground
state and on angular momentum considerations such
that the transition form the isomeric level to the
intermediate level is favored and then of course that
the transition from this intermediate level to the
ground state is favored. This set of processes if of
course the definition of triggering. The final transition
from the intermediate level can either proceed directly
to the ground state or to the ground state by a series of
γ-ray transitions, i.e. a cascade.
We have selected eight isomers to be used in thish
study of triggering probabilities and mechanisms.
They are in increasing mass order: 87mSr, 103mRh,
111m
Cd, 115mIn, 189mOs, 193mIr and 199mHg. The values of
t1/2 for these isomers ranges from 42.6 min for 199mHg
up to 57.4 d for 125mTe. All of these isomers can be
produced via fast neutron inelastic scattering, n,n’γ
from the ground state directly to isomeric level or from
the ground state to excited states which are strongly
coupled to the isomeric level. In certain instances we
hope to be able to determine the probabilities for the
these two processes as a function of neutron energy to
proceed in most of these selected isomers. In all cases,
we have prepared samples of these isomers previously
in our laboratory by exposing suitable amounts of
nuclei to our fast neutron beam and counting the
normal decay of the particular isomer. The cross
sections for producing these isomers (regardless of
which mechanism is responsible) is varies from 0.2 – 2
barns in the energy range of 0.3 – 3 MeV. This is an
energy range easily accessible with our acceleratorbased fast neutron source.
In addition to fast neutron triggering studies which
will be applied to all of the selected isomers, there
exists the possibility that some of these isomers may
be triggered using bremsstrahlung x rays. These
isomers must possess energy levels that are separated
by less than 200 keV in energy so that the
bremsstrahlung
x-ray
source
available
at
NSWC/Carderock can be used to study triggering.
Properties Of The Selected Isomers
The isomers listed above fall into two kinds of
groupings based on a generalized assessment of their
nuclear properties. These properties are a function of
the mass of the nuclide, A and where it appears in the
periodic table, as a function of Z. The first group
contains nuclei with A < 116 and Z < 50. The second
group has A > 125 and Z > 51. In fact, besides 125mTe
the majority in this group has 188 > A < 200. It is this
particular region of the periodic table where isomeric
structures and their relationship to the overall trend in
nuclear shape and level structure becomes increasingly
complex and hence deserving of study.
Experimental MethodsTo Determine
Triggering Probabilities and Mechansims
The
experiments
to
be
performed
at
NSWC/Carderock to determine if forced-gamma
emission i.e.”triggering” is either occurring or has
occurred can be divided into two types: a) the method
of determining ‘burnup” which is a measurement of
the specific activity of the isomer before and after
exposure to the triggering radiation source. If
triggering has occurred (and minimal or zero
production of the isomer has occurred simultaneously)
then the specific isomer activity should be less then
what would be expected from the normal γ decay . b)
In-beam measurements of γ-ray transitions taking
place while the external radiation is applied to the
isomer.
The first group consisting of the lighter–mass
isomers are so-called spin isomers (3). These nuclei
have single unpaired nucleon spins which are couple to
specific values of the orbital angular momentum based
on the properties of the orbitals that they occupy.
These orbitals are generally “out-of-place” in terms of
their relative positions regarding the energy and
angular momentum schemes taking place inside their
respective nuclei. An inversion can be thought to be
taking place where these “intruder” states become
lower in energy that the normal situation that would be
predicted. This inversion places the isomeric level in a
280
inversion of the normal patterns can occur in terms of
angular momentum (now collective or rotational
angular momentum values) and energy sequencing.
position where it is isolated energetically from states
with similar values of angular momentum, i.e. states to
which γ-ray transitions would be most favored given
the typical selection rules. These selection rules
generally place severe restrictions on electromagnetic
transitions which require changes in angular
momentum with values greater than 3 (4).
Collective states nearby to each other may then
interact in such a way as to produce mixing, i.e. lose
their distinctive angular momentum qualities so that
electromagnetic transitions have a greater probability
of occurring then what we have been predicted at
lower energies. These effects may be further enhanced
as the interactions between single particle motion
become more strongly coupled to the collective
motions at the highest observable energies. The
possible γ-ray transitions would become less hindered
due to considerations and restrictions based on
changes in angular momentum. This mixing would
then allow for rapid γ-ray transitions down to the
ground state.
As an example consider the 43 min 111mCd isomer
whch has the isomeric state at 396 keV above the
ground state. The isomeric level has a spin of 11/2while the ground state is at spin ½+. A transition
between the two states requires the emission of a γ ray
with 3 units of angular momentum and this is the
primary reason for the 43 min value of t1/2. However a
level at only 20.5 keV exists above the isomeric level
with spin of 7/2+. This level at 416.7 keV is strongly
connected to the first excited state at 245 keV with
spin of 5/2+, the downward γ-ray transition between
them is 100%. This means that of all the so-called
spin isomers selected only this particular 20.5 keV
transition has the potential to bring about triggering
using the bremsstrahlung x-ray source at our disposal
due to the reasonably small change in angular
momentum between the isomeric state and the
triggering level and the small value of ∆E.
These kind of phenomena would be expected in
nuclei in which shape coexistence is possible as well
as transitions between nuclear shapes. Especially
attractive is the situation where quasi-continua are
either known or would be expected to exist. It is these
kind of states brought about by the nuclear dynamics
of shape changes at high spin and high excitation
energies that may provide important and stimulating
information about the mechanisms and probabilities of
triggering. Additionally, this kind of information may
be very valuable in designing triggering schemes for
the potentially technologically attractive isomers such
as: 178m2Hf, 242mAm, and 177mLu. All of these isomers
have collective and rotational angular momentum
considerations attached them in terms of their isomer
levels and for potential triggering modes.
The other spin isomers mentioned have energy
spacing between the isomeric level and possible
triggering levels that is not possible to use the x-ray
source at NSWC to bring about triggering. For these
other spin-isomers the only alternative is to use the
n,n’γ mechanism. Fortunately, fast neutrons with
significantly larger energy are available using the
NSWC tandem Pelletron and fast neutrons are capable
of providing larger inputs of angular momentum via
inelastic collisions than photo-absorption can. As
much as 8 units of angular momentum can be
transferred using neutrons in the 1 MeV energy range
while with photons the limit is 3 units.
All of the isomers of this second grouping, the
collective/rotational isomers have nuclear excited
states with sufficiently small values of ∆E that they
can be reached using the NSWC bremsstrahlung
source so that they can not only be studied via the
n,n’γ process (in burnup mode) but they can also be
studied with triggering photon excitation in both the
burnup and coincident in-beam mode.
In the case of the second grouping of isomers the
angular momentum considerations based on isomer
formation, stability and potential de-excitation are
more complicated. This is due to the more complex
interaction and arrangement of nuclear levels based on
angular momentum coupling which goes beyond the
simpler considerations of
single particle-orbital
angular momentum coupling. Collective angular
momentum of the entire nucleus becomes significant
and introduces new quantum numbers which in turn
place new restrictions on the possible electromagnetic
transitions that are possible. This phenomena of
collective motion results in typical band structures
based on collective excitations (typically rotational
excitations) as was the case with the spin isomers
Finally these isomers would be ideal to study at
high excitation energies, ca. 2-3 MeV, with the most
energetic neutrons available from our acceleratorbased source. As previously mentioned in these types
of nuclei the possibilities exist that these high-energy
states based on band structures have the desired
properties of mixing levels and would lead to an
enhancement in the forced-gamma emission process.
281
The available fluxes of fast neutrons from the two
reactions are ca. 109-1010 n/cm2/s at a position right
behind either the Li or Be target (a distance of about 1
cm). For instances where it may be worthwhile to
investigate isomer production or burnup with neutron
energies lower than 300 keV , suitable moderators
such as: high-purity graphite or D2O are available.
Experimental Approach
The two major experimental facilities available at
NSWC for these studies are 1) the 3 MV tandem
Pelletron accelerator which produces fast neutrons by
either the 7Li(p,n)7Be reaction and the 9Be(p,pn)24He
reaction. These reactions have both been used
extensively in the past for a variety of neutrons
experiments and exposures over a period of nearly ten
years of continuous operation. These two fast neutronproducing reactions will be used first to prepare
quantities of the particular isomer via the n,n’γ
process.
In the case of photo-absorption type measurements
of triggering the isomers to be studied are:111mCd,
125m
Te, 189mOs, 191mIr, 195mPt and 199mHg. The other
isomers have values of ∆E too large to be useful with
the x-ray source available at NSWC (7). In all of these
cases both burnup first and then in-beam coincidence
measurements will be performed.
After suitable times of irradiation depending on the
production cross sections and the value of t1/2 for the
particular isomer, the samples will have their specific
isomer activity measured at NSWC with a Ge(Li) γray detector for an appropriate time depending on t1/2.
Then the sample will be reintroduced into the fast
neutron beam produced by either of the protoninduced reactions mentioned previously.
The NSWC bremsstrahlung source is capable of
delievering 7 x 1010 photons/cm2/sec at a distance of
10 cm from the W target. It is a Phillips MG 225 x-rat
source which has and endpoint energy of 225 keV and
a maximum intensity of photons at 160 keV.
Conclusions
Again depending on the amount of isomer material
produced and its characteristic t1/2 it will re-irradiated
with fast neutrons. This may be done with different
samples at a variety of different neutron energies.
Then the samples will have their isomer activity
remeasured to determine if burnup has taken place and
if it has to what extent. In this fashion a traditional
excitation function (or for this kind of study a “deexcitation function”) will be generated. We reiterate
that although doing in-beam measurements of the γray spectra in particular doing γ-γ coincidence
measurements would be the ideal method of providing
definitive
information
concerning
triggering
mechanisms it is not possible in this instance primarily
due to the relative small concentration of isomer (ppmppb’s) in the total mass of sample.
An attempt will be made using the facilities of
NSWC/Carderock to study systematically some of the
fundamental issues and parameters involved in the
triggering of nuclear isomers. A variety of isomers
each with slightly different nuclear properties will be
used. All of these isomers can be produced in
sufficient quantity at the NSWC tandem accelerator
facility without resort to extraordinary means or
expense. After producing them using the n,n’γ method
the isomers will be studied systematically by exposing
them to a range of fast neutron energies for deexcitation function measurement and a selected
number having the right energy spacings will be
studied for x-ray triggering probabilities.
References
This means that only indirect evidence for the
states participating in triggering will be obtained.
Furthermore, studies of the behaviour of the isomer
production cross section using the n,n’γ process will
have to be undertaken first as a function of energy to
ascertain what the “build-up” factor for each isomer is
as a function of neutron energy before determining the
burnup factor as a function of neutron energy. All of
the selected isomers will be produced using the n,n’γ
method and all will be measured for burnip by
exposure to neutrons suitable to cause de-excitation.
We have discussed previously how fast neutrons are
far less selective in causing excitation then photons so
that a much larger array of potential triggering states
and mechanisms would be available through their use
then with x rays.
1. Belic, D., et al, Phys. Rev. Lett. 83, 52425245(2000).
2. Ahmad, I., et al, Phys. Rev. Lett. 87, 07253072255(2001).
3. Evans, R. D., Chapter 6, Effects of Nuclear
Moments and Parity on Nuclear Transitions, in
The Atomic Nucleus, New York, McGraw-Hill
Pub., pp. 229-236(1955).
4. Marmier, P. and Sheldon, E., Chapter 9, Radiative
Transitions in Nuclei in Physics of Nuclei and
Particles, Volume 1, New York, Academic Press,
pp. 414-460,(1969).
282
283