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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963
MASTER®&-0.1987
SEP 2 2 1968

FEASIBILITY OF CERTAIN EXPERIME.VTS |
RELEASED FOR ANNOUNCEMENT
USING UNDERGROUND NUCLEAR EXPLOSIONS
IN MUCLEAR SCIENCE ABSTRACTS
EFSIT TRICES
J. W. T. Dabbs
Oak Ridge National Laboratory, Oak Ridge, Tennessee
HC $1.00, M1,53
SUMMARY: Two types of experiments are discussed; (a) Experiments with nuclei
oriented at very low temperatures, and (b) direct measurements of fission life-
times. In (a), it is concluded that such experiments, especially fission experi-
ments, probably should not be performed using an underground nuclear explosion
as a neutron source, because of unavoidable heating associated with the rapidity
of the experiment; it is found that only a small number of unusual cases are at
all feasible. In (b), a new experiment is proposed in which the passage of a
recoiling compound nucleus through a crystal lattice may provide direct time-of-
flight determinations of the lifetime against fission. The experiment utilizes
the recently discovered "blocking" or "anti-channeling" effect in crystals. The
feasibility of the experiment is discussed, with the conclusion that such neutron
fission experiments are possible only with underground nuclear explosions as nev-
tron sources.
(a) EXPERIMENTS WITH NUCLEI ORIENTED AT VERY LOW TEMPERATURES
Several experiments involving fission of nuclei oriented at low tempera-
tures have been found to require very long counting times at reactors for their
accomplishment(1). As a possible extension of these measurements, certain ques-
tions regarding the use of underground nuclear explosions have been investigated. .
Perhaps the primary difference between nuclear explosions and other methods of
producing timed bursts of neutrons is the extreme rapidity of the entire process;
regrettably, this feature appears to place almost insuperable obstacles in the
path of most oriented fission experiments.
The basic problem is that during the few milliseconds following the explo-
sion, enough heating is introduced by the f'ission fragments themselves to remove
or seriously to reduce any nuclear alignment or polarization existing prior to the
explosion. The lattice temperature of any reasonable target will rise to temp-
eratures of ~ 30°K in a polarized neutron transmission experiment where essentially
all fragments are stopped in the target. Only if the nuclear spin-lattice
- 1 -
relaxation time were at least one second at 30°K would such an experiment be feasi-
ble; this is a rather unlikely circunstance. For cases where, let us say, only 1
percent of the fragment energy is deposited in a thin target, no substantial advan-
tage accrues. Since the lattice specific heat is, in almost all cases, a Debye
specific heat which has the form
C = 464 que la cal/mol K
(la)
the integral is of the form
Q = k(T* - T)
(10)
and a reduction of G by a factor 100, as indicated above, will only reduce the
final temperature of the lattice in the present case to loºk.
A reduction of the lattice heating by a factor of 10° would reduce the
temperature rise to 0.3°K, which would probably be tolerable in most cases. This
probably can be accomplished in cases of simple capture experiments where the
nucleus deexcites only by X-ray emission. This is also true for simple neutron
scattering where no competing charged particle reactions occur. In all cases,
rapidly acting shutters would be required to protect the samples from the initial
burst of electromagnetic radiation from the explosion, and from undesired portions
of the neutron spectrum.
In view 01 the difficulties inherent in this approach, it seems wiser at
present to make use of the higher burst intensities which are now becoming avail-
able at electron linear accelerators, where such heating problems essentially do
not occur, and to restrict the types of experiments to those which are feasible
with intensities available at such machines.
The author gratefully acknowledges helpful conversations with B. C. Diven,
L. Aamodt, P. Seeger, and others at Los Alamos, and with J. A. Harvey of ORNL.
(6) DIRECT MEASUREMENTS OF FISSION LIFETIMES
The possibility of the measurement of extremely short lifetimes in nuclear
reactions by time-of-flight within single crystals has been suggested by Tulinov(2)
and by Gemmel and Holland (3). The proposed method makes use of the so-called
"blocking" or "anti-channeling" effect, in which a charged particle (in this case
a reaction product) passes sufficiently near to a row or plane of atoms within the
crystal to be deflected strongly from its original direction by Rutherford scat-
terings. A decay product from a nucleus which is recoiling (transversely) from
its initial lattice position may be delayed in its emission sufficiently that its
path, while initially in the same direction, may pass the members of the row or
plane instead at a large enough distance to be "channeled" through the crystal,
or at least be not deflected.
Experimental observations with emitted alpha particles(4), -reaction-produced
- 2 -
protons(3), and elastically scattered protons(2,3,5) have given conclusive proof
of the existence of the "blocking" effect, which typically appears as a decrease
of more than 75 percent in intensity over a narrow angular range in the neighbor-
hood of the direction of the row on niore than 35 percent near a plane, perhaps
with a small enhancernent in the intensity at slightly larger angles. The total
angular width B over which this "extinction" occurs may be estigated from con-
siderations of screened coulomb scattering with rarious assumptions regarding
lattice vibrations etc., and for most cases of interest yields values 6 Cl.
At the moment, there is still some uncertainty regarding the conditions
under which "row" and "planar" blocking are important(2,5). Although both effects
always occur, it appears that larger dips in intensity occur along rows (or axis
direction in the crystal) whereas narrower patterns are observed in the various
planar directions. In the latter case, pattern widths as small as 0.1° are found(3).
We shall confine our attention to the planar case hereafter, because of intensity
considerations.
We consider here a situation somewhat different from the previous sugges-
tionsi2,3) for observing very short lifetimes in nuclear reactions. If the recoil-
ing compound nucleus moves through distances comparable to lattice spacings before
emission of the reaction products, blocking by rows or channels adjacent to the
one in which the compound nucleus originated can also occur. If the reaction pro-
duct is emitted nearly along the planar direction by a compound nucleus which is
recoiling in a transverse direction, a decrease in intensity may then occur when
vrt sine met
(10)
where v. is the recoil velocity, I the time before emission, the recoil angle
with respect to the planar direction, man integer, and ly the transverse spacing
between planes.
As an example, consider neutron induced fission in single crystal Uom
or "vo, where the incident neutron direction is about 5-6 degrees away from a
[111] axis; while the detector array is exactly aligned with a (perpendicular)
(111) plane. We estimate or assume the following:
Be ~ 0.003 (0.2) oxygen plane
By = 0.007 (0.5) uranium plane
V = 5.9 x 10" YE cm/sec (E in ev)
T = 1/F6~6 x 10-15 sec (for 2550 near thermal)
27 = 1.65 Å (to oxygen plane)
= 3.1 Å (to uranium plane)
For the <>uo, case, one would then expect dips in intensity at E values given by
En met / 5.8 x 1032) 2
(26)
~ 220 m2 kev.
or at 220, 880, 1980 ... kev. Because of the structure of the crystal in this
- 3 -
or at 20, Boro Kev
example, these values represent alternately scatterings by oxygen planes and ura-
nium planes; effects of heavy and light fragments would of course be indistin-
guishable.
Intensity considerations are approximately as follows. We assume a detector
subtending a solid angle of roughly 28, x 30B, 1.e. 0.5 cm x 30 cm at a 60 cm dis-
tance. The sample would be 0.7 cm x 6 cm x 10°* cm thick, cut along the (100)
planes, and mounted at -- 40° so as to intercept a 3 cm beam of neutrons from an
underground nuclear explosion(6), For an assumed of 15 barns, the rate at which
fragments reach the detector array would be approximately 5000 per microsecond, the
statistical accuracy would be expected to be 3 percent for resolving times (o.lusec)
which give energy resolutions AE'E of about 0.01. These rates are near the limits
of feasibility, since the count rates for oxygen planes are expected to show only
& 40 percent decrease over a small portion (20 percent) of the detector area. For
the uranium planes, a 40 percent drop over the central half of the detector may be
expected, however. The above calculations strongly indicate that the only feasible
neutron source for such an experiment is an underground nuclear explosion.
The detectors in the detector array could be made in a configuration of the
same general type as the position sensitive semiconductor detector(7) but with a
central strip electrode and two grounded edge strip electrodes on the back. The
front electrode would contact the entire active area. The current induced by
electron-hole pair formation within the detector would then divide via a transverse
back resistance according to the distance of the fiesion event from the centerline
of the detector, and yield a position signal from the ratio of the central electrode
current to the total. In this way, a self-normalizing signal, with a relatively
high degree of cancellation of statistical errors should be available.
The major difficulty with the proposed experiment appears to be the spread
in fission lifetimes. This can be expected to cause a serious broadening of the
minima (in the detector current ratio) as a function of neutron energy. If this
effect is sufficiently large to make the expected minima unobservable, resort may
be had to the initial rise (with energy) in the ratio, which would occur at a neu-
tron energy of roughly 1 keV for the lifetime assumed above. Here, a counting
rate of 150 counts/microsecond would be expected, with a 3 percent accuracy in
intensity and an energy resolution 4 E/E of about 0.03. This latter measurement
would involve more complicated estimates of the flight path (-10-9cm) and corres-
ponds exactly to the previous suggestions(2,3).
If both effects could be observed, additional useful information could be
obtained by intercomparison. Another possibility, especially in the case of su,
is that two different lifetimes, corresponding to the two compound spin states,
might be seen.
The author gratefully acknowledges several helpful conversations with
-4.
Tf both

0. S. Oen, M. K. Robinson, and H. Lutz.
REFERENCES
* Research sponsored by the U. S. Atomic Energy Commission under contract with
the Union Carbide Corporation.
(1) J. W. T. Dabbs, F. J. Walter, and G. W. Parker, Proc. Int. Conf. Phys. Chem.
Fission, Salzburg, Austria, March 1965, V.1 (IAEA, Vienna, 1965).
(2) A. F. Tulinov, Dokl. Akad. Nauk SSSR 162, No. 3, 546-8; Trans. Sov. Phys.
Dokl. 10, No. 5, 463-5 (1965).
A. F. Tulinov, v. S. Kulikauskas, and M. M. Malov, Physics Lett. 18, 304 (1965).
(3) D. S. Gemel and R. E. Holland, Phys. Rev. Lett. 14, 945 (1965).
(4) B. Domeij and K. Björkqvist, Phys. Lett. 14, 127 (1965).
(5) A. F. Tulinov, B. G. Akhmetova, A. A. Puzanov, and A. A. Bednyakov, JETP Letters
2, No. 1, 48 (1965); English Translation, p. 30.
(6) The estimates iramediately following are based on conversations with B. C. Diven,
P. Seeger, L. Aamodt and others at Los Alamos, and on an unpublished Los Alamos
report compiled by W. K. Brown. This information is gratefully acknowledged.
(7) A silicon surface-barrier detector of this type is manufactured by Nuclear
Diodes, Inc., Highland Park, Illinois, U.S.A.
LEGAL NOTICE
This report was prepared 18 an account of Government sponsored work. Nollber who Uallod
suales, por the Commission, nor any person acung od bebalf of the Communion:
A. Makes any warranty or representation, expressed or implied, with rospoct to the accu-
racy, completencsn, or usefulness of ibo information contalaod in this report, or that the wo
of any information, apparatus, method, or process disclosed in this report may not infringe
privately owned rights; or
B. Assumes any llabillues with respoct the use of, or for damages rosul dog trom the
use of any laformation, apprnows, morbod, or process disclosed in this report.
As used in the abovo, "person acting so behalf of the Commission" includes way om-
ployco or contractor of the Commission, or employee of such contractor, to the extent Want
such employee or con rector of the Commission, or employee of such contraclor proparos,
disseminates, or provides access to, any laformation pursuant to his employment or contract
with the Commission, or his employment with such contractor.
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DATE FILMED
10/21 /66