Evidence for shallow positron traps in a neutron-irradiated Al single crystal Page: 1 of 3
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BNL—31141
Presented at 6th Int'l
Positron Annihilation,
Texas, April 3-7, 1982
Conf. on"
Arlington,
MASTER
DE82 013871
BNL 31141
CD " S 51 0 ^ 3^
EVIDENCE FOR SHALLOW POSITRON TRAPS IN A NEUTRON-IRRADIATED A1 SINGLE CRYSTAL
P. J. Schultz and I. K. MacRenzle
Department of Physics, University of Guelph
Guelph, Ontario, Canada NIG 2W1
K. G. Lynn, R. N. West,* and C. L. Snead, Jr.
Brookhaven National Laboratory
Upton, New York 11973, U.S.A.
Variable energy positrons have been used to determine the dependence on temperature of
positron diffusion out of a neutron-irradiated single crystal of Al. The results are
Interpreted In the context of a one-dlmenslonal diffusion model which Includes bulk
annihilations as well as trapping at voids and other mlcrostructural defects In the
bulk material by way of a removal rate <eff of freely diffusing positrons. The data
show a strongly negative dependence on temperature below 125 K for <eff> Indicating
the presence of some additional phenomenon which we attribute to positron localization
in shallow, presumably radiation-induced, traps In the crystal.
INTRODUCTION
Recent positron studies3*2 of neutron-
irradiated metals containing voids have shown
chat the fraction of positrons which annihilate
In the longest-lived states normally associated
with positrons trapped at voids decreases
significantly as the specimen temperature Is
lowered below ^200 K. Nieminen et al.1 ex-
plained this effect, which was observed In
aluminum. In terms of an Intrinsic positron-
void trapping rate that has a strong positive
temperature dependence at low temperatures.
Schultz et al. studied neutron irradiated
molybdenum and suggested that other defects
produced by neutron Irradiation provide shallow
trapping centers from which the positrons can be
thermally detrapped at relatively low tempera-
tures. Which of these Interpretations Is most
applicable Is currently unresolved since
neither of the studies in question could clearly
resolve any of the potentially large number of
additional lifetime components In the Irradiated
specimens.
Further study of this problem is possible with
variable energy positron beams which permit a
relatively direct measurement of the temperature
dependence of the positron diffusion length
(L+) In the near-surface (51000 &) region. The
diffusion length Is related to the diffusion
coefficient (D+) by
^ ■ ^w1'2
where t"|j (■ <eff) is the total rate of removal
of positrons from the freely diffusing states.
D+ (L+) Is expected to exhibit a (T”1'*)
dependence on temperature since the diffusion
>10 K Is dominated by acoustical-phonon scat-
tering. 3-5 Our results above "'125 K are con-
sistent with a D+ varying as T-1'^ and a <eff
having a modest positive temperature dependence
similar to that Inferred from the results of
more conventional positron studies of neutron-
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Irradiated metals.1 Keff has a strong negative
dependence on temperature below 125 K, consis-
tent with detrapping of a significant fraction
of positrons from shallow traps. A model des-
cribing a temperature-dependent competition be-
tween the probability of positron localization
in shallow traps, in deeper traps including
voids, or annihilation from the freely diffusing
state provides a good representation of the
data.
DISCUSSION
Our measurements consisted of determining the
posltronium (Fs) fraction re-emieted from a
single crystal of Al containing voids as a func-
tion of sample temperature and Incident positron
energy. From these we deduce the parameter E0,
which is related to L+. by constants which depend
on the positron stopping profile viz:
Eo-K-i<VW1/2- <»
Due to space limitations, we must leave a full
description of the data analysis together with
experimental details and derivation of the model
to the more complete presentation of these
results which can be found In ref. 6.
The Al crystal used (Al-3 in ref. 7) was
Irradiated to a fast neutron fluence of 'Ll.2 *
1021 n/cm2 In the Oak Ridge High-Flux Isotope
Reactor. The void distribution had a mean
diameter of about 400 i at a concentration of
about 5*10lu cm-3.7
Figure 1 shows E0(T) for one of the data sets.
In high-purity well-annealed alngle crystals of
Al at room temperature, E0 has been found to be
">3 keV.8 The solid curve In Fig. 1 representing
an Impurity concentration of C^~1000 at.pptr was
calculated assuming a psaudopotentlal dif-
ference3 of 0.1 Ry. It Indicates that the
weakening of the dependence of D+ on tenpera-
ture caused by Impurity scattering la not
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Schultz, P. J.; MacKenzie, I. K.; Lynn, K. G.; West, R. N. & Snead, C. L. Jr. Evidence for shallow positron traps in a neutron-irradiated Al single crystal, article, January 1, 1982; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc1074853/m1/1/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.