Creator:Stanley B. Austerman and John E. Hove
Date Created:October 3, 1955
Place Created:Downey, California
Keywords:graphite,liquid helium temperatures
Context:article prerinted from the Physical Review
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Irradiation of Graphite at Liquid Helium Temperatures
Stanley B. Austerman and John E. Hove
Reprinted from The Physical Review, Vol. 100, No. 4, pp. 1214-1215, November 15, 1955
Reprinted from The Physical Review, Vol. 100, No. 4, 1214-1215, November 15, 1955
Printed in U. s. A.
Irradiation of Graphite at Liquid Helium Temperatures
Stanley B. Austerman and John E. Hove
Nuclear Engineering and Manufacturing, North American Aviation, Downey, California (Received October 3, 1955)
IN a recent paper,1 a model for the damage produced in graphite by particle bombardment was presented. This was based to a considerable extent on information obtained from neutron and proton irradiations at nominal liquid nitrogen temperatures; it was further postulated that the damage retained at this temperature represents essentially all the damage formed. This postulate implies that none of the damage centers move, or are altered, below liquid nitrogen temperatures and that irradiation at liquid nitrogen temperature does not affect the damage centers already formed. To verify this assumption, it is necessary to irradiate at still lower temperatures and the present note describes recent bombardments of graphite (type AWG) at liquid helium temperatures with 1.25-Mev electrons at integrated fluxes of about 20 and 40 microampere-hours. Following irradiation, the specimen was given one-minute annealing pulses at successively higher temperatures to room
80 l?0 160 200 240 Z80 ANNEALING TEMPERATURE
Fig. 1. Pulse annealing of two graphite samples irradiated at liquid helium temperature with 1.25-Mev electrons. Resistivity measured at 4°K.
Fig. 2. Comparison of pulse annealing behavior with that observed in previous proton and neutron irradiations of graphite at the temperatures indicated. Apt, refers to the value at 78°K.
temperature and resistivity measurements made at 4°K following each annealing pulse. Two samples were irradiated, one showing a change of 7% in resistivity, the second a change of 4%. During the first run (7% change), the sample temperature was not observed during irradiation. There was some concern about possible specimen heating, so the second sample was irradiated with a reduced beam current and the temperature continuously monitored during irradiation; for the most part the temperature remained very close to 4°K, occasionally reaching a maximum of about 10°K. Because of the similarity of the two annealing runs, it is deduced that the sample temperature also remained low during the first run.
The two annealing curves are shown in Fig. 1. As can
°k)
—ELECTRON IRRADIATIONS (ABOUT 4°K)
PROTON IRRADIATIONS (ABOUT 103° K) -A- 0.fe5 --O- 7.9 MICROAMPERE-HOURS 12.8 MICROAMPERE-HOURS -X- NEUTRON IRRADIATION (ABOUT I20°K)
40 SO 120 160
ANNEALING TEMPERATURE (
2	LETTERS TO
be seen, there is practically no effect below about 80°K. A small amount of annealing may take place, but is too small to warrant discussion, especially when compared to the striking behavior at higher temperatures. Note the abruptness of the annealing between 80°K and 110°K. The actual temperatures during the nominal "liquid nitrogen" irradiations were in the range of 103°K (cyclotron) to 125°K (Brookhaven Reactor), which are high enough that most of the annealing in the region 80°K-110°K must have taken place during irradiation. Figure 2 compares our results with those obtained previously by Deegan2 following irradiation with protons and neutrons. The neutron irradiation corresponds approximately to 3X1018 fast neutrons per cm2, while one microampere-hour is 2.3X1016 protons (8 Mev) per cm2. The smaller peaks of the latter data are presumably due in part to the annealing at the higher irradiation temperatures, although there is also some dependence of the annealing behavior on the total exposure. On the basis of this comparison, it may be concluded that, although the assumption of reference 1 (that all the damage is retained at the nominal liquid nitrogen exposures) is not literally justified, probably no new type of damage center is introduced by the lower
THE EDITOR
temperature irradiation. In other words, the previous annealing experiments revealed some of the behavior that we have reported, but not all of it. From the standpoint of radiation damage studies, it is of interest to note that, if the specimen temperature is carefully held below 80°K, a liquid nitrogen temperature irradiation is sufficient to inhibit all thermal annealing of the damage centers.
At the present time, we do not wish to discuss the annealing mechanism in the 80-110°K region except to point out that presumably it is due either to a release of trapped electrons or a decomposition of interstitial clusters, both of which will increase the resistivity in graphite.
We wish to express our appreciation to T. G. Berlin-court for much invaluable discussion and assistance. In addition we wish to acknowledge discussions with D. B. Bowen and the help of H. Kenworthy and L. Bienvenue in performing the irradiations.
1 G. R. Hennig and J. E. Hove, "Interpretation of Radiation Damage in Graphite," presented at the International Conference on the Peaceful Uses of Atomic Energy in Geneva on August IS, 1955 (unpublished).
* G. E. Deegan (unpublished).
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