Low Temperature Pulse Anneal of Brookhaven Irradiated Graphite

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Page 1 of ig This document is The changes in electrical and therraal resistivity and thermoelectric power of neutron irradiated AVIG // graphite exposed in the Erookhaven low temper ature facility, have been ragawnarcdv as a-f-»wwtion - of- annealing temperature in Pre irradiation measurements were made for electrical resistivity at room temperature. The specimens were then sent to the Brookhaven Reactor where they were irradiated in the liquid nitrogen test hole where the temperHanford equivalent. The samples were removed from the test hole and returned under liquid nitrogen. They were then mtanted in the ap aratus previously discussed,'5 the entire specimen holder having first been cooled to 78° K and both the sample and the apparatus were kept under liquid nitrogen while The results presented here are for pulse anneals in which the samples were given pulses of five minutes duration, in steps of 25° K, in the temper resistivity measurements were made at 78° -K and ^he thermoelectric power measurements at 90° K, The formula for obtaining thermo-electric power is 4T (/IvA), Er is thermal e m f in /^\/and T is the temperature difference across the specimen in 0K„ 90'"' K was choosen as the average temperature for this measurement because it is the lowest temper-ture at which the instruments will remain steady enough to secure an accurate reading and still allow for a temper ture gradient of approximately 8° K across the specimen. The three camples were comparatively identical in all respects except exposure time.

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The change in electrical resistivity vs. exposure in micro amphere hours is shown in Figure 1, also shown on the same plot are the results of exposure time converted from MW hrs to /< to hrs. in order that a direct comparison of the amount of change of the various properties could be made with specimens irradiated in the cyclotron. The conversion factor used is differing at 13/^hrs by about k%. The thermal resistivity vs. exposure is in very good agreement as may be seen in Figure 2« The thermoelectric power shown in Figure 3 seems to differ completely for the two types of irradiation, as there is complete lack of agreement. The reason for this is not apparent at present. However, measurements are being made on additional samples to determine the validity of this data. Figure ij. shows the electrical resistivity of the three samples. The measurements are olotted as . (^.o is the undamaged resistivity at liquid nitrogen temperature and is the resistivity at ?o K after exposure but before anneal. The values of C ,5 and alter each a: noal is given in Table 10 The total change in l/X is very small in all three exposures making the accuracy of the thermal resistivity gr« hs relatively The unusually large amount of scatter particularly in the 7^ /D sample, in the thermoelectric power is believed to be due to experimental error rather than to any peculiarities of the samples and a recheck is being run on this before any attempt is made to compare it with other data (Figure 6, 1, Work done by George E. Deegan. 3, Deegan, G. E., "Post Irradiation Measureraents of Cyclotron Bombarded Graphite" pages 30 through 32 in "Radiation Effects Quarterly