Determination of Oxygen in Gallium Arsenide by Neutron Activation knalysis An E x c h a n g e of C o m m e n t s SIR: An article r n your journal by Bailey and Ross ( I ) lhas given data on the determination of oxygen in gallium arsenide by neutron activation analysis. I a m working in the field of osygen determination on the surface and in the interior of metal foils by the same reaction sequence , Li6 (n,a )t , 016(l,n)F1*, and have computed and esperimentally verified the total activation cross sections of this sequenw for several Li-0 systems ( S , 4 ) . I t is my opinion that the results concentration us. depth” in Table I and Figure 5 of the quoted work (1) are incorrect because they are computed with an activation cross section of 0.5 mb. being constant with the depth of material. I n fact, this cross section decreases very strongly with the depth in gallium arsenide (roughly exponentially) and becomes zero after one range of tritons being R = 1.2 X inch (computed by Bragg-Kleeman’s rule). I have carried out the computation of the activation cross section as a function of the depth arising from the excitation function of the reaction OlG(t.n)FLs and the stopping function of tritons (3). For a 0.002inch gallium arsenide foil wrapped in lithium metal on both sides, these values are as follows:
1.9 X 10-4-inch aluminum foils were closely packed and activated from one side by tritons produced by lithium fluoride embedded in a polystyrene foil. After irradiation in the reactor the F” formed by the surface oside of the aluminum was radiochcmically separated and counted. The activities corrected for background, chemical yield, and radioactive disintegration decreased with the foil number approximately in the same manner as the computed activation cross sections.
Foil number
Activity,
1 2 3
633 218 102 57.3
4
c.p.m.
Activation cross section, mb. 0 154 0 073 0 036 0.015
ADDENDUM
Unfortunately it was impossible to follow the decrease of the FL8activity by further foils because of the relative small surface oxygen content of the foils. Therefore, the experiments will be continued with other materials. Lorenzen and Konig ( 5 ) have also found a strong decrease of F18 activity us. depth. They determined the excitation function of the reaction 016(t,n)F’* with tritons of decreasing energies produced by stopping in increasing depths of organic material. The shape of their curve in Figure 3 is the same as given by the above computed values, if one takes into account the different triton ranges in the organic material and in gallium arsenide, respectively. In the experiments of Bailey and Ross ( I ) it should be difficult to carry away such very thin layers by etching without smearing over the strong decreasing F18activities. The problem is probably the same as pointed out by Davies, hIcIntyre, and Sims ( 2 ) in etching still smaller (by a factor of 10 to 100) layers. They determined the range of Cs13’ ion3 in germanium and found “that chemical etching is not a reliable method of determining small depths of penetrat,ion.” The t,oo high value of the activation cross section used by Bailey and Ross is probably compensated by too high F1* activities. That would perhaps, but not necessarilv, result in a correct value of the osygen bulk concentration.
In an analogous case, I have verified the decrease of the triton activation with the depth in aluminum. Four
(1) Bailey, R. F., Ross, D. A., ANAL. CHEM.35, 791 (1963).
Depth in GaAs, inches X lo3 0 0.06 0.12 0.24 0.36
Activation crosssection, mb 1.09 0.76 0.59
0.48
0.60 0.71 0.83 0.95 1.00
0.37
0.23
0.13 0.062 0.023 0.0075 0.0030 0.0026
(Beyond 1 X inch the function increases symmetrically.) Obviously a constant value of u == 0.5 mb. gives an erroneous picture of the function “ 0 1 6 concentration us. depth” and an erroneous value of the bulk oxygen concentration. I t was imposs ble to recalculate the authors’ results because of any discrepancies of the depth values between Table I and Figure 5.
LITERATURE CITED
( 2 ) Davies, J. A., McIntyre, J. D., Sims G., Can. J . Chem. 40, 1605 (1962). (3) Leonhardt, W., Dissertation “Ilie
Bestimmung von SauerstoB und Fluor durch Aktivierung im Reaktor,” Technische Universitat Dresden, July 1962. ( 4 ) Leonhardt, W., Kernenergie 5 , 166 (1962). (5) Lorenzen, K., Konig, W., 2. Suturforsch. 16a, 933 (1961). G. LEONHARDT WOLFGANG
Deutsche Akademie der Wissenschaften zu Berlin Zenntralinstitut fiir Kernforschung Rossendorf
SIR: I n response to the comments above, the same type of esperiment was rerun, using samples of “known” oxygen concentration. S o extreme falloff was found as suggested by Dr. Leonhardt’s letter. There was an apparent small decrease in the triton flux and this was within the area of the esperimental error. Again the final result was checked against the result of t,he MS-7 spectrometer and the values corresponded closely. I have no esplanat,ion of the discrepancy between Dr. Leonhardt’s calculations and my results. I n reply to the addendum, I must admit that chemical et’ching may not be the most reliable method of determining small depths of penetration and that too high a value of the activation cross section could have been compensated for by too high F18 activities is a possibility. Howel-er, if the F18 act’ivities were too high this could only be caused by a ‘(smearing” across the depth of penetration. This I find rather hard to accept for the following reasons: first, over half of t,he samples were examined microscopically, both before and after etching and no large change in the measured surface area was found; second, the samples of different penetration depth were reproducible ; third, the agreement with the &IS-7 data; and fourth, several samples that had their et’ching interrupted three times, when they were rinsed with water just to eliminate the possihility of smearing, showed the same results as those that were etched to depth and then processed. As a result, while I agree that Dr. Leonhardt is probably correct that there is some error involved by using a constant cross section, it is about the only point on which I find that I can agree with him. RICHARD F. BAILEY Radio Corporation of America Princeton, N. J. VOL. 36, NO. 9, AUGUST 1964
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