Klema, E. D.

McCammon, G.

Ritchie, R. H.

E.D. Klema

R.H. Ritchie

G. McCammon

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RECALIBRATION OF THE X-10 STANDARD GRAPHITE PILE

UNT Digital Library

I UNITED STATES ATOMIC ENEtOY COMMISSION AECp-3590 RECALIBRATION OF THE X-10 STANDARD GRAPHITE PILE By E. D. Klema R. H. Ritchie G. McCammon October 17, 1952 [Site Issuance Date] Oak Ridge National Laboratory •̂ i O ^ 4 T..h„..l »•'""• »•'• »'^«- ' " " " " . LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Alices any warranty or representation, express or implied, with respect to the a c -curacy, completeness, or usefulness of the infonnation contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not in -fringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission" includes any em-ployee or contractor of the Commission to the extent that such employee or contractor prepares, handles or distributes, or provides access to, any information pursuant to his em-ployment or contract with the Commission. ^ 2 - 7 ~ / DISCLAIMER  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER  Portions of this document may be illegible in electronic image products.  Images are produced from the best available original document.  PHTSICS This report has been reproduced direct from copy as submitted to the Technical Information Service. PEINTED IN USA FRIGE 10 CENTS Available from the Office of Technical Services Department of Commerce Washington 2^, D. C. I- Other Issues of this report may -^^^ T3«ar t h e number aElNL-1398. I— •? •__ ~ • • • I Work p e r f oxmed under ^ • ^ 2 ^ ^ Contract No. W-7li05-ei]g-26. Date Declassified: August 6, 19^3 o o * « ^ Arrangements for reproduction of this document In whole or In part should be made directly with the author and the organiza-tion he represents. Such reproduction Is encouraged by the Uhlted States Atomic Energy Commission. ABC, Oak Ridge, Teniu.-W37381 RECALIBBATIOH OF THE X-10 SHASMSD GRAPHITE PILE Abstract The thermal neutron flux In slot number 11 of the X-10 standard graphite pile has been measured by counting the 7-rays from activated gold foils vlth a sodium iodide scintil-lation counter of known efficiency. Because of an indicated discrepancy In the cedibratlon of the X-10 standard graphite pile as ccapared vlth the standard piles of other labor-atories, ̂ ^̂  the X-10 pile has been recalibrated by a different method from that of the previous calibration.^^ In the present experisient slot nuidier 11, 115 on* frtm the neutron source, in vhlch the cadmium ratio as measured vlth indium foils is > 3OO, was recalibrated by counting the r-rays from a gold foil exposed for sev-eral days in that slot vlth a sodium iodide scintillation counter of known efficiency. Gold foils are particularly suitable for such absolute measure-ments since the Au-̂ ^̂  decays almost exclusively by the 7-ray at 1̂ 11 kev.^^^ Thus if one counts a gold foil vlth a counter of known efficiency for this energy 7-ray, he can determine the absolute disintegration rate'of the active atoms In the foil. The aluminum foil holder used in this vork vas 1. Alan D. Conger and Norman H. Giles, Jjr., ORRL-l^, p. 12. (î r̂am Table 1, the thermal flux In the treatment box vas measured as l) 1.09'x 109 neutrons/cB^/sec from the Oak Ridge Standard Pile eallbiration, or 2) 1.29 X 107 from the Argonne thermal column ealibration.) 2. T. Amette and H. Jones, CP'2Bok, 3. A. R. Brosl, B. H. Ketelle, E. Zeldes and E. Tairstein, Fhys. Rev. 81*̂, 586 (1951). AECD-3590 3 ?̂ ? .^^ k AECD-3590 sufficiently thick to stop the beta particles from the gold. The efficiency of the Nal scintillation counter, eqploying a cryst-al 1-1/2 inches in diameter and 1 inch long vlth a ^19 photcnultlpller, was determined by ^-7 eoineidenee counting as follows. A small sample of i gold vas irradiated in the X-10 graphite reactor and counted in the ^-7 coincidence counter of the ORNL Chemistry Division to establish its abso-lute disintegration rate. This sample vas then measured in the high-pressure ionization ehaaiber of the Chemistry Division in order to cali-brate the chamber. The use of the intermediate s t ^ of the high-pressure ion ehaiA>er vas necessaxy since the round 1-mil gold foils used in the calibration of the standard pile, being 1-1/4 inches in diameter, vere too large for absolute 9-7 counting. The Nal counter vas then calibrated by measuring an active 1-1/4 inch gold foil in the hi^-pressure ion chaidoer and counting it on the nil counter. In the final measurements, a calibration of the nil counter was itade on the same day on iriblch the foil exposed in slot number 11 vas counted. The calibration of the hic^-pressure ion chamber iremalns very constant vlth time, and it can be used to ealibirate other counters at any time. The half-life of the measured activity vas checked on both the hi(^-pres8ure ion chamber and the Nal counter to make sure the eonreet activity vas being observed. The absolute disintegraticm rate of the gold foil exposed in slot nutfser 11 vas obtained by cotintlng on the Nal counter as described above. * •. The calculated saturated activity needs to be corrected only for the ^2.7 3 AECD-3590 5 (k) perturbing effect of the foil on the thermal neutron flux. Bothe^ ' has developed a correction for self-protection and perturbation effects using diffusion theory. More recent vork by T. H. R. Skyrme'^^ utilizes transport theory. He considers an infinite Isotropleally scattering medium throughout vhlch thermal neutrons are produced uniformly. The introdtictlon of a circular foil of radius a and thickness X (units of mean free path in the foil) into the medium is considered to introduce an equivalent negative disc source of zero thickness, radius a, and strength equal to the nuadber of captures taking place in the foil. 13ie effect of this negative source v^on the flux at the foil is th«i found and the average reduction in the number of captures in the foil is given to second order in HC . In the present vork this correction vas made on the basis of Skyrme's theory and amounted to 4.2^. The number of captures occuxring in the foil is calculated, in both the Skyrme and the Bothe theories, by assuming a monocnergetie flux of Isotroplcally distributed neutrcms incident on the foil. The appropriate energy is that corresponding to the average velocity of an (6) assumed Maxvellian distribution. One may see from the vork of Zahn, i^o takes into account the energy distribution for several different angular distributions, that this procedure gives accurate results here. The thermal flux vas calctilated from the corrected saturated k, Bothe, Zelt. f. I^sik 120, kyj (1943). (Translated in CP-2964.) 5. MS 91. 6, C. T. Zahn, Phys. Rev. ̂ , 63 (1937). y-2-7 V 6 AECD-3590 activity of the gold foil according to the following formula: AAg <t> th E NQ m o^ lAere A Is the atomic weight of gold Ag is the saturated activity of the gold foil E Is the efficiency of the Nal counter NQ IS Avogadro's number m is the mass of the gold foil o^ is the absorption cross section for gold. Tbe previous detezminatlons of flux in the standard pile have re-lied ̂ pon an empirical fitting of Indium resonance activation data by a sum of three gausslans (arising in the "age" theory for the slowing down of fast neutrons in the pile), calculation of the number of neutrons arriving at thermal energy (taken to be 0*023 e.r.) from the empirical expression, and the use of dlfruslon theory to determine the resulting spatial distribution of thermal neutrons. Ihe flux found in this manner is very insensitlre to the exact "thermal energy" value chosen. Thoa^ diffusion theory assumes slow neutrons to be monoenergetlc, actually the distribution of neutron velocities v l U reserisle more or less closely the eharaeterlstle Nexwelllan form. In obtaining the final figure for the flux in slot nuifter 11 by the absolute method, a Maz-vellian distribution of velocities vlth a most probable value correspond-ing to room teiperature (2200 m/see or 0.025 e.v. energy) has been ^ f '^^ ^ AECD-3590 7 assumed. Thus in calculating the neutron flux giving rise to the ob-served activation in the gold foil (assumed to have a cross section a — ) the following effective activation cross section has been used: oo J M(v) a(v) ^^^^ "0.025 e.v. effective " -r^ " f M(v) dv 1.128 J o wheire M(v) dv • e"^ ' O' v3dv and v. is the most probable velocity. v2 v3dv and \Q The measured cross section of gold at a velocity of 2200 m/sec of 95 bams has therefore been reduced to Bk bams for the calculation under the above assumption. The thermal neutron fltix in slot number 11 calculated as de-scribed above is 703 neutrons/cm?/sec. This value is to be conqpared vlth the value of 654 neutrons/cm /sec as given in the previous cali-bration described in CP-2804. If flux is to be determined in a given field In which one can assume a Haxvellian distrlbutl(m corresponding to room temperature, one has only to multiply the activation of a thin foil exposed in the field by the ratio of the known flux in the standard pile to the acti-vation of the foil irtien placed at that point in the pile. If a thin absorbing material is placed in the calibrated field, the activation resulting from the known flux must be calculated using a cross section b probable energy if the absorber ha. a i 1.128 • given by *"** °^°^ J^"*^^^* ^""^^ if the absorber ha. a i cross 7. H. Pcmerance, Phys. Rev. 8^, 6kl (1951)• ^-^7 8 AECD-3590 section, or by averaging the known cross section over the assumed Max-vellian distribution if the 1 lav does not hold. V If one vlshes to determine the flux vlth a thick foil in a medium vhlch has a diffusion length different from that of the stand-ard pile, the perturbation by the foil of the fluxes In the pile and in the medliua in question vlll be different. A correction for this effect should be made using the Bothe or the Skyrme theory. It is a pleasure to acknovledge our Indebtedness to Dr. A. R. Brosl of the ORNL Chemistry Division for his aid in the ^-7 coinci-dence and the high-pressure ion chamber measurements. We vlsh to thank Mr. David G. Ott of the Oak Ridge School of Reactor Technology, irtio made the preliminary measurements, and Mrs. Elizabeth Johnson, who assisted in carrying out the final meas\irements. ^2-7 -p 

Recalibration of the X-10 Standard Graphite Pile

Recalibration of the X-10 Standard Graphite Pile

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