When assaying a mixture of radioisotopes, the Compton distribution from higher energy gamma rays must be “peeled” off or subtracted from the observed photopeak. In counting radioisotopes which have more than one gamma ray in coincidence, it is much more accurate to integrate the counts under the sum-peak and to use the product of the efficiencies in determining the absolute disintegration rate. Many features of the decay scheme must be taken into consideration in converting the photopeak count rate to absolute disintegration rate. These include coincident gamma rays, how the j3 (or E.C.) branching populates the various gamma levels, and the d e gree of electron conversion. Calibration of Positions Above the 3 X 3 Inch Well-Type and Solid Crystals. Known arbitrary positions above the crystals were calibrated using the same techniques as were used in the well crystal. d correlation of the photopeak countr ing efficiency could be made with crystal-to-source distance using the following equation : E , = ep.Jo.d
where : E, Jg
= = =
6
=
tp
photopeak detection efficiency intrinsic photopeak efficiency fraction of the total number of gamma rays incident on the crystal (i.e., geometry) the deviation of the intrinsic peak efficiency from that expected from purely geometric considerations
For a 3 X 3 inch crystal, the geometry f o may be calculated by the equation:
fP
=
d
m d 2d14.5 dz
+
where d is the distance from the crystal in centimeters. The intrinsic photopeak efficiency t p is the probability a gamma ray will produce a pulse under the photopeak if it strikes the crystal. These efficiencies are shown as a function of gamma ray energy on Figure 1. The upper curve is for the 3 X 3 inch solid cryatal. The lower curve, which is the efficiency for counting in a 3 X 3 inch well crystal, surprisingly also corresponds closely t o the intrinsic peak efficiency for the 3 X 3 inch well-type crystal. To determine the efficiency for any point along the axis of these
I
IO04
0.70‘
,
,
, ,
2
4
,
,
,
6
, 8
,
,
,
1
1 0 1 2
Distance from Crystal Surface (cm.)
Figure 2. Variation of intrinsic photopeak efficiency with distance from surface (a). 3 X 3 inch well-type crystal. (b). 3 X 3 inch solid crystal. Variation i s normalized at zero distance from crystal face
crystals, one must multiply the intrinsic peak efficiency ep by the geometry subtended by the crystal with respect to the sample f o and by 6, which is the deviation of the intrinsic peak efficiency from that expected from just a geometry consideration. The variation of 6 with distance is shown in Figure 2. This parameter arises from the fact that the effective geometry of the crystal does not vary in the same way as the calculated geometry. As is seen in Figure 2, 6 passes through a minimum a t a distance of about 3 cm. from the crystal face for 3 X 3 inch crystals. This effect may be qualitatively accounted for as follows for a 3 X 3 inch solid crystal: Gamma rays from a sample placed at the surface of the crystal or a t a large distance from it pass through minimum crystal distances of l l / z and 3 inches, respectively. This is to be compared with a sample placed a t a small distance from the crystal (e.g., 3 cm.) where the minimum crystal distance through which a gamma ray may pass approaches zero inch. Therefore, the effective geometry in the latter instance should be thought of as being less than the calculated geometry using the intrinsic peak efficiency a t zero distance from the crystal as the point of reference. In assaying a sample external to the crystal it is very important to determine the crystal-to-source distance accurately because of the sensitivity with which the photopeak efficiency varies with distance. Since variation of photopeak efficiency with distance may be as high $8 574 per millimeter, it is frequently difficult to obtain accuracies In addition angular better than *5%. correlation of coincident gamma rays may introduce errors in the efficiency with which the sum-peak is counted.
Although calibrated crystals such as those just discussed are secondary standards, they have the distinct advantage of being reproducible in form and size, and consequently are reproducible in efficiency. They may be used generally for gamma-emitting isotopes, many of which cannot be assayed accurately by other means. The accuracy (*3 to 5%) with which the data are presented should make the 3 X 3 inch well- and solid-type NaI crystal useful in many applications where quantitative radioisotope assays are important, LITERATURE CITED
( 1 ) Champion, P. J., Intem. J . A p p l .
Radaation Isotopes 4,232 (1959). (2) Colby, L. J., Jr., Cobble, J. W., ANAL.CHEM.31,798 (1959). (3) Davis, R. C., Bell, P. R., Lazar, N. H., I R E Trans. on Nuclear Sei. NS-3, No. 4 , 8 2 (1956). (4) Gunnink, R. Colby, L. J., Jr., Cobble, J. w., ANAL.&EM. 31,796 (1959). (5) Heath, R. L., IDO-Rept. 1640S, (1957). (6) Pate, B. D., Yaffe, L., Can. J. Chem. 33,15,610,929, 1656 (1955). (7) Seliger, N. H., Schwebel, A., Nucleonics 12, 54 (1954). (8) Vegors, Stanley N. Jr., Msrsden,
Louis L., Heath, R. L., IDO-Rept.
16370 (1958). (9) Wolicki, E. A., Sastrow, R., Brooks, F., N R L Rept. 4833 (1956).
RECEIVEDfor review March 22, 1961. Acce ted June 22, 1961. Work supported by S. Air Force Office of Scientific Research under Contract 49(638)-815. Contribution No. 205 from United States Rubber Co., Research Center, Wayne, N. J.
6.
Correcf ion Infrared Studies of Some New Polycyclic Hydrocarbons and Their Derivatives Containing the Cyclopropyl Ring In this article by S. A. Liebnian and B. J. Gudzinowicz [ANAL.b ~ 33, . 931 (19Sl)], on page 933, Table I, compound I11 should be 6’,6‘- dimethylspiro [cyclopropane - 1,2’ - norpinane] instead of 6‘,6’,dimethylspiro [cyclopropane-l,2,-norpinane].
