Backscattering of alpha particles from thick metal backings as a function of atomic weight

International Journal of Applied Radiation and Isotopes, 1968, Vol. 19, pp. 517-522. Pergamon Press. Printed in Northern Ireland

Backscattering of Alpha Particles from Thick Metal Backings as a Function of Atomic Weight j . M. R. H U T C H I N S O N , C. R. NASS, D. H. W A L K E R and W. B. M A N N National Bureau of Standards, Washington, D.C. Alpha-particle backscattering from thick metal backings has been studied using two separate counters with geometries of 2,r and approximately l~r steradians. LA RI~TRO-DISPERSION DES PARTICULES ALPHA DES ADOSSEMENTS DE MI~TAL EPAIS COMME FONCTION DU POIDS ATOMIQUE On a ~tudi* la r~tro-dispersion des particules alpha des adossements de mdtal ~pais en employant deux compteurs s~par~s ayant des gdom~tries de 2 ~r et d'environ 1rr st~radians. OBPATHOE PACCEHHIIE AJII~(I)A-tIACTHI~ H3 TOJICTbIX METAJIJIHqECHHX IIO~KJIA~(OK HAH (I)YHHIIHH ATOMHOFO BECA I/IsyqaJiocb o6paTnoo pacceHHHe am,$a-qacT~q ~t8 TO:mT~X M~ra~a~qecRax no~K~a~o~ IIpHMeHHH ~Ba OT~eJII~HI~.-XctleTqHHa (3 reoMeTp~el~ 2~r H IIpH6~IHI~HTegIbHO1~" cTepa~HaHI~I. RUCKSTREUUNG VON ALPHATEILCHEN AUS DICKEN METALLBELAGE ALS FU~NKTION DES ATOMGEWICHTES Die Alphateilchenrtickstreuung aus dicken Metallbel~igen wurde unter Benutzung ,con zwel getrennten Z~]llem mit einer Geometrie yon 2~r und yon etwa 17r Steradian untersucht. 1. I N T R O D U C T I O N of x-particles at small angles to the metal surface. (Although the counter is referred to VERY little theoretical or experimental work related to x-particle backscattering from thick as a " l r r x " counter, the solid angle subtended metal backings, on which thin sources are from the source position is 27r/2"531 steradians, based on the dimensions of the counter.) deposited, has appeared in the literature. The National Bureau of Standards has, for From the standpoint of radioactive source calibration, such studies are useful for relating x- nearly 20 years, been issuing such solid aparticle emission rates including backscattering particle standards which have been calibrated into a 2~r geometry, to the total disintegration in a 2rr proportional counter. It was realized rate of the s o u r c e . JAFFEY (1) and W A L K E R (l) that the calibrations of such sources, on monel, have shown that backscattered x-particles come should be corrected by about 2 percent for off predominantly at small angles with respect backscattering, for the radionuclides used, to to the plane of the source. The tacit acceptance give the actual disintegration rates, although of this fact led to the development in 1960 of the effect might also have been slightly comthe "lTrx" counter by ROBINSON(a), which was pensated for by some self-absorption in the designed for the purpose of making absolute solids of the source. Some two years ago it was decided to develop calibrations of thin solid x-particle sources, on metal backings, which measurements would a " l r t x " counter, following the design of be free from error due to the backscattering ROBINSON(a~, with some modifications, in order 517

518

J. M. R. Hutchinson, C. R. Naas, D. H. Walker and IV. B. Mann

to improve the accuracy of the alpha-particle standards issued by the National Bureau of Standards. This brief paper is to report on the design of that counter, and also to describe some comparative measurements carried out with the NBS "Irr0~" and 21r0~ counters. These measurements give directly the magnitude of the backscattering correction which users of the older standards may wish to incorporate into their calibrations, and also a measured value for the geometrical efficiency of the counter. The comparative measurements consisted of counting a group of sources on various source mount materials first in one counter and then in the other and determining the ratio of the rates for each source. The results can be analysed as follows. T h e number of ~-particles emitted into the 2rr0c counter, N2~, is composed of those directly emitted, N0/2 , where N 0 is the total ~-emission rate of the source, and those backscattered from the source backing (N0/2)B, where B is the probability that if a particle is emitted downward it will be backscattered. T h e number of 0~-particles detected in the " l r r ~ " counter is ff~. Thus the ratio of the counting rates into the 2rr0t to the "lrr0~" counter is:

(a) 2~r counter TEMMER and W Y G K O F F (4)and W A L K E R (s)have described the 2~r counter. It is a gas-flow proportional counter with hemispherical inner wall with a 4-in. radius. T h e counting gas is 90 percent argon, I0 percent methane mixture. Sources wcrc placed in the counter, and discrimination plateaus were taken. (b) " l i r a " counter

The " l r r ~ " counter, shown schematically in Fig. 1, is essentially of th• same design and dimensions as described by ROE~SON (8~. Instead of zinc sulphide, however, a 0.020 in.thick disc of plastic scintillator was mounted on a 5 in.-dia, elcctronmultiplicr phototube, which was positioned approximately 2 in. from the source. As in Robinson's counter, a baffle was located in the center of the disc scintillator and was of such a shape and size that small inaccuracies in source positioning cause negligible changes in the solid angle subtended by the detector. The two important modifications that have been made are (1) the use of the plastic scintillator in the place of zinc sulphide for 0c-particle detection; and (2 / instead of evacuating the chamber, the air is displaced by hydrogen gas. T h e advantage of the plastic 2V•Ir at. - x(1 + . ) (1) scintillator is that it is easy to mount, although it was necessary to use optical coupling glue where K is the ratio of the geometrical efficiency (R313) because, when silicone grease was of the 2tr to the "l~'0d' counter. B depends used, small bubbles were found after a period upon t h e backing material and the initial of several weeks, between the detector and energy of the emitted 0~-particle. phototube. The disadvantages, compared to In the experiment described here, measure- the zinc sulphide screen, which were unimments were performed for ten different backing portant in our application, are increased gammamaterials. Using equation (1), K and B were ray and fl-particle efficiency, and lower light determined by means of a least squares fit. o u t p u t . The advantage of flowing hydrogen It was also found, however, that the effect of gas through the system is that it is at all times the backing material on 0~-particle backscattering at atmospheric pressure. T h e expense and was critically dependent on whether the ~- complications associated with a v a c u u m system particle source material, the chloride of arc thus avoidcd. A drawback of the hydrogcn polonium-210, was adsorbed on to a polished system is that there is approximately a I - M c V surface or whether the source was held in a energy lossfor some alpha-particlesin traversing collodion film deposited on the surface. the hydrogen, which causes a reduction in pulse height and a shortening of the plateau. A thin 2. E X P E R I M E N T A L ARRANGEMENT but optically reflectinglayer of aluminium was AND PROGEDURE evaporated onto the surface of the detector to T h e two counters used in the experiment prevent detection of unwanted scintillations from the gas. were:

Backscattering

of alpha #articles from

thick metal backings as

a function

519

of atomic weight

.YLINDRICAL 3iAMBER

PHOTOTUBE

PLASTIC SCINTILLATOR

FIG. 1.

Schematic diagram of “1 na” counter.

When these modifications were made, discrimination plateaus with slopes less than O-1 percent from 30-70 V in the amplifier output which considered were obtained, were satisfactory. The count rates, as functions of vertical and horizontal displacements, respectively, of the source mount are shown in Figs. 2 and 3. 290 .

.**

.’

.

26qr I

0

2

HORIZONTAL

4

6

DISPLACEMENT,

6

D

mm

FIG. 3. Count rate versus horizontal displacement.

2s08 0

VERTICAL

FIG, 2.

2 DISPLACEMENT,

mm

Count rate versus vertical displacement.

3. EXPERIMENTAL RESULTS AND DISCUSSION Data have been obtained fkom two groups of sources, namely, (1) those adsorbed on to unpolished and highly polished backings, and

520

J. M. R. Hutchinson, C. R. Nass, D. H. Walker and W. B. Mann

T~a3r ~ 1. Ratio of counting rate in 2 ~r counter to rate in "1 ~r~" counter for different mounting materials Source mount Beryllium Aluminum Stainless steel Iron Nickel Monel Brass Copper Palladium Silver Cadmium Platinum Gold

Effective atomic mass number A

Po 2x° on unpolished mount

Po 210 on polished mount

Po sa° in collodion on polished mount

9.02 26.98 55-27 55.84 58.71 60.11 63.84 63-57 106.7 107-88 112.41 195.09 197.0

2.453 2.532 2.548 2-555 2.563 2"579 -2"454 2.523 2"582 -2.617 2.606

2.521 2.520 --2-560 2.583 -2"582 -2-585 2.572 2.621 2.604

2.522 2-545 2.555 --2.570 2"562 2.557 -2"587 2-582 2.637 2.623

2.6~

L~J

I-2.6(

RATIO OF GEOMETRICAL

0 I-

~ •

EFFICIENCIES

EXPERIMENTAL POINTS CRAWFORD'S CALCULATION

2.55

-

Ll0 0 I-2.5(

0

I

20

T

I

40

60

I

80

I00

z

FIo. 4. Counting rate in 2~r counter divided by counting rate in "l~ra" counter for Po m° sources impregnated in collodion on polished mounts plotted against Z. Points from Crawford's calettlation, indicated by O, are seen to agree with the experimental points. (2) sources consisting o f a c o l lo d i o n solution o f p o l o n l u m - 2 1 0 d e p o s i te d in a thin la y e r o n to h i g h l y polished m e t a l backings. T h e results for the t wo groups a r e g i v e n in T a b l e 1. I n Fig. 4, values o f N 2 , / N ~ for g r o u p (2), a r e p l o t t e d as a function o f Z. A least squares fit was m a d e to the points using e q u a t i o n 1,

w h e r e B is assumed to h a v e the f o r m : B = blZ

(2)

b 1 is a constant. V a l u e s o f K a n d b x w e r e d e t e r m i n e d b y m e a n s o f the fit to be 2-525 q0-002 a n d (1.30 -4- 0.08) 10 -3 respectively. T h e q u o t e d errors c o r r e s p o n d to the e s t i m a t e d

Backscattering of alpha partides from thick metal backings as a function of atomic weight 2.65

I

J

I

521

1 0

LO

2.6C


0 t--

2.5E

1= (.q LL

0 0 I-

0

0

2.5(3 - -

0 POLISHED & UNPOLISHED

2.4

.,~ 0

I

20

"

I

I

40

60

I

80

z

FIo. 5. Counting rate in 2 ~r counter divided by the counting rate in the"l ~rot" counter for Po~x°sources adsorbed on to polished and unpolished mounts vs. Z. The solid line indicates, for comparison only, the least squares fit to the points in Fig. 4. standard errors (c.£ Ref. 6). T h e fit is shown in Fig. 4 where the points have been weighted equally. T h e error bars on the points correspond to one standard deviation. T h e values for the effective atomic n u m b e r for the three measured alloys were calculated using Zeffeetlve= Y~n~Zi, where n~ are the atomic fractions of the elements whose atomic numbers are Z~. T h e only theoretical calculation in the literature of B is by C R . A W F O R D (8), who derived the following relationship :

rXn,A ~ ] 113 B = b,

LI~n,AI/~A

(3)

where b~ is a constant and A~ are the atomic weights of the constituents. Values of N'a,/N"I obtained using Crawford's theory, in which his calculated values of B have been multiplied b y 1.05, are seen to agree with the data.

I n Fig. 5 values of Na,/N, are plotted against Z for polished and unpolished source mounts. Sources on unpolished metal backings show a very large scatter. T h e latter result is probably to be expected because absorption of 0c-particles emitted at grazing incidence m a y be haphazardly absorbed in the minute craters of an irregular surface. T h e results from the polished surfaces without collodion show an unexpectedly large spread. I t is possible that, even in polished surfaces, significant irregularities still exist so that by putting the polonium chloride in a collodion film which remains just above the surface, the result, seen in Fig. 4, could be to smooth out the effects of the irregularities. I t is clear that more research is needed to clarify the problem of surface effects, but a least squares extrapolation obtained from the results in Fig. 4 is sufficient to justify confidence in the calculated geometrical efficiency of the new National Bureau of Standards "1 rr0(' counter.

5 22

J . M . R . Hutchinson, C. R. Nass, D. H. Walker and W. B. Mann CONCLUSIONS

(i) A new "1~'0(' counter has been constructed using a plastic scintillator a n d a h y d r o g e n gas atmosphere which gives reproducible results on thin sources to a precision of -4-0.1 percent. (ii) T h e a c c u r a c y o f the geometrical factor for the new counter has been checked to 4-0.3 percent using a procedure o f extrapolating N 2 , / N , to Z = 0. (iii) I t is d e d u c e d that small angle backscattering from thick metal backings is pred o m i n a n t l y a multiple scattering p h e n o m e n o n . T h e results shown in Fig. 4 show that the d a t a are consistent with Crawford's calculation both as to functional dependence of B on A a n d the m a g n i t u d e o f B. (iv) Surface effects c a n n o t b e r e m o v e d merely b y electroplating on to polished surfaces as d e m o n s t r a t e d b y Fig. 5. (v) Measurements using unpolished mounts show that, in some cases, surface effects are as large as backscattering effects. (vi) Values of K and b I were determined.

Acknowledgement--The authors wish to thank Dr. H. P. ROBINSONof the Lawrence Radiation Laboratory of the University of California for providing drawings of his " 1 ~ " counter, and also for carrying out intercomparative measurements on a polonium-210 source without collodion on polished monel, which agreed with ours to within 0.3 percent.

REFERENCES 1. JAFFEY A. H. private communication. 2. WALKERD. H. Int. d. appl. Radiat. Isotopes 16, 183 (1965). 3. ROBINSON H. P. The Metrology of Radionuclides, p. 147, International Atomic Energy Agency, Vienna (1960). 4. TEMMERG. M. and WYCKOFFJ. M. Phys. Rev. 9'2, 913 (1953). 5. C ~ w m ~ J. A. Theoretical Calculations Concerning Back-scattering of Alpha Particles, edited by GLENN T. SEABORO et al. in The Transuranium Elements, National Nuclear Energy Series, Manhattan Project Technical Section, Division IV-Plutonium Project Record, Vol. 14. Part II, p. 1307, McGraw-Hill, New York (1949). 6. Ku H. H. Statistical Concepts in Metrology, in American Society of Tool and Manufacturing Engineers, Handbook of Industrial Metrology, p. 53, Prentice-Hall, New York (1967).