Angular distributions of photoelectrons ejected from uranium by 1.118 MeV gamma rays

13.A-,3 .B

Nuclear Physics 22 (1961) 306---315; © North-Holland Publishing Co., Amsterdam permission from the publisher Not to be reproduced by photoprint or microfilm without written

ANGULAR DISTRIBUTIONS OF PHOTOELECTRONS EJECTED FROM URANIUM BY 1 .118 MeV GAMMA RAYS SOLVE HULTBERG t, TOM O'CONNOR and JOSEPH H. HAMILTON Physics Department, Vanderbilt Uaiversity, Nashville, Tennessee tt Received 12 September 1960 Abstract: At a gamma-ray energy of 1118 keV, we have studied longitudinal angular distributions of photoelectrons ejected from the K-, L-, and M+N+ . . .-shells of uranium . The application of these angular functions to the calculation of the photoelectric correction factor f is discussed . It is important to obtain / at an energy such as 1 .118 MeV to increase the accuracy, around 1 MeV, in the curve of / versus energy . A smooth-curve relationship, in a log-log diagram, has been found between the gamma-ray energy by and the angle 9 m of maximum photoelectric cross-section . It is pointed out that the Swedish computer BESK now undertakes calculations of / at four gamma-ray energies : 412, 662, 1118 and 1332 keV (assuming rectangular symmetry and taking source size into account) . This new calculation increases the accuracy of / values such that the overall accuracy in / between 400 and 1400 keV is now considered to be of the order of 1 % .

1 . Introduction An extended knowledge of photoelectric angular distributions has proved to be of particular importance for the determination of absolute values of internal conversion coefficients according to the internal-external conversion method of Hultberg and Stockendal 1) . Investigations of such angular functions is also of interest from a theoretical point of view since in this way detailed information is obtained about the nature of the photoeffect . For instance, it is important to establish as accurately as possible the ratio r&/ZK between the integrated photoelectric cross-section of the whole atom (ta) and that of the K-shell (rK) 1) . This ratio makes possible an accurate comparison of the theoretical tK and the ZK as derived from absorption measurements which give is as the primary information. In the deduction of gamma-ray intensities from an analysis of photolines the so-called photoelectric correction factor f has been introduced 1) . A study of the behavior of f as a function of energy reveals that, in many cases, f will go through a maximum at an energy in the neighborhood of 1 MeV . To increase the accuracy of the interpolatifm around 1 MeV of the curve off versus energy we studied the photoelectric angular distributions of the uranium K-, L-, and t On leave from the Nobel Institute of Physics, Stockholm, Sweden . tt Work supported in part by a grant from the National Science Foundation . 306

ANGULAR DISTRIBUTIONS OF PHOTOELECTRONS

307

M+N+ . . .-shells at a gamma-ray energy of 1 .118 MeV. The gamma-rays were obtained from the decay of Sc4 g. 2. Experimental Arrangements and Results This work was carried out with the Vanderbilt iron-free, double-focusing spectrometer 3) . A side window, continuous flow, G. M. counter was used as the detector. Special lead shielding was used to minimize the background in the counter from direct radiation.

TOP VI EW Fig. 1. Experimental arrangement inside iron-free double-focusing spectrometer for the study of photoelectric angular distributions . By turning a dial outside the spectrometer, the gamma-ray source can be rotated around the converter.

The experimental arrangement used with the spectrometer is shown in figs. l and 2. The gamma-ray source consisted of 25M9 of Sc203, packed into a 2 x 2 mm cylindrical cavity of an aluminium container and irradiated for two days with a neutron flux of 2 x 1014 neutrons cm -2 sec-1 at Oak Ridge National Laboratory . The source activity was about 230 mc. Tae distance a between the source and the converter was 25 mm . By means of a dial (see fig. 2) outside the spectro-

308

SôLVL SVLTHERG et ta.

meter, the source could be rotated around the converter which remained fixed. The position of the gamma-ray source with respect to the converter is defined by the angle 8, according to the schematic diagram of fig. 3. o"IAL

Fig. 2. Side view of source rod which is removable from the spectrometer . The Duodial, source converter and worm gear are indicated.

Aperture (baffle) contre From R .__r 1d _- _ . _, T - - ='.~ S "axis" Spectrometer a

m

Fig. 3. Schematic diagram, defining the position of the gamma-ray source with respect to the normal through the centre of the converter. The normal is defined as B = 0.

The gear arrangement shown in fig. 1 was such that 6 changed by 360° when the dial was rotated through exactly 40 turns, each of which thus corresponded to a variation in 8 of 9°. The circumference of the dial had 100 subdivisions so that 0 could easily be set with an accuracy of 0.09° which is far

ANGULAR DISTRIBUTIONS OF PHOTOELECTRONS 1118K

3000

309

ANGLE O° 2=92

2000 C/M 1000 0 Lr..

8200 83,00 8400 600

. 8600 8700 8800 8900 9000 9100 9200 POTENTIOMETER SETTING

r 9300 9400

Fig. 4. Photoline spectrum, as recorded at 0 = 0. The gamma-ray energy is by = 1118 keV . A circular uranium converter was used (diameter 14.0 mm). Resolution about 1 %.

TABLE 1 values of the photoelectric angular functions, per unit solid angle, of photoelectrons Numerical K-, L-, M+N+ . . .-shells at by = 1118 keV . Uncorrected experifrom the uranium and ejected mental functions Je are given (cf . Discussion) 0°

K-shell

L-shell

M -i-N -}- . . .-shell

3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 55 60 65 70 75 80 85 90 100

740 738 735 720 700 665 6,20 570 515 460 405 345 230 240 205 170 140 120 95 68 50 35 22 14 6 2 0

181 181 178 173 164 154 140 125 110 95 82 70 59 49 39 32 25 20 15 9 5 1 0

62 62 61 61 60 58 56 53 48 42 3ti 30 23 18 13 10 7 3 1 0

0

100

,I~

50

100

150

200

250

o ri ï96, 30° 60° 0 30 60

1

1118 !t

1

30°

«Î II~

1 °

1118 L

'

961

o

50

100

0°

1 °

--ir

°

-

1118 M+N . . .. .

Fig. 5. Uncorrected experimental angular distributions Je of photoelectrons ejected at by = 1118 keV. (A) from the uranium K-shell, (B) from the L-shell, and (C) from the M+N+ . . .-shells. Estimated uncertainties in the individual measurements are indicated . Ordinates represent intensities per unit solid angle (fixed setting of spectrometer aperture) .

0°

200 -I

300 -i

400 -I

300-

600-

700

800

ANGULAR DISTRIBUTIONS OF PHOTOELECTRONS

better than required for these experiments. During the measurements the dial was always rotated in only one direction to avoid possible uncertainties from any small play in the gear. ' 1, The experiments were carried out in a manner similar to previous works 2) . For every setting of the angle B, a complete spectrum of K-, L-, and M+N+ . . . photolines was recorded . The L- and M+N+ . . . photolines were no longer recorded after their intensities were found to be indistinguishable from background. A typical spectrum is shown in fig. 4 for B --.- 0°. We used a circular uranium converter (diameter 14.0 mm) with a thickness of 3.9 mg/em2. The continuum around the peaks was carefully recorded to allow an accurate interpolation of the background under the peaks. Because of the relatively poor resolution (about 1 %), the L and M-a- N-}- . . . lines were not completely resolved. With the help of the peak heights which could be reasonably well estimated, it was possible, however, to obtain the L and M--}-N+ contributions separately . By plotting the photoline areas (per unit momentum interval) versus 0, photoelectric angular distributions were obtained for the uranium K-, L-, and M+N+ . . .-shells, as given in fig. 5. In fig. 5 we have indicated errors that represent estimated uncertainties in the photoline areas (5 % and up) . The angular functions J,,,(0) of fig. 5 represent intensities per unit solid angle_ (spectrometer aperture fixed) and they are presented numerically in table 1 . It should be observed that the K, L, and M+N+ . . . functions of table 1 are mutually correct on a relative scale so that K/L, K/ (M+N+ . . .), etc. ratios of photolines and of total absorptions in the different shells can be formed. The. latter ratios (rl/r2) are given by eq . (3) and the former ratios can be written (ref. 1)) : 'ri fi r (Ay )2 'r2 f2 where A is the photoline intensity and the subscripts 1, 2 refer to specific shells (K, L or M+N . . . ) .

3. Discussion

In fig. i we have obtained certain angular func'ions by plotting the intensities sense of of three groups of photolines versus the angle 8 which we use in the the fig. 3 to describe the emission direction of a photoelectron with regard to - ,he notat on incidence direction. of the gamma ray. We have already introduced serves to stress the J,.(0) for these angular functions where the subscript e angular functions experimental nature. Theoretically, it is possible to calculate J(e) which represent the probability per unit solid angle that a photoelectron

31 2

SÖLVE HULTBERG Bt

al.

be emitted in the direction ®, if we define the incidence direction of the gamma ray as 9 = 0. We then pose the following question . What is the relation between the functions Je (8) and J (0)? From fig. 3 it is clearly understood that Je and J cannot be identical. This follows from the fact that obviously our converter has to have a certain lateral extension (i.e. a finite radius) and likewise a finite thickness . Both these facts mean that we cannot, in our experiments, define the relative angle 8 sharply. The converter radius introduces a "geometric" distortion 1) since our measurements only give a type of mean value of J(e) inside an angular interval (0-d1 6, 6-+ 2 6) where d16, d20 are given by the maximum displacements from the converter centre O of the interaction point R (cf. fig. 3) . Obviously, this kind of diffuseness in 6 has a maximum at 6 --= 0 and a minimum at B = 90°. The thickness of the converter manifests itself by scattering of the photoelectrons so that they will emerge from the converter surface in' directions that will differ in general from the initial directions . This introduces a further confusion of angles and we may define it as the "scattering" distortion 1) . Both the geometric and the scattering distortions tend to smooth out (rapid) variations in the angular functions so that differences between minima and maxima will be less pronounced in Je(6) than in J(6) . The degree of this smoothing-out effect will be strongly dependent on the particular choice of the parameters c and d (cf. fig. 3) and quite naturally we will have Je -+ J as c and d tend toward zero. To infer J(6) from Je (6) we might then undertake special experiments where the behavior of J. (O) is studied as a function of c and d which would then permit an extrapolation to c = d = 0 1). Such a procedure is necessary for carrying out a detailed comparison with theory. In the present case, however, we did not undertake such "extrapolation" experiments for the following reasons. No theoretical predictions are yet available for heavy elements. It turns out that the correction factorf 1) is very insensitive (to within about 1 % in most cases; see also ref. 4)) to the exact values of c and d that characterize a particular angular distribution. This means that we can compute the desired f factors to a satisfactory accuracy without having to find the angular distribution that would correspond to c = 0 and to the particular d value used in the experiment . Let us assume that the converter is thin enough so that no photoelectrons are absorbed inside the converter material. Such an assumption is justified here since no appreciable broadening of the measured photolines was observed . Then we can write, according to ref. 1) fô Je (8) sin 0 d8 = K f"J(0) sin 8 d8,

(2)

where K is a constant during the measurements . Essentially, eq. (2) states that our measured function Je is merely a redistribution (distortion) of J, apart from the constant K . Using the definition of the integrated photoelectric

ANGULAR DISTRIBUTIONS OF PHOTOELECTRONS

cross-section r of a specific shell, we obtain for the ratio sections of shells 1 and 2 1) Tl

[fÔfe(0)

[ fOJe(8)

T2

313 r1/Z2

of such cross-

sin 0 dO] 1 Sin

0 dO]2

Evaluating (3) from table 1, we obtain at by = 1118 keV : i-- = 5.3A=0.3 ;

ix

= 14 .0±0.9;

TÉ zK

TÉ

== 2.7±0 .2 ;

rf f+lv+ . . .

Ta

= 1 .26~O.OI .

These values are in good agreement with those found earlier at three other energies') . It is interesting to notice that, at high energies, Pratt s) finds theoretically zx/zL = 5.3 and tL/zM+x+ . . . = 2 .7 for uranium. This confirms that the photoelectric absorption ratios are remarkably independent of energy. 100;

.r~=.... ....... .

S

80

as

60 40

em

20

mm 3081I011N1111""f" M0011111111AP11

10° 8

6 ITT1TiTIII

4

2

1

0

~nm um~uiu~~_

0.1

0.2

0.4

0.60.8 1

2

rumnm 4

6

11111

810

h Y in MeV

Fig. 6. The angle em of maximum K-shell photoelectric emission from uranium, plotted as function refs . 1 . e .') . of the energy by of the incident photons. The points are experimental and are taken from

From the .%e(0) functions of fig. 5, we cannot easily determine the -angle infer 0,n(1 11 '` from 0m of maximum photoele, ,tric emission . We can, however, fig. 6 which shows 0m as a function of the gamma-ray energy in a log-log dia-

31 4

SÖLVE HVLTBERG

et al.

gram. These data in fig. 6 are fitted nicely by the smooth curve shown. This curve makes accurate predictions at other energies rather easy. This simple relationship between 8m and by has not been demonstrated before. The experimental information for fig. 6 has been obtained from refs. z, 6, 7) . At the threshold energy Nagel g) finds for the K shell that 0. = 97°. This is seen to be consistent with an eictrapolation of the curve of fig. 6 which yields Om = 970±30 at by = 116 keV (the K-shell threshold of uranium) . From the latter diagram we obtain 8.(1118) = 12.7°.

0, 00

400

____-_--------

keV -

1800

11200

Fig. 7. Typical f values calculated for 3 x 16 mm$ source with a 5 x 21 mm$ converter which were used in the Vanderbilt spectrometer in the study of conversion coefficients o) . The small dashed curve is a possible extrapolation of the f values made without the point at 1118 keV obtained from the present work.

It should be pointed out that the functions of table 1 are now available for f-factor calculations at the Swedish electronic computer BESK 2) so that f can be obtained from BESK at the four gamma-ray energies by = 412, 662, 1118 and 1332 keV and for three shells (K, L and M-}-N+ . . .) at each energy. The calculations at 1118 keV help to increase the general accuracy in the curve of f versus energy, as can be appreciated from fig. 7 which illustrates a particular case. The overall accuracy in f may be taken to be of the order of 1 % in the energy region 400-1400 keV . It should be emphasized, however, that to obtain this accuracy each of the parameters entering the calculation of f must be carefully determined from the experimental set-up.

ANGULAR DISTRIBUTIONS OF PHOTOELECTRONS

315

In the near future fls9 x, L, LI+LII , L,,, and f208 x will be available g) (subscripts indicate energy in keV and shells) . All this applies to uranium as converter material. However, the Z dependence of f is probablyvery weak, at least above Z = 78 1°). Application forms for f calculations can be obtained from the Nobel Institute of Physics, Stockholm 50, Sweden. References 1) S. Hultberg and R. Stockendal, Arkiv för fysik 14 (1959) 565; S. Hultberg, Arkiv för fysik 15 (1959) 307 2) S. Hultberg and 7.. Sujkowski, Phys. Rev. Letters 3 (1959) 227 3) Q. L. Baird, Ph. D. Thesis, Vanderbilt University (1958) ; J . C. Nall, Ph. D. Thesis, Vanderbilt University (1958); J . C. Nall, Q. L. Baird and S. K. HaSnes, Phys. Rev. 118 (1960) 1278 4) R. Stockendal, Arkiv för fysik 17 (19C',) 579 5) R. H. Pratt, Phys. Rev . 119 (1960) 1619 6) J . H. Hamilton, S. F. Frey and S. Hultberg, Nuclear Physics (to be published) 7) Z. Sujkowski (private communication) 8) B. Nagel, Arkiv för fysik 18 (1960) 29 9) W. F. Frey, J . H. Hamilton and S. Hultberg, Nuclear Physics (to be published) 10) C. de Vries, Ph. D. Thesis, Amsterdam (1960) ; parts are published in Nuclear Physics 1.8 (1960) 428, 446, 454