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Measurements on the continuous beta-spectra, the conversion lines and the auger-lines of U(X1 + X2)
Stoker, P. H. Heerschap, M. Ong Ping Hok
Physica
XIX
433-444
1953
MEASUREMENTS ON THE CONTINUOUS BETA-SPECTRA, THE CONVERSION LINES AND THE AUGER-LINES OF U(X, +X2) by P. H. STOKER Communication
*), M. HEERSCHAP
from the Natuurkundig Laboratorium Amsterdam, Nederland
t) and ONG PING HOK van de Vrije Universiteit,
Synopsis Measurements on the p-spectra of U(X, + X,) in a 16.2 cm doublefocusing spectrometer, (resolution of 1.5%) are described. Of the electronlines 25 could be ascribed to 11 y-rays, while the lines below 21 keV could be interpreted as L-Auger lines on account of energy and intensity considerations. The Fermi-Kurie analysis of the continuous p-spectra gave 5 components with endpoints at 2305, 1500 and 580 keV for UX, and at 193 and 103 keV for UX,. The relative intensities were respectively 90%, 9% and 1 y0 for UX, and 66% and 34% for UX,. The 91 keV y-ray of UX, is E2, while the 8 10 keV y-ray of UX, may be Ml. A tentative disintegration scheme with spin and parity assignments to some of the levels is discussed.
1. I&roductiolz. In a previous paper ‘) measurements on the p-spectrum of U(X, + X,) with a thin mica-foil as source carrier and performed in a semi-circular spectrometer were described. Since backscattering against the mica-foil distorted the low energy region of the spectrum, we now used a thin film of plastic material “) and remeasured the spectrum not only in the semi-circular but also in a 16.2 cm double-focusing spectrometer “) at a resolution of 1.5 Oh. A classification of the electron lines found, as internal conversion and Auger lines is given in section 4. After a Fermi-Kurie analysis (section 5) of the continuous spectrum, a tentative decay scheme is discussed in section 6. *) Now at the Potchefstroom University for C.H.E., Potchefstroom, South-Africa. t) Now at the Nederlandse Springstoffenfabriek, Muiden, Nederland.
Physica XIX
433 20
434
P. H. STOKER,
M. HEERSCHAP
AND ONG PING HOK
2. Preparation. of the source. The UX activity was extracted from some kilograms of uranyl-nitrate, by means of an ether-water separation, followed by a passage through an ion-exchange column (wofatit KS.). It was then dissolved in concentrated HCl and brought on a pep film 2), using insulin to define the source area. After it had been dried in vacuum, a formvar film, upon which a thin layer of Al had been evaporated, was placed against the backside of the foil to make it conductive. The total weight of the source backing was less than 0.1 mg/cm’. The mean thickness of the active material was 0.3 mg/cm2, mainly due to FeCl, from the wofatit. 3. The measurements of the &ybectra. The p-spectra of U(X, + X2) were measured several times in the double-focusing spectrometer at a resolution of 1.5%, each time with the same result. The radial width of both the source and counter window was 9 mm. The thickness of the counter window, made of formvar, was about 70 /c gr/cm2. No correction for absorption of the low energy electrons in this window was applied. The field measuring system was calibrated on the F- and I-lines of ThB, using the He-values of L i n d s t r ii m “)-
Fig.
1. The
b-ray spectrum of UX,.
In figure 1 the spectrum of UX, with many low energy lines is shown. Vertical lines indicate the statistical accuracy of the low
THE
BETA-SPECTRA
OF u(x,
+
435
x,)
energy part of the spectrum. At higher energies the fluctuations are about the same or smaller than the diameter of the points. Figure 2 shows some conversion lines found on the UX, spectrum. 125,.
Fig. 2. Conversion
lines on the /?-spectrum
of UX2.
blp.93J
Fig. 3. The conversion
lines of the 91 keV y-ray
of UX,
at 0.3%
resolution.
The conversion lines of the 91 keV y-ray of UX, were also measured with a resolution of 0.3%. In these measurements even the O-conversion showed up (see fig. 3).
436
P. H. STOKER,
M. HEERSCHAP
AND
ONG
PING
HOK
4. The electron lines.
a. Internal conversion of the y-rays. In table1 the energy values of the electron lines in UX, are tabulated and ascribed to conversion of y-rays, in such a way, that a minimum number of y-rays is obtained. The values between parenthesis are considered to be less certain. TABLE Conversion Line (Fig. 2)
%
E,-(keV)
11 i .c k i m
532 571 505 667 548 737
1
(588) 702
24.2 27.8 22.0 37.7 25.3 45.7 (29.5) 41.6
0 P It 4 I s t LI v
833 864 (788) 923.3 1030 1056 1086 1111 1219 1693 ( 1895) (2078) (2147)
57.6 61.8 (51.9) 70.2 86.0 90.1 94.6 99.1 117.3 209
I
lines of US, Level
E, (keV)
IllI NI LI Ill1 =I NI
29.6 29.2 43.1 43.1 46.4 47.1
=I M, NI Al I NI
62.7 47.0 43.0 63.0 63.2
=I MI NI illI NI Ii LI
91.3 91.4 91.5 100.0 100.5 229.3 229
=I IllI
316 316
e-is-
( UlY,)
8.3 2.1 0.57
x IO-’ x IO-2 x IO-2
(253) (295) (311)
The i-line has been taken as the L, converted y-ray of 47.0 keV owing to the following considerations: le. since many conversion lines overlap in this neighbourhood, it is difficult to determine the exact peak-value, 2e. together with the 43.1 keV y-ray a cascade transition may be formed that has the strong 91.4 keV y-ray as a cross-over. On the photograph taken by L. M e i t n e r “) the lines t and u appeared also faintly. When they are the M and N conversion lines of a 100 keV y-ray, the L-conversion line must appear in the region between the strong lines q and r. The 229 keV y-ray has been taken as starting from an excited
437 . state of UZ and not of UII because a better fit was obtained with the ionization energies of the Pa-atom. The L- and M-conversion lines of a 395 keV y-ray, given by B r a d t and S c h e r r e r “) for the isomeric transition from UX, to UZ have not been found, although we measured with a strong source especially on the upward slope of the UX, spectrum. From the statistical accuracy of these measurements (1.4%) we calculated for the ratio : e;+Jt!?- (UX,) < 0.3 x 10M3,while B r a d t found a ratio of 0.75 x 10-3. THE
BETA-SPECTRA
OF u(x,
TABLE Conversion Line (Fig. 3) A
H@ I
3641
1
3984 4019 3765 (4120) (4175) (3885)
n C II E I; G
E,(keV) )
694
1
x,)
II
lines of UX, Level
I
.I<
1
ii-
788 799 729 W6) (841) (761)
+
E, (keV) ai0 811 a05 a45 a48 a47 a77
e-/B- (UJY.2) 5.2 x IO1.0 x IO-3
In figure 3 the intense electron lines of UX, are shown, while their interpretation is given in table II. b. A u g e r e f f e c t. The lines below 21 keV (a to f in fig. 2) have not been interpreted as y-conversion lines, because it has been possible to ascribe them to Auger electrons on account of energy and intensity considerations. The greatest probability on K-Auger electrons results from the excitation of the K-shell by the 8 10 keV y-ray. (e;/@-( UX,) = 0.005 ; cf. table II). The semi-empirical formula of S t e f f e n “) gives for the K-Auger transition probability : A, = C/(Z3.5 + C); c = 2.2 x lo5
This gives for U with 2 = 92, A, = 0.03. From various possible Auger transitions it has been found by E 11 i s ‘), that the K --f L,,, L,,, transition in Th(B + C) and Ra(B + C) is the most intense one. This transition will give in our case 1.6% above the continuous spectrum which is about the statistical accuracy. The points round w in figure 2, found several times in our measurements, indicate a line of the right energy (73 keV) for this transition.
438
P. H. STOKER,
M. HEERSCHAP
AND
ONG
PING
HOK
The excitation of the L-shell, however, is far greater than that of the K-shell (cf. line Q in fig. 1). From table I follows, that mainly L,-conversion occurs, not only of the 91 keV y-ray, but also of the other low-energy y-rays. By Cost e r-K r o n i g (C.K.) transitions “) also the L,,, shell may be excited. Moreover, the L,, and L,,, shell are also excited by the emission of Ku, and Ku, X-radiation g), amounting to 25% and 54% respectively of the total number of K-conversion electrons. The different Auger transition probabilities may now be estimated in the following way. .TXl3LE L-Auecr
r
-
Line (fig. 2)
E/f (ke\Y) esp. A
8.2
9.4
12.2
14.2
15.2 18.3
-
III
lines of C’S,
7
-
EA (keV) theor. --8.4 8.1 8.3 9.5 9.65 9.8 12.55 12.5 12.51 12.65 14.0 14.34 14.51 15.55 18.66 18.44 18.32
P I’ P, B P I’, B P B P I’ I-’
K i n s e y lo) has found theoretically for fI = 0.2 and C,, III = 0.6 while S t e p h e n s o n 11) determined fIII experimentally as 0.45 (fI and fIII are the fluorescence yield of the L, and L,,, shell and C I, III is the probability for the C.K. transition L,- L,,,, M or N). From these values follows, that 0.2 L, and 0.33 L,,, Auger electrons occur for every L, conversion electron. The number of L,,-Auger electrons is negligible due to the fact, that C,, II is small. Comparison of these values with our experimental data has not yet been possible, owing to the strong absorption in the source (0.3 mg/cm2). Furthermore the overlap of the lines makes it difficult to find the continuous spectrum in this region.
THE BETA-SPECTRA
OF u(x,
439
+ x,)
In table III the measured peak values are compared with the energies of the various possible Auger transitions, as given in column 4. The letters B and P in this column indicate respectively, that these transitions have been found by B u t t r2) in Th(B + C) and Ra(B + C) or will be very probable theoretically, according to Pin c h e r 1 e 13). The difference between column 2 and 4 can be explained by the peakshift, caused by absorption in the source and counterfoil. The mean difference in the lines a, b en c (0.2 keV) was taken as the possible shift for the interpretation of d, e and f as Augerlines. Whether all the lines u to f are really due to Auger transitions must be decided by further experimental work. 5. The beta-specta of UX, and UX,. The experimental
curve of the ,Sspectrum of UX, showed an increasing number of electrons above its endpoint, as it was known from previous measurements I). This is also seen in the Fermi-Kurie plot as given in fig. 4C.
-c
E I”
II”
Fig. 4. The Fermi-Kurie
analysis
of the ,&spectrum b-spectrum
of ux’z. Ux’z.
This increase was ascribed to low-energy electrons of UX,, that reach the counter by trochoidal orbits at a large magnetic field. This is made possible by the radially decreasing magnetic field in the double focusing spectrometer. The intensity of these electrons should be very sensitive to the pressure in the chamber because of their long orbit. Now the points in figure 4, C were obtained at a pressure of lo-’ mm Hg, so further measurements were performed
440
P. H. STOKER,
M. HEERSCHAP
AND
ONG
PING
HOK
above 2 MeV with increasing pressures till 0.1 mm Hg. At a pressure of 5 x lop3 mm Hg, all the trochoidal electrons were just absorbed and the high energy part of the UX, spectrum was now remeasured (cf. the Fermi-Kurie plot in fig. 4, B). Comparison of the curves B and C gives a deviation at about 2 MeV. From the width of the smallest baffle and from the magnetic field strength the energy of these electrons was estimated to be below 30 keV. In curve A the FermiKurie plot of the mean values of the measurements, corrected for the trochoidal effect is represented. The p-spectrum of UX, was analysed into three components with endpoints at 2305, 1500 and 580 keV and relative intensities of 90%, 9% and lyo respectively. The deviation at 600 keV from the second Fermi-Kurie plot could not be attributed to the thickness of the source on account of the semi-empirical formula of H a m i 1 t o n and Gross14). T-
Fig. 5. The Fermi-Kurie
analysis of the /?-spectrum
of UX,.
The endpoint of UX, was found at 193 keV (fig. 5). Although the deviation from the straight line at about 80 keV may be due to the thickness of the source, the Fermi-Kurie-analysis was continued and a second component with an endpoint at 103 keV was found. The intensity ratio of these two components is 2 : 1, the low energy component being much less intense than found previously l). A reconstruction of the p-spectrum of UX, from the Fermi-Kurieanalysis gave the dashed line in figure 2, indicating an excess of low
THE
BETA-SPECTRA
OF
u(x,
+
441
x,)
l
energy electrons. The area of the UX, spectrum thus obtained exceeded that of UX, by 8%, which may be due to backscattering. In table IV the five /3-components with their endpoints and relative intensities are listed, while the last column gives the log ft-values calculated according to the curves of I; e e n b e r g and T r i g g 15). TABLE The /?-components Component
IV of US,
Energy of endpoint (IteV)
I II III IV V
Fig. 6. Tentative
2305 I.500 580 193 103
decay
and US,
Intensity
I
scheme
log rt
67% 33 %
of UX,
and U.3’1.
6. Discussion. A tentative decay scheme of U(X, + X,), in which only the intense y-transitions are considered, is given in figure 6. Dashed lines indicate possible transitions to other levels. Because UX, and UII are even-even nuclei their ground states will have spin zero and even parity. The 193 keV t!?-component of UX, has a log ft-value of 6.56 and so may be .once forbidden. (LII = 1; yes). From
442
P. H. STOKER,
M. HEERSCHAP
AND
ONG
PING
HOK
this follows, that the 2305 keV ,Ccomponent of UX, has parity change with AI = 1 and must also be once forbidden. This is not excluded by the log ft-value, which is just between the allowed and forbidden region 16). If we assume that this transition is allowed, the same difficulty about the parity of the UX, level will arise, that has already been discussed by Feather and Richardsonl’). The L-conversion coefficient of the 91 keV y-ray of UX, is 0.25 and is according to G e 11 m a n et al. 18) either El of E2. This value is however to be taken as a lower limit, because the intensity of the 103 keV p-component may be too large (cf. section 5) and a possible y - y cascade of 43 and 47 keV is neglected. Therefore the 91 keV y-ray is very likely E2, ivhich has also been found by B r a d t and S c h e r r e r “). The “excited state of UX,” will thus have odd parity and spin 1 (or 2). This makes the 103 keV /?-component of UX, once forbidden in agreement with its log ft-value. In table VI the theoretical z;/ e; ra t ios and conversion coefficients for the possible multipole assignments of the 810 keV y-ray of UX, are compared with the experimental values. TABLE Multipole radiation E2 nr 1 M2 M3
- (ezded
5.2
enp
assignment (ah.) exp
0.057
V of the 810 keV-ray 1 ‘“XII”
(ad 20) theor. 0.0139 0.104 0.165 0.284
/ , Bzzl/%~-Yzz) 0.37 0.045 0.026 0.013
Since the ground state of UII has spin zero, a mixture of E2 and M 1, which will give the best agreement with the experimental values, is excluded by the selection rules. As the 1500 keV /?-component (cf. fig. 4) E2 radiation cannot be 37% of all UX, disintegrations is also excluded. M2 and M3 are also very unlikely on account of intensity, parity and spin considerations. So we conclude, that the 810 keV y-ray will be M 1. The second partial @-component will then be 4.5% of the total number of UX, disintegrations, which lies between our experimental value of 9% and that of B r a d t et al. 21) (1.2%). M easurement on the external conversion of y-rays of 800 keV and above, gave too low a yield to draw any definite conclusions, but gave the impression, that the intensities of these y-rays are greater than given by B r a d t and S c h e r r e r.
THE
BETA-SPECTRA
ON u(x,
+
x,)
443
The spin and parity assignment (1, +) of the 810 keV excited state of UII and the log ft-value of the /?-component of 1500 keV are in accordance with a once forbidden transition. The 580 keV /?-transition may be allowed on account of its low log ft-value. The two y-rays of 845 and 877 keV which follow this transition are not the same as those found by B r a d t et al. 21). Their decay scheme with more than one F-component at about 1500 keV are not excluded by our results. The high energy y-ray of 1500 keV, found by S i z o o and C o u m o u 22) may be the cross-over transition of these two y-rays. Whether the isomeric transition from UX, to UZ is formed by the 216 keV y-ray or the 395 keV y-ray, proposed by B r a d t and S c h e r r er will be decided by remeasurements of the UZ spectrum, for which preparations are being ‘made 23). Acknowledgements. We wish to thank Prof. Dr G. J. S i z o o, under whose direction this work was carried out, and Prof. Dr C. C. J o n k e r for the many discussions on this problem. Thanks are also due to Dr F. B a r e n d r e g t for preparing the radio-active material. We greatfully acknowledge the help given by Mr H. W. H o r em a n with the measurements and calculations. Finally we thank Mr L. D o r s m a n for his help and Mr W. J o n g s m a for his technical assistance. Received
13-3-53.
REFERENCES 1) H c c r s c 11 n p, M., 0 n g P i II g H o k, mtl S i z o o, G. J., Physica l(I (19.50) 767. 2) II e e r s c h n p, M., Thesis, .4mstrrdnnr V.U. (1951); H e e r s c h a p, II. and S t o k e r, P. H., submittecl for publication in Physica. 3) L i II d s t r 6 m, G., Phys. Rev. WI (1951) 465. 4) M c i t 11e r, L., Z. Physik 17 (1923) 54. 5) B r a d t, H. and S c he r r e r, P., Helv. phys. Acta IU (1945) 435. 6) S t e f f e II, R. AI., 1-I u b e r, 0. and H u 11, I, c 1, F., Hclv. phys. Acta f:! (1949) 167. 7) E 1 I i s, C. D., Proc. ray. Sm. London A 1%) (1933) 336. 8) Co s t e r, D. and Ii r o n i g, R. d e L., Physica P (1935) 13. t 111y c r, F. Ii., Phys. Rev. .‘,I (1937) 913. 9) R a 111b c r g, 1:. G. and Rich IO) I< i II s e y, B. B., Canadian J. Res. %A (1948) 404, 421.
444 II) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23)
THE
BETA-SPECTRA
OF
M(X, + X,)
Physikalische Tabellen, Bd I, 1. Teil (1950, Springer Landolt-Barnstein, Verlag), p. 320. B u t t, D. I<., Proc. phys. Sot. A (if (I 950) 986. Pincherle, L.,NuovoCimento I? (1935) 81. H a m i 1 t o II, D. R. and G r o s s, L., Rev. sci. Inst. 21 (1950) 912. Feenberg, Eand Trigg, G.,Rev.mod.Phys.Z(1950)399. Fe ingold, A. M., Rev. mod. Phys. 2:) (1951) 10. Feat her, N. and Richardson, H. 0. W., Proc. phys. Sot., London, (il (1948) 452. C e 11 m a n, H., G r i f f i t h, B. A. and S t a n 1 e y, J. P., Phys. Rev. 80 (1950) 866. G o 1 d h a b e r, M. and S u n y n r, A. W., Phys. Rev. 8:s (1951) 906. R o s e, M. E., et al., K-Shell Internal Conversion Coefficients, Revised Tables, (Oakridge, 1951); Phys. Rev. 8:s (1951) 79. B r a d t, H., H e i n e, H. G. and S c II e r r e r, P., Helv. phys. Acta l(i (1943) 455. S i z o o, G. J. and C o u m o u, D. J., Physicn :I (1936) 921. Bare II d re g t, F. and To II), Sj., Physica 17 (1951) 817.
Physica
XIX
433-444
1953
MEASUREMENTS ON THE CONTINUOUS BETA-SPECTRA, THE CONVERSION LINES AND THE AUGER-LINES OF U(X, +X2) by P. H. STOKER Communication
*), M. HEERSCHAP
from the Natuurkundig Laboratorium Amsterdam, Nederland
t) and ONG PING HOK van de Vrije Universiteit,
Synopsis Measurements on the p-spectra of U(X, + X,) in a 16.2 cm doublefocusing spectrometer, (resolution of 1.5%) are described. Of the electronlines 25 could be ascribed to 11 y-rays, while the lines below 21 keV could be interpreted as L-Auger lines on account of energy and intensity considerations. The Fermi-Kurie analysis of the continuous p-spectra gave 5 components with endpoints at 2305, 1500 and 580 keV for UX, and at 193 and 103 keV for UX,. The relative intensities were respectively 90%, 9% and 1 y0 for UX, and 66% and 34% for UX,. The 91 keV y-ray of UX, is E2, while the 8 10 keV y-ray of UX, may be Ml. A tentative disintegration scheme with spin and parity assignments to some of the levels is discussed.
1. I&roductiolz. In a previous paper ‘) measurements on the p-spectrum of U(X, + X,) with a thin mica-foil as source carrier and performed in a semi-circular spectrometer were described. Since backscattering against the mica-foil distorted the low energy region of the spectrum, we now used a thin film of plastic material “) and remeasured the spectrum not only in the semi-circular but also in a 16.2 cm double-focusing spectrometer “) at a resolution of 1.5 Oh. A classification of the electron lines found, as internal conversion and Auger lines is given in section 4. After a Fermi-Kurie analysis (section 5) of the continuous spectrum, a tentative decay scheme is discussed in section 6. *) Now at the Potchefstroom University for C.H.E., Potchefstroom, South-Africa. t) Now at the Nederlandse Springstoffenfabriek, Muiden, Nederland.
Physica XIX
433 20
434
P. H. STOKER,
M. HEERSCHAP
AND ONG PING HOK
2. Preparation. of the source. The UX activity was extracted from some kilograms of uranyl-nitrate, by means of an ether-water separation, followed by a passage through an ion-exchange column (wofatit KS.). It was then dissolved in concentrated HCl and brought on a pep film 2), using insulin to define the source area. After it had been dried in vacuum, a formvar film, upon which a thin layer of Al had been evaporated, was placed against the backside of the foil to make it conductive. The total weight of the source backing was less than 0.1 mg/cm’. The mean thickness of the active material was 0.3 mg/cm2, mainly due to FeCl, from the wofatit. 3. The measurements of the &ybectra. The p-spectra of U(X, + X2) were measured several times in the double-focusing spectrometer at a resolution of 1.5%, each time with the same result. The radial width of both the source and counter window was 9 mm. The thickness of the counter window, made of formvar, was about 70 /c gr/cm2. No correction for absorption of the low energy electrons in this window was applied. The field measuring system was calibrated on the F- and I-lines of ThB, using the He-values of L i n d s t r ii m “)-
Fig.
1. The
b-ray spectrum of UX,.
In figure 1 the spectrum of UX, with many low energy lines is shown. Vertical lines indicate the statistical accuracy of the low
THE
BETA-SPECTRA
OF u(x,
+
435
x,)
energy part of the spectrum. At higher energies the fluctuations are about the same or smaller than the diameter of the points. Figure 2 shows some conversion lines found on the UX, spectrum. 125,.
Fig. 2. Conversion
lines on the /?-spectrum
of UX2.
blp.93J
Fig. 3. The conversion
lines of the 91 keV y-ray
of UX,
at 0.3%
resolution.
The conversion lines of the 91 keV y-ray of UX, were also measured with a resolution of 0.3%. In these measurements even the O-conversion showed up (see fig. 3).
436
P. H. STOKER,
M. HEERSCHAP
AND
ONG
PING
HOK
4. The electron lines.
a. Internal conversion of the y-rays. In table1 the energy values of the electron lines in UX, are tabulated and ascribed to conversion of y-rays, in such a way, that a minimum number of y-rays is obtained. The values between parenthesis are considered to be less certain. TABLE Conversion Line (Fig. 2)
%
E,-(keV)
11 i .c k i m
532 571 505 667 548 737
1
(588) 702
24.2 27.8 22.0 37.7 25.3 45.7 (29.5) 41.6
0 P It 4 I s t LI v
833 864 (788) 923.3 1030 1056 1086 1111 1219 1693 ( 1895) (2078) (2147)
57.6 61.8 (51.9) 70.2 86.0 90.1 94.6 99.1 117.3 209
I
lines of US, Level
E, (keV)
IllI NI LI Ill1 =I NI
29.6 29.2 43.1 43.1 46.4 47.1
=I M, NI Al I NI
62.7 47.0 43.0 63.0 63.2
=I MI NI illI NI Ii LI
91.3 91.4 91.5 100.0 100.5 229.3 229
=I IllI
316 316
e-is-
( UlY,)
8.3 2.1 0.57
x IO-’ x IO-2 x IO-2
(253) (295) (311)
The i-line has been taken as the L, converted y-ray of 47.0 keV owing to the following considerations: le. since many conversion lines overlap in this neighbourhood, it is difficult to determine the exact peak-value, 2e. together with the 43.1 keV y-ray a cascade transition may be formed that has the strong 91.4 keV y-ray as a cross-over. On the photograph taken by L. M e i t n e r “) the lines t and u appeared also faintly. When they are the M and N conversion lines of a 100 keV y-ray, the L-conversion line must appear in the region between the strong lines q and r. The 229 keV y-ray has been taken as starting from an excited
437 . state of UZ and not of UII because a better fit was obtained with the ionization energies of the Pa-atom. The L- and M-conversion lines of a 395 keV y-ray, given by B r a d t and S c h e r r e r “) for the isomeric transition from UX, to UZ have not been found, although we measured with a strong source especially on the upward slope of the UX, spectrum. From the statistical accuracy of these measurements (1.4%) we calculated for the ratio : e;+Jt!?- (UX,) < 0.3 x 10M3,while B r a d t found a ratio of 0.75 x 10-3. THE
BETA-SPECTRA
OF u(x,
TABLE Conversion Line (Fig. 3) A
H@ I
3641
1
3984 4019 3765 (4120) (4175) (3885)
n C II E I; G
E,(keV) )
694
1
x,)
II
lines of UX, Level
I
.I<
1
ii-
788 799 729 W6) (841) (761)
+
E, (keV) ai0 811 a05 a45 a48 a47 a77
e-/B- (UJY.2) 5.2 x IO1.0 x IO-3
In figure 3 the intense electron lines of UX, are shown, while their interpretation is given in table II. b. A u g e r e f f e c t. The lines below 21 keV (a to f in fig. 2) have not been interpreted as y-conversion lines, because it has been possible to ascribe them to Auger electrons on account of energy and intensity considerations. The greatest probability on K-Auger electrons results from the excitation of the K-shell by the 8 10 keV y-ray. (e;/@-( UX,) = 0.005 ; cf. table II). The semi-empirical formula of S t e f f e n “) gives for the K-Auger transition probability : A, = C/(Z3.5 + C); c = 2.2 x lo5
This gives for U with 2 = 92, A, = 0.03. From various possible Auger transitions it has been found by E 11 i s ‘), that the K --f L,,, L,,, transition in Th(B + C) and Ra(B + C) is the most intense one. This transition will give in our case 1.6% above the continuous spectrum which is about the statistical accuracy. The points round w in figure 2, found several times in our measurements, indicate a line of the right energy (73 keV) for this transition.
438
P. H. STOKER,
M. HEERSCHAP
AND
ONG
PING
HOK
The excitation of the L-shell, however, is far greater than that of the K-shell (cf. line Q in fig. 1). From table I follows, that mainly L,-conversion occurs, not only of the 91 keV y-ray, but also of the other low-energy y-rays. By Cost e r-K r o n i g (C.K.) transitions “) also the L,,, shell may be excited. Moreover, the L,, and L,,, shell are also excited by the emission of Ku, and Ku, X-radiation g), amounting to 25% and 54% respectively of the total number of K-conversion electrons. The different Auger transition probabilities may now be estimated in the following way. .TXl3LE L-Auecr
r
-
Line (fig. 2)
E/f (ke\Y) esp. A
8.2
9.4
12.2
14.2
15.2 18.3
-
III
lines of C’S,
7
-
EA (keV) theor. --8.4 8.1 8.3 9.5 9.65 9.8 12.55 12.5 12.51 12.65 14.0 14.34 14.51 15.55 18.66 18.44 18.32
P I’ P, B P I’, B P B P I’ I-’
K i n s e y lo) has found theoretically for fI = 0.2 and C,, III = 0.6 while S t e p h e n s o n 11) determined fIII experimentally as 0.45 (fI and fIII are the fluorescence yield of the L, and L,,, shell and C I, III is the probability for the C.K. transition L,- L,,,, M or N). From these values follows, that 0.2 L, and 0.33 L,,, Auger electrons occur for every L, conversion electron. The number of L,,-Auger electrons is negligible due to the fact, that C,, II is small. Comparison of these values with our experimental data has not yet been possible, owing to the strong absorption in the source (0.3 mg/cm2). Furthermore the overlap of the lines makes it difficult to find the continuous spectrum in this region.
THE BETA-SPECTRA
OF u(x,
439
+ x,)
In table III the measured peak values are compared with the energies of the various possible Auger transitions, as given in column 4. The letters B and P in this column indicate respectively, that these transitions have been found by B u t t r2) in Th(B + C) and Ra(B + C) or will be very probable theoretically, according to Pin c h e r 1 e 13). The difference between column 2 and 4 can be explained by the peakshift, caused by absorption in the source and counterfoil. The mean difference in the lines a, b en c (0.2 keV) was taken as the possible shift for the interpretation of d, e and f as Augerlines. Whether all the lines u to f are really due to Auger transitions must be decided by further experimental work. 5. The beta-specta of UX, and UX,. The experimental
curve of the ,Sspectrum of UX, showed an increasing number of electrons above its endpoint, as it was known from previous measurements I). This is also seen in the Fermi-Kurie plot as given in fig. 4C.
-c
E I”
II”
Fig. 4. The Fermi-Kurie
analysis
of the ,&spectrum b-spectrum
of ux’z. Ux’z.
This increase was ascribed to low-energy electrons of UX,, that reach the counter by trochoidal orbits at a large magnetic field. This is made possible by the radially decreasing magnetic field in the double focusing spectrometer. The intensity of these electrons should be very sensitive to the pressure in the chamber because of their long orbit. Now the points in figure 4, C were obtained at a pressure of lo-’ mm Hg, so further measurements were performed
440
P. H. STOKER,
M. HEERSCHAP
AND
ONG
PING
HOK
above 2 MeV with increasing pressures till 0.1 mm Hg. At a pressure of 5 x lop3 mm Hg, all the trochoidal electrons were just absorbed and the high energy part of the UX, spectrum was now remeasured (cf. the Fermi-Kurie plot in fig. 4, B). Comparison of the curves B and C gives a deviation at about 2 MeV. From the width of the smallest baffle and from the magnetic field strength the energy of these electrons was estimated to be below 30 keV. In curve A the FermiKurie plot of the mean values of the measurements, corrected for the trochoidal effect is represented. The p-spectrum of UX, was analysed into three components with endpoints at 2305, 1500 and 580 keV and relative intensities of 90%, 9% and lyo respectively. The deviation at 600 keV from the second Fermi-Kurie plot could not be attributed to the thickness of the source on account of the semi-empirical formula of H a m i 1 t o n and Gross14). T-
Fig. 5. The Fermi-Kurie
analysis of the /?-spectrum
of UX,.
The endpoint of UX, was found at 193 keV (fig. 5). Although the deviation from the straight line at about 80 keV may be due to the thickness of the source, the Fermi-Kurie-analysis was continued and a second component with an endpoint at 103 keV was found. The intensity ratio of these two components is 2 : 1, the low energy component being much less intense than found previously l). A reconstruction of the p-spectrum of UX, from the Fermi-Kurieanalysis gave the dashed line in figure 2, indicating an excess of low
THE
BETA-SPECTRA
OF
u(x,
+
441
x,)
l
energy electrons. The area of the UX, spectrum thus obtained exceeded that of UX, by 8%, which may be due to backscattering. In table IV the five /3-components with their endpoints and relative intensities are listed, while the last column gives the log ft-values calculated according to the curves of I; e e n b e r g and T r i g g 15). TABLE The /?-components Component
IV of US,
Energy of endpoint (IteV)
I II III IV V
Fig. 6. Tentative
2305 I.500 580 193 103
decay
and US,
Intensity
I
scheme
log rt
67% 33 %
of UX,
and U.3’1.
6. Discussion. A tentative decay scheme of U(X, + X,), in which only the intense y-transitions are considered, is given in figure 6. Dashed lines indicate possible transitions to other levels. Because UX, and UII are even-even nuclei their ground states will have spin zero and even parity. The 193 keV t!?-component of UX, has a log ft-value of 6.56 and so may be .once forbidden. (LII = 1; yes). From
442
P. H. STOKER,
M. HEERSCHAP
AND
ONG
PING
HOK
this follows, that the 2305 keV ,Ccomponent of UX, has parity change with AI = 1 and must also be once forbidden. This is not excluded by the log ft-value, which is just between the allowed and forbidden region 16). If we assume that this transition is allowed, the same difficulty about the parity of the UX, level will arise, that has already been discussed by Feather and Richardsonl’). The L-conversion coefficient of the 91 keV y-ray of UX, is 0.25 and is according to G e 11 m a n et al. 18) either El of E2. This value is however to be taken as a lower limit, because the intensity of the 103 keV p-component may be too large (cf. section 5) and a possible y - y cascade of 43 and 47 keV is neglected. Therefore the 91 keV y-ray is very likely E2, ivhich has also been found by B r a d t and S c h e r r e r “). The “excited state of UX,” will thus have odd parity and spin 1 (or 2). This makes the 103 keV /?-component of UX, once forbidden in agreement with its log ft-value. In table VI the theoretical z;/ e; ra t ios and conversion coefficients for the possible multipole assignments of the 810 keV y-ray of UX, are compared with the experimental values. TABLE Multipole radiation E2 nr 1 M2 M3
- (ezded
5.2
enp
assignment (ah.) exp
0.057
V of the 810 keV-ray 1 ‘“XII”
(ad 20) theor. 0.0139 0.104 0.165 0.284
/ , Bzzl/%~-Yzz) 0.37 0.045 0.026 0.013
Since the ground state of UII has spin zero, a mixture of E2 and M 1, which will give the best agreement with the experimental values, is excluded by the selection rules. As the 1500 keV /?-component (cf. fig. 4) E2 radiation cannot be 37% of all UX, disintegrations is also excluded. M2 and M3 are also very unlikely on account of intensity, parity and spin considerations. So we conclude, that the 810 keV y-ray will be M 1. The second partial @-component will then be 4.5% of the total number of UX, disintegrations, which lies between our experimental value of 9% and that of B r a d t et al. 21) (1.2%). M easurement on the external conversion of y-rays of 800 keV and above, gave too low a yield to draw any definite conclusions, but gave the impression, that the intensities of these y-rays are greater than given by B r a d t and S c h e r r e r.
THE
BETA-SPECTRA
ON u(x,
+
x,)
443
The spin and parity assignment (1, +) of the 810 keV excited state of UII and the log ft-value of the /?-component of 1500 keV are in accordance with a once forbidden transition. The 580 keV /?-transition may be allowed on account of its low log ft-value. The two y-rays of 845 and 877 keV which follow this transition are not the same as those found by B r a d t et al. 21). Their decay scheme with more than one F-component at about 1500 keV are not excluded by our results. The high energy y-ray of 1500 keV, found by S i z o o and C o u m o u 22) may be the cross-over transition of these two y-rays. Whether the isomeric transition from UX, to UZ is formed by the 216 keV y-ray or the 395 keV y-ray, proposed by B r a d t and S c h e r r er will be decided by remeasurements of the UZ spectrum, for which preparations are being ‘made 23). Acknowledgements. We wish to thank Prof. Dr G. J. S i z o o, under whose direction this work was carried out, and Prof. Dr C. C. J o n k e r for the many discussions on this problem. Thanks are also due to Dr F. B a r e n d r e g t for preparing the radio-active material. We greatfully acknowledge the help given by Mr H. W. H o r em a n with the measurements and calculations. Finally we thank Mr L. D o r s m a n for his help and Mr W. J o n g s m a for his technical assistance. Received
13-3-53.
REFERENCES 1) H c c r s c 11 n p, M., 0 n g P i II g H o k, mtl S i z o o, G. J., Physica l(I (19.50) 767. 2) II e e r s c h n p, M., Thesis, .4mstrrdnnr V.U. (1951); H e e r s c h a p, II. and S t o k e r, P. H., submittecl for publication in Physica. 3) L i II d s t r 6 m, G., Phys. Rev. WI (1951) 465. 4) M c i t 11e r, L., Z. Physik 17 (1923) 54. 5) B r a d t, H. and S c he r r e r, P., Helv. phys. Acta IU (1945) 435. 6) S t e f f e II, R. AI., 1-I u b e r, 0. and H u 11, I, c 1, F., Hclv. phys. Acta f:! (1949) 167. 7) E 1 I i s, C. D., Proc. ray. Sm. London A 1%) (1933) 336. 8) Co s t e r, D. and Ii r o n i g, R. d e L., Physica P (1935) 13. t 111y c r, F. Ii., Phys. Rev. .‘,I (1937) 913. 9) R a 111b c r g, 1:. G. and Rich IO) I< i II s e y, B. B., Canadian J. Res. %A (1948) 404, 421.
444 II) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23)
THE
BETA-SPECTRA
OF
M(X, + X,)
Physikalische Tabellen, Bd I, 1. Teil (1950, Springer Landolt-Barnstein, Verlag), p. 320. B u t t, D. I<., Proc. phys. Sot. A (if (I 950) 986. Pincherle, L.,NuovoCimento I? (1935) 81. H a m i 1 t o II, D. R. and G r o s s, L., Rev. sci. Inst. 21 (1950) 912. Feenberg, Eand Trigg, G.,Rev.mod.Phys.Z(1950)399. Fe ingold, A. M., Rev. mod. Phys. 2:) (1951) 10. Feat her, N. and Richardson, H. 0. W., Proc. phys. Sot., London, (il (1948) 452. C e 11 m a n, H., G r i f f i t h, B. A. and S t a n 1 e y, J. P., Phys. Rev. 80 (1950) 866. G o 1 d h a b e r, M. and S u n y n r, A. W., Phys. Rev. 8:s (1951) 906. R o s e, M. E., et al., K-Shell Internal Conversion Coefficients, Revised Tables, (Oakridge, 1951); Phys. Rev. 8:s (1951) 79. B r a d t, H., H e i n e, H. G. and S c II e r r e r, P., Helv. phys. Acta l(i (1943) 455. S i z o o, G. J. and C o u m o u, D. J., Physicn :I (1936) 921. Bare II d re g t, F. and To II), Sj., Physica 17 (1951) 817.