Conversion-electron spectroscopy of short-lived nuclides

N U C L E A R INSTRUMENTS AND METHODS

II 4 (I974)

341-347;

© NORTH-HOLLAND

PUBLISHING

CO.

CONVERSION-ELECTRON SPECTROSCOPY OF SHORT-LIVED NUCLIDES* D. R. Z O L N O W S K I and T. T. S U G I H A R A

Cyclotron Institute, Texas A & M University, College Station, Texas 77843, U.S.A. Received 28 May 1973 and in revised form 1 August 1973 A simple technique for obtaining conversion-electron spectra of short-lived activities with a Si(Li) detector is described. The method employs the helium-jet technique to produce sources repetitively for subsequent counting through a 0.90 mg/cmz aluminized Mylar window. The effects of the Mylar window on resolution and line shape were investigated and shown to give acceptable results for electron energies as low as 85 keV. The

most probable energy loss for electrons of energies 85, 109, 253, 292, and 389 keV was found to agree reasonably well with theoretical predictions. Conversion-electron spectra of 177Hfm (1.I s) and 152Tbm (4.2 min) are shown. For the latter nucleus, transition multipolarities deduced from experimental internal-conversion coefficients were found to support spin and parity assignments made previously.

1. Introduction

than several minutes, standard methods of source preparation are not possible and magnetic spectrometers cannot be used. While some short-lived isotopes can be studied in equilibrium with a longer lived parent, this is not generally possible, and experimenters have been forced to try more exotic techniques such as the use of pneumatic transport systems1). Regrettably, few

The measurement of the internal-conversion electron spectra of short-lived activities has in general proved a difficult task. When half-lives are significantly less * Supported in part by the U.S. Atomic Energy Commission and the Robert A. Welch Foundation.

l II £/sec.

SOURCE,TRANSPORTSYSTEM

ION PUMP~

I~/sec.

ON PUMP

TO

.IFIER

MECHANICAL PUMP SOURCE " ~ - ~ LLECTIONJ SITE t HE-JET j' CAPILLARY STEPPING MOTOR

ROUGHING POR

Si ( L i ) DETECTOR

Fig. 1. Schematic drawing of conversion-electron spectrometer when used in conjunction with the source transport of the helium-jet system.

341

342

D. R. Z O L N O W S K I

A N D T. T. S U G I H A R A

of the approaches have met with significant success, resulting in a serious lack of information on the multipolarities of transitions in the decay schemes of most short-lived activities. This indecision about spin and parity assignments has caused the determination of nuclear structure to suffer. In a promising approach to the problem, an on-line mass separator has been used in conjunction with a Si(Li) detector and a tape-transport system to permit counting many sources2-4). However, the unavailability of such devices for the majority of experimenters shows up the need for a less complex, yet still effective, technique. This paper reports on such a method. The approach involves the use of recoil sweeping with helium gas to produce sources of short-lived activities repetitively for subsequent counting with a Si(Li) detector that is positioned behind an aluminized Mylar window. The effect of the Mylar window on the quality of spectra obtained is investigated and typical conversion-electron spectra are shown for activities with half-lives as short as 1.1 s.

covering a 1.27 cm diameter circular opening. The flange face behind the source being counted, as shown in fig. 2, serves as a 0.79 mm aluminum window for obtaining simultaneous ),-ray spectra or for Ge(Li)-Si(Li) coincidence experiments. The position of the detector is varied by the extension and compression of a stainless steel bellows. The maximum permitted travel of the detector is 7.6 cm and source-to-detector distances as close as 6 m m can be employed. A clean vacuum (typically less than 10-7 torr) is maintained in the detector chamber by the use of two ion pumps (1 1/s and 11 l/s). The details of the helium-jet technique and the variable source positioner have been described in refs. 5-7. The relative electron detection efficiency for the Si(Li) detector was determined using sources of 75Se (ref. 8), 169yb (ref. 9), 2°7Bi (refs. 10 and 11), and 17°Lu (ref. 12). The electron intensities adopted for the first three nuclides are listed in table 1. In the case of 17°Lu, which provided information on the high-energy portion of the curve, the adopted electron intensities were deduced from the measured y-ray intensities 12) and theoretical internal conversion coefficients13). The experimental data are summarized in table 2. This approach is justified on the basis that the chosen transitions must be pure multipoles according to the most recently proposed decay scheme12). The relative efficiency curve is given in fig. 3. Initially the 75Se source" was used to obtain the low-energy portion of the curve. The relative efficiencies for detecting the 97K, 121K, 265K, 304K, and 401K lines were found to be constant and arbitrarily set equal to

2. Experimental apparatus A schematic drawing of the electron spectrometer is shown in fig. 1. A 3 mm x 2 c m 2 Si(Li) detector with a resolution of 2.17 keV at 975 keV is employed. The physical relationship between the detector and the source-collection site at the low-pressure end of the capillary tube of the helium-jet system is also shown. Successive sources are transported on a tape to a position in front of the detector which is situated behind a 0.90mg/cm 2 aluminized Mylar window

Ge (Li) ~-0.79 mm / . . . . . . . . . . . . . . . . . . . . . .

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TRANSPORT TAPE

SOURCE POSITION j Si (Li)

Fig. 2. Cross-sectional view at the source detection site.

CONVERSION-ELECTRON

343

SPECTROSCOPY

TABLE 1 Relative efficiency of Si(Li) detector. Source

Conversion line

75Se c

Electron energy (keV)

97K 121K 265K 304K 401K 93K 110K

169yb d

207Bi e,f

85 109 253 292 389 34 51

130K

71

198K 308K 570K 1063K 1770K

139 249 482 975 1682

IK a

Relative area b

Relative efficiency

645 (25) 154.0 (23) 100 16.1 (8) 3.78 (26) 517 (50) 2602 (260) 410 973 (78) 35.3 (35) 22.0 (5) 100 (2) 0.352 (28)

656 154.0 93.2 16.1 3.65 449 2580 410 908 35.3 22.0 91.7 0.204

1.02 ~_1.00 0.93 1.00 0.97 0.87 0.99

(4) (4) (5) (7) (9) (10)

1.00 (10) 0.93 (9) ~_ 1.00 ----1.00 0.92 (5) 0.58 (5)

a Relative intensities and errors as given in ref. cited in column (1). b Relative peak area determined experimentally; scale chosen for each nuclide such that relative efficiency = (relative area)/Ii. Statistical error less than 1%. e tl:ef. 8. d R.ef. 9. e Ref. 10. f Ref. 11. TABLE 2 Relative efficiency of Si(Li) detector at high energies with a 17°Lu source. Conversion line

Electron energy (keV)

ir , a

419K 2126K 2364K 2748K 2783K

358 2065 2303 2687 2722

11.2 (3) 111.0 (35) 32.4 (10) 46.3 (20) 22.4 (10)

ctK b ( 10-4 )

M1:550 E1:3.5 E1:3.0 E1:2.4 M1:5.4

Inferred IK

----100 6.31 1.59 1.80 •.96

(20) (5) (8) (9)

Relative area e

Relative efficiency

=- 100 2.60 (9) 0.467 (60) 0.504 (45) 0.435 (39)

----1.00 0.41 0.29 0.28 0.22

(2) (4) (3) (2)

a Pef. 12. b Per. 13, multipolarity assignment from ref. 12. e Relative peak area determined experimentally;' scale chosen such that relative efficiency = (relative area)/(inferred IK).

u n i t y . T h e d a t a o b t a i n e d f r o m t h e o t h e r s o u r c e s were subsequently added to the curve by appropriate normalization to a portion of the curve considered known.

3. The effect of the Mylar window Because of the use of thin-window Geiger counters with beta-ray spectrometers, the properties of thin fihns have been studied extensively in the past to determine a correction curve for electron trans-

mission14'15). I n t h e p a r t i c u l a r a p p l i c a t i o n d e s c r i b e d h e r e , a 0.90 m g / c m 2 a l u m i n i z e d M y l a r film w a s e m p l o y e d b e c a u s e o f its a b i l i t y t o w i t h s t a n d a p r e s s u r e d i f f e r e n t i a l o f 1 a t m a c r o s s t h e 1.27 c m d i a m e t e r circular opening. The 169yb data taken with and w i t h o u t t h e w i n d o w s h o w n o s i g n i f i c a n t loss i n t r a n s m i s s i o n f o r e n e r g i e s as l o w as 51 k e V ( l l 0 K line), a result consistent with the transmission curve for a 1.07 m g / c m 2 M y l a r foil g i v e n i n ref. 15. In the passage of electrons through matter, energy

344

D. R. Z O L N O W S K I

AND

loss occurs principally through excitation and ionization of the atoms. Energy loss due to bremsstrahlung may also occur but is negligible for energies below 1 MeV. The theory for losses due to inelastic collisions has been reviewed by Knop and Paul16). Since it is experimentally difficult to measure the mean energy loss per centimeter of path, it is customary to compare the most probable energy loss, AE, with the expression given by Landau 17) in which

AE = ax

Il n imvzax 2 ( 1-[ 3z)

flZ+K-A].

T. T. S U G I H A R A

TABLE

Electron energy (keV)

3.04 2.40 1.34 1.27 1.19

J ~ ~ ~ ~11

i

3minx2

i

~ J J ~ r] I

cm 2 Si(Li)

RELATIVE

(2) (4) (3) (5) (10)

2.47 2.03 1.21 1.13 1.01

C a l c u l a t e d f r o m eq. (1).

The aluminized Mylar foil was taken to consist of a layer of aluminum of thickness 80 gg/cm 2 and a layer of Mylar of thickness 820/~g/cm 2 (6.35/~m). The mean excitation energies of atomic electrons for Mylar and aluminum were taken to be 72.6eV and 163eV, respectively18), and their densities 1.29 g/cm 3 and 2.70 g/cm 3, respectively18). For the purposes of applying eq. (1) it was assumed that effects of the aluminum and Mylar layers were additive. Despite the fact that the nominal thickness of Mylar was adopted as exact, and no special care was taken to ensure perpendicular incidence of the electrons on the foil, the agreement with theory is rather good, indicating that eq. (1) provides reasonable estimates of energy loss in thin foils. The effect of the window on resolution is shown in fig. 4 for the K-shell conversion of several transitions observed in the decays of 75Se and 2°7Bi. Even for [

i

M o s t p r o b a b l e e n e r g y loss (keV) Exp. Theor. a

85 109 253 292 389

(1)

Here a = O.153(pZ/Afl 2) MeV/cm, x is the thickness of the absorber in cm, m and v are the mass and velocity of the incident electron, fl is v/c, I is the mean excitation energy of the atomic electrons, K = 1.12 and A is a correction term arising from the polarizability of the absorber and is negligible for the electron energies under consideration here. The quantities p, Z, and A are the density (g/cm3), atomic number, and atomic weight, respectively, of the absorber. The most probable energy loss for several Mylar foils thicker than 3.3 mg/cm 2 has been measured previously 18) and found to agree reasonably well with theory. To study the effect of the Mylar window employed in this work, conversion-electron spectra of 75Se and 2°7Bi were taken with and without the Mylar window at a source-to-detector distance of 2.0 cm. The sources used were prepared by evaporation of the activities on Mylar backings. The most probable energy loss determined from the centroid shifts in the K-conversion lines of the 97, 121, 265, 304, and 401 keV transitions in 7 5 S e decay are summarized in table 3 and compared with the predictions of eq. (1).

3

M o s t p r o b a b l e e n e r g y loss in 0.90 m g / c m 2 a l u m i n i z e d M y l a r w i n d o w by i n t e r n a l - c o n v e r s i o n e l e c t r o n s f r o m 75Se source. A m p l i f i e r gain c o r r e s p o n d e d to a p p r o x i m a t e l y 0.12 k e V / c h a n n e l .

i

i

I

EFFECT OF MYLAR WINDOW ON RESOLUTION

EFFICIENCY

5.0

>

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I

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I

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ELECTRON ENERGY (keV)

I EO0

Fig. 3. R e l a t i v e e l e c t r o n efficiency c u r v e for a 3 m m × 2 c m 2 Si(Li) detector, c o n s t r u c t e d f r o m d a t a g i v e n in t a b l e s 1 a n d 2. A l o g a r i t h m i c a b s c i s s a scale w a s c h o s e n to s e p a r a t e the several l o w - e n e r g y d a t a p o i n t s f r o m each other.

I

I

I

400

600

800

ELECTRON ENERGY(keV) Fig. 4. Effect o f a 0.90 m g / c m 2 M y l a r w i n d o w o n s y s t e m resolut i o n ( f w h m ) o f a Si(Li) detector.

CONVERSION-ELECTRON e l e c t r o n s w i t h e n e r g y as l o w as 85 k e V ( 9 7 K line f r o m 75Se) t h e r e s o l u t i o n d e t e r i o r a t e s o n l y b y a f a c t o r o f t w o . F o r e l e c t r o n s w i t h e n e r g i e s a b o v e a b o u t 300 k e V t h e c h a n g e in r e s o l u t i o n is a p p r o x i m a t e l y 2 0 % . The actual changes in the individual line shapes r e s u l t i n g f r o m t h e M y l a r w i n d o w a r e s h o w n in fig. 5. In order to simplify the comparison, background has been subtracted and the peaks normalized to contain t h e s a m e n u m b e r o f e v e n t s . T h e line s h a p e s f o r t h e t w o h i g h e r e n e r g y t r a n s i t i o n s i n d i c a t e v e r y little differe n c e in b a s i c s h a p e b e t w e e n t h e w i n d o w a n d n o window data. The two lower energy peaks show some a d d i t i o n a l , b u t n o t severe, b r o a d e n i n g .

SPECTROSCOPY

SeZ5 97 K

(.0 I'--

o

r- 3.04 key

1

/ I

345

Se 75

- .]

[~ 2.40key 1.26keV FWHM

~ ~1.26 keY FWHM

I

Se 75 265 K

,

,

l I.34 keV

I

Ri 207 ~

"

"

0.81keV

m

1

d LLI ,'r"

FWHM ~i.40 keV

4. Results To date, several conversion-electron spectra of s h o r t - l i v e d a c t i v i t i e s h a v e b e e n i n v e s t i g a t e d in t h i s l a b o r a t o r y . T h e s p e c t r u m o f 4.2 m i n 152Tb m s h o w n in fig. 6 w a s o b t a i n e d b y c o u n t i n g 375 s e p a r a t e s o u r c e s f o r 28 s e a c h . T h e s o u r c e s w e r e p r o d u c e d b y t h e ' 5 ' E u ( ~ , 3 n ) 152Tb m r e a c t i o n a n d r e p e t i t i v e l y t r a n s ported to the detector location using the tape transport deviceT). T h e e l e c t r o n i n t e n s i t i e s ( c o r r e c t e d f o r d e t e c t o r efficiency) a r e s u m m a r i z e d in t a b l e 4. T h e K - s h e l l i n t e r n a l c o n v e r s i o n coefficients listed w e r e o b t a i n e d b y

2b

40

~o

,

L

8o

2b

40

#

~b

CHANNEL NUMBER

Fig. 5. Line shapes and energy losses for conversion lines o~tained with and without a 0.90 mg/cm 2 Mylar window between the source and detector. The peaks are corrected for background and normalized to contain the same number of events.

TABLE 4

Conversion-electron intensities, conversion coefficients, and multipolarity assignments of transitions in the decay of 4.2-rain 152Tbm

Ey (keV)

1K

I~, a

159.59 235.41 277.15 283.29 344.26 351.9 385.9 411.16 427.6 440.4 471.95 519.60 526.9 532.8 586.3 615.4 647.6 726.2 1106.3 1166.9

261 (13) 11.1 (6) 15.2 (9) 55.2 (30) _----10.45 0.74 (8) ~ 1.9 6.27 (32) ~0.25 1.87 (14) 2.95 (18) 0.92 (I1) 2.19(14) 0.75 (8) 0.56 (6) 0.36 (6) 0.74 (6) 0.41 (5) ~0.07 0.19 (4)

260 (13) 69 (3) 155(10) ~ 1000 337 (20) 23 (3) 52 (6) 321 (12) 18 (3) 14 (3) 211 (11) 89 (10) 29 (6) 71 (8) 28 (2) 71 47 53 61

K conversion coefficient (units 10 3) Exp. Theor. b

(5) (6) (6) (5)

a Ref. 19. b Ref. 13. e See text for details of normalization.

1000 (80) 161(11) 98 (9) 55 (4) ~ 31.0 32 (5) ~36 19.5(12) ~14 134 (30) 14.0(11) 10.2 (18) 76 (16) 10.5 (16) 19.9 (26) 10.4 (11) 8.8 (16) ~ 1.3 3.1 (7)

E3 : 1140 MI: 172 MI: 111 E2 : 56 E2 : 31.0 E2 : 29.3 E2 : 23.5 E 2 : 19.1 E2 : 17.2 MI: 30.2 E 2 : 13.2 E2 : 10.3 MI: 19.2 E2 : 9.7 MI: 14.7 MI: MI: E2 : MI:

11.4 8.6 1.9 2.8

Multipolarity

E3 MI M1 E2 ~ E2 e E2 (E2) E2 (E2) (E2+MI)+E0 E2 E2 (E2+MI)+E0 E2 (E2+ M 1 ) + E 0 E0 M1 MI (E2) MI

346

D. R. Z O L N O W S K I

A N D T. T. S U G I H A R A

106 1

o

05

152Tb m(4.2 min) CONVERSION ELECTRON

SPECTRUM

~

!~

P,

!t

• [/11

~

Z 0 i0 3 -

. - - ~..,..:~,~.,,,,~ ~

l0 a -

,,.

+

~'-,-"e->,3, o,~.,%,.; g iO3

•

I0 z

780

I 0 860

. ..__ ~.~',..'~'~'~...f.O~,d,~,..c.~.,.j,e.z~.d..ct..

~:

• .~,.~.

~. ....

,,..,,~.¢~..~-d.;.¢.,~...,

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.,-.~...

=

. . . .

i

r

i

i

I

I

200 960

500 IO60

400 1160

500 1260

600 1560

700 1460

CHANNEL NUMBER Fig. 6. Conversion-electron spectrum o f 4.2-rain 152Tbm. A total o f 375 sources were counted for 28 s each.

combining these data with the previously reported p r a y intensities of Bowman et a1.19). The two intensity scales are normalized such that the conversion coefficient of the well-known 344-keV 2 + ~ 0 + transition in 152Gd is E2. Comparison with theoretical values la) yields multipolarities which support previously proposed spin and parity assignments19). Some indication of the capability of this technique is given in fig. 7 which shows the conversion-electron spectrum ofl77Hfm. The half-life of this isomer is only 1.1 s. The activity was produced by the 176yb(cq3n) 177Hfm reaction with 40-MeV alpha particles. Each of 695 sources was collected for 4 s and subsequently counted for 1 s after movement to the counting site. In principle, activities as short as 0.5 s could be studied with the present technique. When the half-life of an activity is more than a few minutes, the helium-jet technique is convenient but not necessary. The present method of conversion-electron spectroscopy has one principal limitation. Since the detector is exposed to all types of radiation simultaneously,

nuclides with strong /~- or c~-particle branches can present difficulties. This is especially true for those cases where the transitions of interest are of higher energy and hence weakly converted. The problems associated with p r a y background are in general not serious• A spectrum taken through a plastic absorber thick enough to stop electrons can be used'to correct for the 7-ray contribution to the total spectrum. The authors express their gratitude to W. W. Bowman and D. R. Haenni for helpful discussions and to M. D. Devous, F. R. Hamiter and M. B. Hughes for assistance in taking data. References 1) j. S. Geiger, R. L. G r a h a m and W. Gelletly, Arkiv Fysik 36 (1967) 197. 2) The I S O L D E Collaboration, The I S O L D E Isotope Separator on-line facility at CERN, ed. by A. Kjelberg and G. Rudstam, C E R N Report 70-3 (1970). 8) M. Finger, R. Voucher, J. P. Husson, J. Jastryzebski, A. Johnson, C. Sebille, R. Henck, J. M. Kuchly, R. Regal,

CONVERSION-ELECTRON

347

SPECTROSCOPY

105

t77Hfm (I.I sec) CONVERSION

÷

ELECTRON

SPECTRUM

I

104

- -

_

o~

~

103

o

_

vv

~1~

04

J

~ l l /~ -A

.

.

.

.

O3 l-Z

Ai

o

(D

102

I0'

0

'

8'0

'

160 '

'

2~0

520 '

'

400 '

CHANNEL

4 ~o

'

5~ 0

'

6~ 0

'

7~ 0

'

8 0' 0

NUMBER

Fig. 7. Conversion-electron spectrum of l.l-s 177Hfm. Each o f 695 sources was collected for 4 s and subsequently counted for 1 s. A b o u t 3 s was required to move a source from the collection site to the detection site.

4) 5) 6) 7) s) 9) t0)

P. Siffert, G. Astner, B. R. Erdal, E. Hagebo, A. Kjelberg, F. Munnich, P. Patzelt, E. Beck and H. Kugler, C E R N Report 70-29 (1970). D. J. Hnatowicz, A. Kjelberg and F. Munnich, Nucl. Phys. A147 (1970) 449. R. D. Macfarlane and R. D. Griffioen, Nucl. Instr. and Meth. 24 (1963) 461. R. D. Macfarlane, R. A. Gough, N. S. Oakey and D. F. Torgerson, Nucl. Instr. and Meth. 73 (1969) 285. W. W. Bowman, T. T. Sugihara and R. D. Macfarlane, Nucl. Instr. and Meth. 103 (1972) 61. E. P. Grigor'ev, A. V. Zolotavin, V. Ya. Klement'ev and R. V. Sinitsyn, Nucl. Phys. 14 (1960) 443. Z. Grabowski, J. E. Thun, M. S. EI-Nesr and W. D. Hamilton, Z. Phys. 169 (1962) 303. G. Hedin and A. Bficklin, Arkiv Fysik 38 (1969) 593.

11) D. R. Zolnowski, Ph.D. Thesis (Univ. o f Notre Dame, 1971). a2) D. C. Camp and F. M. Bernthal, Phys. Rev. C6 (1972) 1040. 13) R. S. Hager and E. C. Seltzer, Nucl. Data A4 (1968); L. A. Sliv and I. M. Band, Alpha-, beta-, and gamma-ray spectroscopy, edited by K. Siegbahn (North-Holland Publishing Company, Amsterdam, 1965) 1639. 14) R. O. Lane and D. J. Zaffarano, Phys. Rev. 94 (1970) 1954. 15) j. S. Geiger, R. L. G r a h a m and G. T. Ewan, Nucl. Phys. 16 (1960) 1. t6) G. K n o p and W. Paul, Alpha-, beta-, and gamma-ray spectroscopy, ed. by K. Siegbahn (North-Holland Publishing Company, Amsterdam, 1965) 1. 17) L. Landau, J. Phys. U S S R 8 (1944) 201. is) p. j. Walsh, Ph.D. Thesis (Univ. o f N o r t h Carolina, 1968). tg) W. W. Bowman, T. T. Sugihara and F. R. Hamiter, Phys. Rev. C3 (1971) 1275.