Nuclear Instruments and Methods in Physics Research 223(1984) 319 324 319 North-Holland, Amsterdam

RELATIVE ALPHA I N T E N S I T I E S OF SEVERAL A C T I N I D E N U C L I D E S *

I r s h a d A H M A D

(hemi~lrv Dit,iston, Argonne National Laboratory, 9700 South Cass Al~enue, Argonne, lllinms t>0439, USA

]'he relative intensities of alpha groups in the decays of 2~U- , 23Spu, 239pu, 24°pu and 241Am have been measured with a high-resolution semiconductor detector, lsotopically enriched and essentially massless sources, prepared by an electromagnetic isotope scparator, were used in this investigation. All spectra were recorded at low geometries, in order to reduce distortions in alpha intensities due to alpha-electron coincidence summing and had resolutions (fwhm) of - 13 keV. For all of the nuclides studied, intensities have been measured with higher accuracies than previously reported. The intensities of the 24o Pu alpha groups obtained in this work are significantly different from the literature values. For other nuclides, our intensities agree with previous measurements.

1. Introduction

Alpha spectroscopy is a fairly old and well estab- lished technique for the study of nuclear structure and the quant i ta t ive analysis of heavy elements. This tech- nique relies on the fact that most nuclides beyond mass 208 decay by alpha particle emission. Alpha spectros- copy is especially useful for the assay of even even act inide nucldes. In these nuclides, a lpha decay is fol- lowed by highly converted internal transitions, which make gamma ray spectroscopy ineffective.

In the early fifties, when t ransuranic elements first became available, a lpha spectroscopy provided a power- ful tool for de termining nuclear energy levels. The alpha spectra of most t ransuranic nucli ' tes were measured with high-resolution magnetic spectrographs. The focus of these investigations was the de terminat ion of the energies of well resolved a lpha groups with high accu- racy. Accuracies of a few percent in alpha intensities were satisfactory in these studies.

In general, a highly accurate measurement of alpha intensities is difficult with a magnet ic spectrograph for two reasons. First, these ins t ruments have low transmis- sion and second, they use emulsion plates to detect the analyzed a lpha particles. Emulsion plates saturate with a few thousand tracks per ram, thus l imiting the counts in each peak. Semiconductor detectors, however, can be operated at high geometries and there is no limit on peak counts.

Now that high-resolut ion (fwhm - 1 2 keV) Au Si surface barr ier detectors have become commercial ly

* Work performed under the auspices of the Office of High Energy and Nuclear Physics, Division of Nuclear Physics, US Department of Energy under contract number W-31-109- I-iNG-38.

0167-5087 /84 /$03 .00 <,~ Elsevier Science Publishers B.V. (Nor th -Hol land Physics Publishing Division)

available, it is possible to measure alpha intensities with high accuracy. The IEAE advisory commit tee recom- mended [1] that the a lpha intensities of several actinide nuclides (see table l ) be measured to a high accuracy. These s tandards will be used for quant i ta t ive analysis. For this reason, we undertook the series of measure- ments described here.

2. Experimental methods

2.1. Sources

Sources of 233U, 238pu, 239pu, 24°pu and 241Am were

prepared in the Argonne electromagnetic separator. The separator provided a very thin source of extremely high isotopic purity. The sources were prepared [2] by de- posit ing 200 eV ions from the decelerated beam of the isotope separator on to 15 mg cm 2 AI foils. The activity was confined to a spot of 2 - 4 mm diameter and had a

Table 1 Most needed alpha intensities in actinides.

Nu- Half-life Required clide (years) accuracy

in alpha intensity (%)

Needed for

234U 2.45 × 10 5 1

235U 7.04× 10 s 1

23~U 4.47×10 9 1 23JNp 2.14 × 10 6 1 238pu 87.74 0.1 239pu 2.41 × 10 4 1 24°pu 6.57x 10 3 0.2 242pu 3.76×10 s 4

mass determination, fuel assay mass determination, fuel assay mass determination mass determination mass determination mass determination mass determination mass determination

lIl. SPECTROMETRY

10 6 , , , , ,~ ~

F°--°°'% /V 104 I / V

i , o , ~ 10 2 ~_ ,.I 1 ......

I Tail = 3~ of peak

1 0

1 0 1400

t L i 1500 1600

Channel Number 1700

Iotal mass of less than 3 #~g. All sourcc~ c,~ccpt the - F u

.',ource were invis ible .

' ~ < ' t / p h a ~pectra

A l p h a spec t r a of the mass s e p a r a t e d sources were

m e a s u r e d wi th a 6 mm d i a m e t e r A t b Si sur face ba r r i e r

d e t e c t o r p l a c e d in a smal l chambe~ e ~ a c u a t e d te 10 "

Tor r . The d e t e c t o r was c o n n e c t e d to ~ low noise pre-

ampl i f i e r , wh ich had a load res is tor of 100 Mr2. The

p r e - a m p t i f i e r o u t p u t s igna l was p roces sed by a combi -

n a t i o n of an a m p l i f i e r a n d a b i a sed a m p l i f i e r a n d w a s

then fed in to a m u l t i - c h a n n e l ana lyze r . A test pulse .

Fig. I Alpha spectrum of an isotope separator prepared 238pu

source measured with a 25 mm 2 Au-Si surface barrier detector.

Source-to-detector distance was ~ 6 cm. The tail at a channel

400 keV below the a~j peak is 1 /390t.) of the a o peak height.

Energ~ scale is - 2 keV per channel.

10 6

10-'

104

' I ' I I

238pu G spectrum

FWHM = 4.0 keY

FWI0.1)M = 8.4 keV

t-

~ 103 ~ ~ ~ --

<3

102 1

. . .

10 • e

L I , i , , 60 120 140 160

ChannelNumber I i 1

80 t00 I80

r # l >

]

]

--4 4

oo• t.

2;0

Fig. 2. A 238 Pu alpha spectrum measured with a double focusing magnetic spectrometer. This spectrum is only shown for comparison.

1. Ahmad / Re~alice alpha intensities of actinide nuchde.~ 321

introduced in the pre-amplifier, was used for energy cal ibrat ion and spectrum stabilization. The experimen-

tal setup had electronic noise (pulser resolution) of 6.0 keV and alpha particle resolution of 12.5 keV. A 23~pu

alpha spectrum, measured at a geometry of 0.05%, is shown in fig. 1. At this low geometry, a lpha and elec- tron count ing rates were low and hence no alpha elec- tron pile-up is visible at the high energy side of the ~0 peak. Also. because of the low geometry, a lpha electron summing is negligible and hence a large peak to valley ratio is obtained. For comparison, a 23gpu alpha spec-

t rum measured with the Argonne double focusing mag- netic spectrometer [3] is shown in fig. 2.

2.3, Source,~ of error

In order to determine intensities of alpha groups with an accuracy of - 0.1%, it is essential to unders tand various effects which could distort the measured intensi- ties. There are two such effects and we will illustrate them by considering the intensities of the 23Spu ~o and

~43 groups. The most impor tan t source of distort ions in alpha

intensities is alpha electron coincidence summing. The silicon detector is sensitive to low energy electrons and hence correlated a lpha particles and electrons give rise to sum peaks. A 23Spu electron spectrum measured with the same detector used for the alpha spectrum is dis- played in fig. 3. In this spectrum, L and M lines, which are - 1 4 keV apart , are well resolved, Since alpha particles popula t ing the 43 keY level in 234U are in coincidence with the 43 L and 43 M electrons, two sum peaks appear in the ;pectrum. These two peaks lie - 20 keV (energy of the 43 L electrons) and - 3 9 keV

10 4 T

10 3

)

~ 102

g o

23Bpu Eieclrons

0 i00 200 300 400 Channel Number

Fig. 3. Elec t ron spectrum of 23Spu measured with the sanle source and detector as the alpha spectrum in fig. 1. Energy scale is ~ 0.32 keV per channel.

1 0 6 ~ - I I I I I ~ 0 [

# "k

°N" I1 ;3Spu ?t ~ t t

I0~ - G .... try 2.0% i ~ f I

I 2' /t i / J / t " , 7

103 i

102 l/ I I L 1560 1580 1600 1620 1640

Channel N u m b e r

Fig. 4. A 231~pu alpha spectrum measured with a 25 mm 2 Au-Si surface barrier detector at a geometry of 2.0¢;. Accidental pileup and alpha-electron sum peaks are present in the spec- trum. In the spectrum analysis, peak shape was derived from a 2~pu spectrum measured at 0.05% geometo.

(energ~ of the 43 M electrons) above the a43 peak. The sum peak with the 43 M electrons falls under the main peak ,% and in the analysis it is counted as a part of the a 0 peak. The 0'43-f-43 L sum peak appears between the ,% and a43 peaks and, with a high-resolu- tion system, it can be resolved, as is done in fig. 4. The net effect of a lpha -electron coincidence summing is to increase the intensity of the ~0 group and to decrease the intensity of the o~43 group.

The intensities of the sum peaks can be calculated from the detector geometry ~2. For example, the inten- sity of the ~a3 + 43 L sum peak, 1<, L, is given by

I ,H : 143 " ~ " I t . , ( l )

where 143 is the intensity of the ~x43 group and lt. is the n u m b e r of L electrons emitted per a43 decay. In 23~pu, 11_ = 0.7 and 1 M = 0.3.

The second effect which distorts a lpha spectra is the accidental pile-up of alpha pulses with electron pulses. The random coincidence pile-up rate. N~,,, j<,., . is given by

N ,,,a,,,, = N<,-,~. • 2r, (2)

where N , A,~. , and 2 r denote the alpha count rate, the electron count rate and the resolving time, respectively. The pile-up of a 0 pulses with the 43 L electron pulses gives rise to a small peak in the spectrum shown in fig. 4. Because of pile-up distortions, spectra should be measured at low count rates. The pile-up effects can also be el iminated electronically, with the help of a pile-up rejector.

111. SPECTROMETRY

322 1 A b r o a d Re/alt~'e a lpha Inten~ttws o~ acl~nlat ~m¢ //d~'~

It is worth pointing out that both the a lpha-e lec t ron

coincidence summing and alpha electron accidental pile-up could be reduced or eliminated, b'r using ~

magnetic field to deflect the Iov, energy electron~ awa\

from the detector.

3. Results

The measured alpha spectra were plotted and analyzed by hand. The peak shape was derived from the

most intense peak in each spectrum. For spectra mea-

sured at high resolution and low geometry, determina-

tion of peak areas is straightforwmd. Since it is difficult

to determine the counts under the peak and its ta i . we

obtained areas from the top to 1/300 maximum for each p e a l as shown in fig. 5. In the spectrum, the tail Ls

not as clearly defined as the main peak. However. this

problem does not introduce large uncertainties in the

intensities, because the area under the tail is small

compared with the counts in the main peak.

For each nuclide, several spectra were measured and analyzed. The 238pu and 241Am spectra were measured

at 0.05% geometry, whereas spectra of other nuclides were measured at a geometry of 0.20%. The average values of intensities from several runs along with uncer- tainties. 1 o. are given in table 2. These intensities have

been corrected for small a lpha-e lec t ron summing. For compartson, previous measurements are included in this

table. Only two references are given for each nuclide:

106 =- 23apu source: 2.0 × 106 o Opm

/"1 ~'>" , l Geometry = 0.05% ,,43 f '~

105 E " "%" ,/

6 / ~'f

10 3 Z /

1 0 2 ~ . . . . . . . . . . . . . .

440 460 480 500 52-0 540 . . . . 56% . . . . ,580 Channel Numioer

Fig. ~. Analysis of a '~Spu alpha spectram The arrow~, arc placed at 1/300 of maximum and denote the channels between which peak areas are determined for a pha intensit'~.

additional references can bc obtained in the "Table ~f isotopes'" [4]. Most original references d~ not contain

errors and hence these are not given m the table. All

previous measurements of the alpha intensities except

those reported in ref. [7] were made with magnetic spectrographs. The 23Spu alpha mtensmes in ref, 17]

were measured with a Au-S i surface barrier detector

and these are in excellent agreement with our values, As can be seen in table 2. our intensities are in general agreement with old measurements, with the exception of

105

== 6 ~Q,. 104

8

240pu

Geometry 0.20% p ~=

= 22 V

"'"'"'""/ /

103 ~ ~ F / : ./,,..'

280 300 320

p A

34O Channe l N u m b e r

360 380 400

Fig. 6 Analysis of a 24°pu alpha spectrum. Arrows have Ihe same s~gmficance as in fib 5

-]

i

1. Ahmad / Relatwe alpha intensities of actmide mwlides 323

Table 2 Summar~ of alpha intensities.

Nuclide Energy (MeV) [4]

Excited state (keY)

Intensity in g per alpha decay

Present work Previous measurements

Intensity' Reference lntensit> Reference

2~k; 4.824 0 4.783 42 4.729 97

>~Pu 5.499 0 5.457 43 5.359 143

>gPu 5.155 0 5.143 13 5.105 52

241)pu 5.168 0

5.123 45 5.014 149

241Am 5.545 0

5.513 33 5.486 60 5.443 103 5.389 159

82.7 _+0.3 83 [51 84.4 161 14.9 +0,2 14.6 13.2

1.85 +0.05 1.5 1.6 70.9 _+0.1 70.7 _+0.2 [7] 72.2 18 I 29.0 _+0.1 29.3 +0.2 27.8 0.106 _+ 0.003 0.1 0.068

69 [9] 73.3 [10] 88.0 +0.3

20 15.1 12.0 _+0.2 11 11.5 72.8 _+0.1 76 [9] 75.5 Ill] 27.1 _+0.1 24 24.4 0.090 _+ 0.005 0.1 0.091 0.36 _+0.01 0.42 [12] (}.25 I13] 0.23 _+0.01 0.24 0.12

84.0 +0.2 84.2 + 1.5 86 13.1 -+0.07 13.6 _+1.4 12.7 1.65 _+0 .08 1.42_+0.15 1.3

24°pu. However, intensities of the main alpha groups

measured in this work are consistently lower than the

more recent measurements with magnetic spectro- graphs. Our intensity of the 24°pu o~ 0 group is substan- tially lower than the two previously measured values

[9,11]. Since we used a highly enriched source prepared

in an isotope separator, we feel that our intensities are correct. Also the 240 Pu alpha spectrum (fig. 6) shows no

sign of any :3'~Pu in it which could distort alpha intensi- ties. It is likely that higher intensities of the group in

previous measurements were caused by the presence of 2)'~Pu in the source.

The intensity of the 240p 0:45 group can be derived from the measured intensity of the 45.2 keV gamma ray

and its theoretical conversion coefficient. The absolute intensity of the 45.2 keV gamma ray has been measured

by Hehner and Reich [14] and Gunnink et al. [15] as 0.0435 and 0.0453%, respectively, with an uncertainty of - 2%. Using the average of the above two values for the intensity of the 45.2 keV gamma ray and the theoretical conversion coefficient [16] of 612 for E2 nmhipolarity, we calculate the intensity of the ot45 group as 27.2 _+ 0.9%. This intensity is in excellent agreement with the value of 27.1 + 0.1% measured in the present work.

4, Comments

The intensities of alpha groups measured in the present work are generally in good agreement with older measurements, with the exception of 24°pu. However, our intensities of the rnain alpha groups in each nuclide

are consistently lower than the more recent measure- ments with magnetic spectrographs. A large discrepancy occurs for the 24°pu alpha intensities between the pre-

sent and previous measurements. It is recommended that an independent measurement of 24°pu alpha inten-

sities be undertaken.

The author wishes to thank the late J. Lerner for isotope separator preparation of sources and A.H. Jaf-

fey and E. Browne for helpful discussions.

R e f e r e n c e s

[1] Proc. Advisory Group Meeting on Transactinium Isotope Nuclear Data, Karlsruhe, Fed. Rep. Germany, November 3-7, 1975, vol. 1 IAEA-186 (IAEA, Vienna, 1976).

[2] J. Lerner. Nucl. Instr. and Meth. 102 (1972) 373. [3] 1. Ahmad and J. Milsted, Nucl. Phys. A239 (1975) 1. [4] C.M. Lederer and V.S. Shirley, Table of isotopes (7th ed.)

(Wiley, New York, 1978). [5] B.S. Dzhelepov, R.B. Ivanov, V.G. Nedovesov and Yu T.

Puzynovich, lzvest. Akad. Nauk. SSSR Ser. Fiz 24 (1960) 258.

[6] S.A. Baranov, M.K. Gadziev, V.M. Kulakov and V.M. Shatinskii, Yad. Fiz. 5 (1967) 518; Soy. J. Nucl. Phys. 5 (1967) 365,

[7] J.C. Soares, J.P. Ribeiro, A. Gonclaves, F.B. Gil and J.G. Ferreira, Compt. Rend. 273B (1971) 985.

[8] S.A. Baranov, V.M. Kulakov, V.M. Shatinskii and Z.S. Gladkikh, Yad. Fiz. 12 (1970) 1105.

[9] F. Asaro and I. Perlman, Phys. Rev. 88 (1952)828. [10] S.A. Baranov, V.M. Kulakov and S.N. Belenky, Nucl.

Phys. 41 (1963) 95.

I11. SPECTROM ETRY

324 I. A h m a d / Relalit,e ulpha itttenstrtc~ ,,~ a ( t m m c nt;¢ It~l~.~

[11] L.L. Gol'din, G.I. Novikova and E.F. Tret'yakov. Phys.

Rev. 103 (1956) 1004. [12] F. Asaro, cited in ref. [4]. [13] S.A. Baranov. V.M. Kulakov and V.M. Shatinsk,,. Nlucl.

Phys. 56 (1964) 252. [14] R.G. Helmer and C.W. Reich, Int. J. Appl. Rad lsot. 32

(1981) 829.

[15] R. Gunnink. J.IL Evans and A. PrinUle. ~. S ERI)A Repo~l UCRL-52139 ~ 1976 ,.

[16] R.S. Hager anti E.C. Seltzer. Nucl Oata A4(19Ob~ I ~ Dragoun Z Plajner and F Schnu~tter. ~b~d \t~ (1o7~ I1Q