Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 187–190, 2002

187

Decay Studies of Neutron-deficient Am, Cm, and Bk Nuclei Using an On-line Isotope
Separator
M. Asai,∗,a M. Sakama,b K. Tsukada,a S. Ichikawa,a H. Haba,a I. Nishinaka,a Y. Nagame,aS. Goto,c K. Akiyama,a,d A. Toyoshima,a,e Y. Kojima,f Y. Oura,d H. Nakahara,d M. Shibata,gand K. KawadehaAdvanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195,JapanbDepartment of Radiologic Science and Engineering, University of Tokushima, Tokushima 770-8509,
Japanc
Department of Chemistry, Niigata University, Niigata 950-2181, Japand
Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japane
Department of Chemistry, Osaka University, Toyonaka, Osaka 560-0043, Japanf
Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japang
Facility for Nuclear Materials, Nagoya University, Nagoya 464-8603, Japanh
Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japan
Received: February 18, 2002; In Final Form: February 18, 2002
The EC and α decays of neutron-deficient Am and Cm nuclei have been studied using a gas-jet coupled on-line
isotope separator. Decay schemes of the EC decay of 235, 236 Am have been constructed, and weak α decays of
233, 235, 236
Am and 237, 238 Cm have been observed. The efficiency of the on-line mass separation of Bk nuclei wasmeasured to be ∼1%. The Qα values, α-decay partial half-lives, and proton-neutron configurations are discussed.

5/2- [523]∗Corresponding author. E-mail: asai@tandem.tokai.jaeri.go.jp.
FAX: +81-29-282-5927.

1118.8

5/2- [752]
5/2 ;3/2+[631]3/2+;1/2+[631](5/2- )+
(5/2 )

5/2+[633]

639.0
535.1

(3/2+)(7/2+)
+
5/2

290.6
265.3
183.7
41.9

+

825.9

183.7 (20)

(5/2+)

1029.6290.6 (100)248.6 (19)265.3 (35)223.5 (42)5/2 [622]+

Neutron-deficient Am and Cm nuclei were produced by
the reactions of 233 U(6 Li, xn)233−235 Am, 235 U(6 Li, 5n)236 Am, and237
Np(6 Li, xn)237, 238 Cm at the JAERI tandem accelerator facility. A stack of twenty-one uranium or neptunium targets set ina multiple-target chamber with 5 mm spacings was bombarded
with a 6 Li beam of about 400 particle-nA intensity. Each tar-

IEC(%) log ft

373.7 (33)351.4 (30)269.7 (38)244.6 (13)

2. Experiments

10.3(6) min(5/2- )
0
235
95Am
EC

828.3 (27)739.0 (36)Spectroscopic studies of heavy and superheavy nuclei give us
information on various nuclear properties in heavy element region. Measured α-decay energies lead to the precise determination of atomic masses which are one of the most essential quantities to define nuclear decay properties, fission and fusion probabilities, and shell stability in heavy nuclei. Gamma-ray spectroscopy following α, β, and orbital-electron capture (EC) decays and fine structure of α-particle spectra reveal excited statesin daughter nuclei, single-particle energies, proton-neutron configurations, and nuclear deformation.
Neutron-deficient Am, Cm, and Bk nuclei predominantly decay by EC, and their α-decay probabilities are very small. This
property makes experimental studies for these nuclei difficult. In
order to measure γ rays following the EC decay and very weakα transitions, it is quite important to isolate the nuclei of interest
from a large amount of other reaction products. The traditional
method to separate heavy nuclei is chemical separation owing
to its high separation efficiency. Another efficient method is inflight separation using recoil separators. However, these methods are not so suitable to apply to the neutron-deficient Am,
Cm, and Bk nuclei; their half-lives are in the order of a few minutes, and their α-decay intensities are very small. In this work,
we have studied EC and α decays of the neutron-deficient Am
and Cm nuclei using the gas-jet coupled JAERI on-line isotope
separator (ISOL)1 and also succeeded in the on-line mass separation of Bk nuclei. Extremely low-contaminated α/γ sourcesfrom the ISOL enabled us to observe weak α/γ transitions withunambiguous mass identification.

get was electrodeposited with an effective thickness of about
100 µg/cm2 . Reaction products recoiling out of the targets were
stopped in He gas loaded with PbI2 clusters, and transported
into an ion source of the ISOL with gas-jet stream through an
8 m long capillary. Atoms ionized in the surface ionizationtype thermal ion source were accelerated with 30 kV and massseparated with a resolution of M/∆M ∼ 800. The overall efficiency of this ISOL system was measured to be 0.3% for 237 Am
produced in the 235 U(6 Li, 4n) reaction.2
For γ-ray measurements, the separated ions were implanted
into an aluminum-coated Mylar tape in a tape transport system,and periodically transported to a measuring position equippedwith two Ge detectors. Gamma-ray singles, γ-γ coincidence,and γ-γ delayed coincidence measurements were performed. For
α-decay measurements, the separated ions were implanted into
10 µg/cm2 thick PVC/PVAc foils set on the periphery of a fourposition rotating wheel. The wheel periodically rotates 90◦ toconvey the implanted sources to three consecutive detector stations. Each of the detector stations was equipped with two Si

642.2 (20)

1. Introduction

41.9
0

< 50%

235
94Pu141

Figure 1. A partial decay scheme of the EC decay of 235 Am.

c 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002> 5.8

188 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Asai

#5/2 [523] 5/2 [633] - #5/2 [523] 5/2 [633] -

+

-

5

-

-

(2 )
(0-)

5

5

+

K=5- isomer

1.2(3) µs

Octupole band
K=0

-

0

-

53
1

Am

6+
Ground state band
K=0+1340.81311.51185.5

866.1
758.1
698.3
158.36 (69)102.82
44.63

+

+

K=0-

1

3.6(2) min
2.9(2) min( 0 ) (1-)( 0+x )
5236
236

1021.6 (39)718.6 (154)560.3 (66)713.3 (89)610.8 (52)698.3 (83)653.68 (117)#5/2 [523]#5/2 [642] K=0 rotational band
#5/2 [523]#5/2 [642] -642.5 (73)582.8 (139)613.2 (80)1037.8 (81)879.7 (152)319.50 (100) M1

K=5-

+

4
+
2
0+

305.81147.4544.63
0EC
IEC(%) log ft

AmEC
68(8)% 4.8(1)26(4)% 5.3(1)97( +3- 8 )% 4.9(1)

+
3( +8
- 3 )% 6.6( - 0.7)
+6( +8
- 6 )% 6.2( - 0.4)

)
0.1(-+80.1) 8.3( -+2.0
< 17%

> 6.0

< 22%

> 5.8

236
94Pu142

Figure 2. A decay scheme of the EC decay of 236 Am.

detectors placed on both sides of the foil to measure α particles
with 85% efficiency. An additional Si detector was placed at
the implantation station just behind the foil. To determine α/EC
branching ratios, a Ge detector was placed at the first detector
station. Alpha and γ-ray singles, and α-γ coincidence measurements were performed. The α decay of 237 Cm was also measured with another experimental setup where the separated ionswere implanted directly into a Si detector. For 238 Cm, the tapetransport system equipped with seven Si detectors was used tomeasure its half-life. Details of the experimental procedures are
given in Reference 3 and in a forthcoming paper.
3. Results and Discussions
3.1. EC Decay of 235,236 Am. Figure 1 shows a partial decay
scheme of 235 Am established on the basis of the observed γ-γ
coincidence relationships. The <50% EC-branching intensity
to the ground state of 235 Pu was deduced from the comparison
between the observed Pu KX-ray intensity and the calculated
one based on the proposed decay scheme.
According to the energy systematics of Nilsson singleparticle states, the 95th proton of the ground state of 235 Am is expected to occupy either the π5/2− [523] orbital or the π5/2+ [642]one. Around the neutron-deficient Am region, most of EC andβ− transitions show log ft ≥ 5.8 except for the transitions between the π5/2+ [642] and the ν5/2+ [633] orbitals whose log ft
values are 5.2 ∼ 5.4.4 If the configuration of the ground stateof 235 Am is the π5/2+ [642], the EC transition to the ν5/2+ [633]ground state in 235 Pu should have a small log ft value of ∼5.3.
The deduced log ft value of >5.8 therefore suggests that the
π5/2+ [642] assignment is unlikely for the ground state of 235 Am.
The EC decay of 237−240 Am is characterized by the intense π5/2− [523] → ν5/2+ [622] transition with log ft = 6.0–
6.2.5–8 The observed 535 keV level in 235 Pu is a candidate forthe ν5/2+ [622] state because of the large EC feeding to thislevel. The 41.9 keV level in 235 Pu is most likely the 7/2+ statein the 5/2+ [633] band. The 291, 265, and 184 keV levels are
considered to be the 5/2− [752] band head, the 5/2+ state in the
3/2+ [631] band, and the 3/2+ state in the 1/2+ [631] band, respectively, owing to the γ-ray branching ratios from these levels
to the 5/2+ [633] band compared with those in the N = 141 isotones.
Figure 2 shows a decay scheme of the EC decay of 236 Am.
Only the ground state band with spins up to 16+ has been known

in 236 Pu (Ref. 4). The 698, 758, and 866 keV levels are considered to be the 1− , 3− , and 5− states in the K π = 0− octupole
band which typically appears at low excitation energy in lightactinide nuclei. The energy spacings and γ-ray branching ratios
are consistent with this assignment. From the KX to γ intensity ratio, the K-internal conversion coefficient of the 320 keVtransition was deduced to be αK = 0.87(21). This reveals thatthe 320 keV transition has an M1 multipolarity, and thus, thespin-parity of 5− is assigned to the 1186 keV level. It has been
also found that the 1186 keV level is a K isomer with K π = 5−
and t1/2 = 1.2(3) µs. Decay curves of γ rays have revealed that
there are two EC-decaying states in 236 Am; one is a high-spinstate with T 1/2 = 3.6(2) min, and the other is a low-spin one with
T 1/2 = 2.9(2) min.
The EC transitions from 236 Am to the 1186, 1312, and
1341 keV levels in 236 Pu show small log ft values of 4.9, 5.3,
and 4.8, indicating that the π5/2+ [642] → ν5/2+ [633] transition
largely contributes to these transitions, that is, the configuration of the 236g,m Am must include either the π5/2+ [642] or theν5/2+ [633] orbital. Since the transition to the 1186 keV 5−
state is allowed, the EC-decaying high-spin state has an odd
parity. These restrictions in conjunction with the energy systematics of Nilsson single-particle states in Z = 95 and N = 141
nuclei lead to the (π5/2− [523]ν5/2+ [633])5− assignment for the
EC-decaying high-spin state, and this 5− state is considered tobe the ground state according to the Gallagher and Moszkowski
coupling rule.9 The candidate for the low-spin isomer is the0− state or its signature partner 1− state with K π = 0− and the
π5/2− [523]ν5/2+ [633] configuration.
Since the 236g,m Am have the π5/2− [523]ν5/2+ [633] configuration and the occupied π5/2+ [642] orbital, the π5/2+ [642] →ν5/2+ [633] transition generates the π5/2− [523]π5/2+ [642] configuration, hence the 1186, 1312, and 1341 keV states in 236 Pu
are assigned to be the π5/2− [523]π5/2+ [642] two-quasiparticle
states. The π5/2− [523]π5/2+ [642] two-quasiparticle states werealso reported in 240 Pu at 1309, 1411, and 1438 keV with spinparities of 5− , 0− , and 2− , respectively.4 The 1309 keV levelis also the K π = 5− isomer, and the 1411 and 1438 keV levels
are interpreted as the K π = 0− (π5/2− [523]π5/2+ [642])0− stateand its 2− rotational band member, respectively.10 If the 1312
and 1341 keV levels in 236 Pu are the same 0− and 2− states, the236m
Am is assigned to be the 1− state with K π = 0− . The intensity ratio between the EC transitions from 236m Am to the 1312

Decay Studies of Neutron-deficient Am, Cm, and Bk Nuclei
and 1341 keV levels is consistent with this interpretation.11
3.2. Alpha Decay of 233−236 Am. The α decay of 233 Am
was unambiguously identified through the observation of the
α-α correlations between the α decay of 233 Am and following five-successive α decays of 229 Np→225 Pa→221 Ac→217 Fr→
213
At→209 Bi. The observed α-particle energy for 233 Am was
6780(17) keV, and the half-life of 233 Am was determined to be
3.2(8) min from the decay curve of the α particles. Pu KX rays
associated with the EC decay of 233 Am were not observed. Taking account of the detection efficiency for the Pu KX rays, the
α-branching intensity was estimated to be I α > 3%.3
For the α decay of 234 Am, Hall et al.12 reported a 6.46 MeV
α group with an α/EC branching ratio of 3.9(12) ×10−4 and a2.32(8) min half-life determined from the decay curve of ECdelayed fission events. In the present experiment, Pu KX-ray
peaks were weakly observed in the mass-234 fraction, but no αpeak was observed at 6.46 MeV; the upper limit of I α < 0.04%
was deduced for the 6.46 MeV α group.
In the mass-235 fraction, the 6457(14) keV α peak whose
half-life was in good agreement with that of the Pu Kα X
rays was clearly observed. The deduced half-life value of10.3(6) min is consistent with the literature value of 15(5) min
by Guo et al.13 extracted from the growth and decay of 235 Pu in
chemically purified Am fractions. The α-branching intensity ofI α = 0.40(5)% was derived from the ratio between the observed
α and Pu KX-ray intensities. No α-γ coincidence event was observed for the 6457 keV peak, indicating that the upper limit of
the expected E1 transition energy between the 5/2− [523] state
and the 5/2+ [633] ground state in 231 Np should be <15 keV.
Hall14 reported a 6.41 MeV α group with an α/EC branching ratio of 4.2(6) ×10−4 for 236 Am. In the present experiment,
however, no 6.41 MeV α peak was observed; the upper limitof the branching intensity for the 6.41 MeV α particles was
I α < 0.002%. Instead of it, a very weak α group with a fewminute half-life and I α = 0.004% was observed at 6150 keV. We
tentatively assign this α group to that associated with 236 Am.
αfor the groundFigure 3 shows α-decay partial half-lives T 1/2
state to ground state transitions in even-even Pu and Cm isotopes
and those for the favored transitions in odd-mass 237−243 Am isotopes as a function of the released α-decay energy E∗α which
is a measured α-particle energy corrected by its recoil energy
αand electron screening.15 The T 1/2
values of 233, 235 Am lie on the15semi-empirical curve fitted to the values of the favored transitions in 237−243 Am, indicating that the α transitions observed

1016

1012

248

Am
241

246

Am

Cm

239

Pu

Am

1010

237
238

108

244

Pu
236

Cm
242

Pu

234

Am
235

Cm
240

6

10

Cm
233

Pu

5000

6000
*
Eα / keV

20

(a)

237

A = 237

Np
237

10

236

Am

237

Cm

Pu

0

Am
Am

104
102

Neutron-deficient Bk nuclei have been studied scarcely because of their short half-lives and small α-branching intensities. The 238, 240 Bk were identified through the observation ofEC-delayed fission events,18, 19 and Cm KX rays associated with
the EC decay of 242 Bk (T 1/2 = 7.0 min) were observed throughthe chemical separation.20 In future plans, we intend to measure the EC decay of 241 Bk produced in the 239 Pu(6 Li, 4n)241 Bk
reaction using the present ISOL system. In order to check theionization efficiency of Bk, the on-line mass separation of 250 Bk(T 1/2 = 3.2 h) produced in the transfer reaction with a 248 Cm target and 18 O projectiles has been performed. The intensity ofγ rays from the mass-separated 250 Bk was compared with thatmeasured by using only the gas-jet transport. The intensity ratio of 2% was obtained. This value does not include only thegas-jet transport efficiency which is about 50% in the present
system. Thus, the overall efficiency is estimated to be ∼1%
which is comparable to that of Am isotopes. The ionization efficiency depends not only on the ionization potential of eachelement but also on the vapor pressure. The ionization potential

243

Cm

Pu
240

4. Future Plans

Counts / 10 keV

242

189

in 233, 235 Am are the favored transition. Since the ground stateof 235 Am has the π5/2− [523] configuration, the favored transition populates the π5/2− [523] state in 231 Np. The π5/2− [523]state in Np isotopes decreases in energy with decreasing neutron number, 60, 49, and <15 keV in 237 Np, 235 Np, and 231 Np,
respectively, corresponding to the decreasing deformation, andin 229 Np it is expected to become further lower. Since the uncertainties of the level energies populated by the α decay of
233, 235
Am are as small as those of the α-particle energies, we
can determine the Qα values of 233, 235 Am within ±20 keV uncertainties. The deduced Qα values are by 100–200 keV smaller
than the evaluated ones by Audi et al.16
3.3. 237 Cm and 238 Cm. Figures 4(a) and (b) show αparticle spectra observed in the mass-237 and 238 fractions separated from products in the 237 Np(6 Li, xn) reaction. The α lines
observed at 5500, 5773, and 6047 keV are attributable to 238 Pu,
236
Pu, and 237 Am, respectively. Taking account of the mass
resolution of the present ISOL system, about 0.2% of massseparated 236 Pu may be observed in the adjacent mass-237 fraction. The 6660(10) keV line was attributed to the α decay of237
Cm. Its mass identification was confirmed by measuring theadjacent mass fractions. The α-particle energy of 238 Cm was revised from the literature value of 6520(50) keV17 to the present
one of 6560(10) keV. The half-life of 238 Cm was determined tobe 2.2(4) h, while that of 237 Cm could not be determined becauseof its less statistics.

7000

Figure 3. Alpha-decay partial half-lives as a function of the released
α-decay energy.

Counts / 10 keV

α-decay partial half-life / s

1014 244Pu

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

20

(b)

10

0
4000

238

238

Cm

A = 238

Pu

5000
6000
7000
α-particle Energy / keV

8000

Figure 4. Alpha-particle spectra observed in (a) the mass-237 fractionand (b) the mass-238 fraction.

190 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
increases with increasing atomic number from Am to probably
No, which makes the ionization efficiency decrease. However, if
the vapor pressure increases, the ionization efficiency increases.
Therefore, it may be expected that Es, Md, and No isotopes arealso ionized efficiently and their EC decays are studied using thepresent ISOL system.
The other plan is α-γ coincidence measurements for 259,261 Rf
and 257 No to assign spin-parities and Nilsson orbitals in N > 153
nuclei. In this experiment, we only use the gas-jet transport
without using the ISOL, and γ, KX, LX, and internal conversion electrons in coincidence with α particles are measured withgood statistics. Then, we deduce level energies and multipolarities of γ transitions from internal conversion coefficients. In
combination with α-particle energies and their hindrance factors, we can extract information on spin-parities and Nilsson orbitals of excited states in daughter nuclei as well as the groundstates of the parents. In addition, we can also determine Qα values without any assumptions on the populated level energies.
The nucleus 261 Rf is located on the α-decay chain starting fromthe nucleus 277 112. The above spectroscopic data provide anexperimental basis to discuss nuclear structure in heavy and superheavy region.
Acknowledgement. We would like to thank the crew of the
JAERI tandem accelerator for generating an intense and stable 6 Li beam. This work was partly supported by the JAERIUniversity Collaborative Research Project.
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