Nuclear Physics A 734 (2004) E65–E68
www.elsevier.com/locate/npe
Study of neutron-rich argon isotopes in β-decay
J.Mrázeka , S.Grévyb , S.Iulianc , A.Butac , F.Negoitac , J.C.Angéliqueb , P.Baumannd ,
C.Borceac , G.Cancheld , W.Catforde , S.Courtind , J.M.Daugasf , Z.Dlouhýa , P.Dessagned ,
A.Knipperd , G.Lehrsenneaud , F.R.Lecolleyb , J.L.Lecoueyb , M.Lewitowiczf , E.Liénardb ,
S.Lukyanov, F.Maréchald , C.Miehed , F. de Oliveiraf , N.A.Orrb , D.Pantelicac ,
Y.Penionzhkevichg , J.Peterb , S.Pietrib , E.Poirierd , O.Sorlinh , M.Stanoiuf , O.Stodelf ,
O.Tarasovg , C.Timisc
a
b
c
f
LPC, Bd Maréchal Juin, F-14050 Caen Cedex, France
IFIN-HH, PO Box MG6, RO-76900 Bucharest-Magurele, Romania
d
e
Nuclear Physics Institute of ASCR, CZ-25068 Řež, Czech Republic
IreS, Université Louis Pasteur, F-67037 Strasbourg Cedex 2, France
Univesity of Surrey, Guildford, Surrey, GU2 7HX, United Kingdom
GANIL, BP 55027, F-14076 Caen Cedex 5, France
g
FLNR, JINR Dubna, PO Box 79, 101 000 Moscow, Russia
h
Institut de Physique Nucléaire, CNRS-IN2P3, F-91506, Orsay Cedex, France
The study [1] of the neutron rich nuclei near the N=28 magic number was performed at
GANIL-France. This letter reports on preliminary results concerning isotopes 44,45,46 Ar.
Schemes of excited levels were deduced from the γ − γ coincidences. Levels above Sn
energy were suggested from the γ−n coincidences for 46 Ar.
1. Introduction
Behaviour of shell closures at extreme isospin values is one of the challenging topics
of nuclear physics. The beam of 48 Ca [60 AMeV] was fragmented on Be target. The
spectrometer LISE3 was used to select 44,45,46 Cl isotopes that were purified by Wienfilter. Isotopes under study were implanted into double-sided Si detector with 16x16
strips of 3 mm width. Two plastic scintillators were used to detect β radiation emitted
by decays of implanted isotopes and their daughter nuclei. The implantation detector
was surrounded by two coaxial 70% HPGe and one EXOGAM four-fold clover detector.
Neutrons were detected with TONNERRE array [2] positioned in distance of 1.2 m from
the implantation detector and their energy was derived from their time of flight.
0375-9474/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.nuclphysa.2004.03.021
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J. Mrázek et al. / Nuclear Physics A 734 (2004) E65–E68
2. Results and discussions
2.1. Decay of 44 Cl
Excited states of 44 Ar were already studied in deep inelastic reaction [3] where 2+ , (4+ ),
(6+ ) sequence was observed. The states 1.11, 1.78 and 2.61 MeV with assignments 2+
+
and probable 2+
2 , 4 were observed in ref. [4]. Transitions 852.5, 965.1, 1157.5, 2010 and
2796 keV belonging to 44 Ar were reported in β-decay study [5]. The level scheme deduced
from the β decay [5] is referred in Nucl. Data Sheets [6].
Our data confirm levels and transitions reported in ref. [6], the location of 2748 keV
transition in the scheme can be deduced only from energy differences. Moreover we propose three new transitions 3649.8 and 4808 keV based on energy differences and 4195.0 keV
transition also from a weak coincidence with 1158.1 keV transition.
2.2. Decay of 45 Cl
Transitions 541.7, 1192.6, 1228.1, 1524.4 and 2751 keV belonging to 45 Ar were reported
in the β-decay study [5]. The recent in-beam study [7] reports 537 -1228 keV sequence
and levels at 1352 and 1914 keV deexcited to the ground state.
We observe in coincidences sequences of transitions 542.7-1228-1524 keV and 542.72752 keV, what apparently forms a common level at 3295.4 keV. We can also see 542.71193 keV coincidence and a weak coincidence between 1193 and 1560.2 keV lines. The
energy of the latter transition corresponds to the energy difference between 3295.4 and
1735.7 keV levels and thus we place the 1560.2 keV line in between these two levels and,
based on the energy difference, we also place 3295.2 keV transition into the scheme.
We can also see quite an important coincidence of 797.7 and 874 keV transitions with
542.7 keV one. However, we have also found weak coincidences of the two mentioned
transitions with 1228 keV one. Since these transitions were also populated in the β-n
decay of 46 Cl, we were able to compare the data from the two different modes of excitation.
We have found that 797.7 keV transition in 46 Cl decay is by 50% more intensive then the
1228 keV, while in the 45 Cl decay the 1228 keV transition is more intensive than the
797.7 keV by a factor almost 10. The intensity of 874 keV transition behaves a similar
way. Thus we suggest two levels 1340.4 and 1416.7 keV and leave an open possibility of
a doublet and unseen transition).
Shell model calculations presented in [7] suggest two close levels 1240 and 1330 keV
−
−
with spins 3/2−
2 and 5/2 , respectively. This 3/22 should (ref. [7]) contain the most
46
of the intruder strength. As odd-odd Cl have probably one neutron in p3/2 orbital, a
simple view offers a possibility that one neutron in 46 Cl f7/2 can turn into proton in f7/2
via allowed β-decay. This would leave one neutron in p3/2 in 46 Ar and after a neutron
emission from f7/2 orbital would generate intruder states in 45 Ar. Thus we can state that
the increase of intensity of 797.7 and 874 keV transitions in 46 Cl β-n decay supports their
possible ν intruder origin with a role of 2p2h proton correlation. A deeper analysis would
clarify a role of proton excitation in these possible intruder states.
Transitions 1416.1, 3783.8 and 3295.2 keV were placed in the scheme using a sum
rule. Very weak coincidences of 3407 and 2214.8 keV transitions with 542.7 keV one are
bases for tentative levels 3950.2 and 2757.5 keV. Transitions 1955.1 and 2986.2 keV were
observed in coincidences with 1340 and 797.7 keV transitions and transition 2986.2 keV
also with 542.7 keV one.
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J. Mrázek et al. / Nuclear Physics A 734 (2004) E65–E68
3783.8
3
3295.2
12
2214.8
1
2752.6
28
1955.1
2.
1524.7 1
17
4325.9
2757.5
(3/2 −)
1416.1 3
1.0
3295.4
1416.7
1340.4
1340 3.4
−
(?3/2 )
3950.2
1770.7
1735.7
1228.0 36
2.0
2011.3
1158.1
874.0 2.7
3.6
2748.0
1588.7
5.
1818.1 5
6.6
2977.0
2748.0
797.7 4
+
2376.0 15
2796.6 85
2
4807.9
1193.0 14
1.0
+
(2 )
5353.0
3.0
965.7 15
1158.1 140 853.2 100
2.0
2010.6 60
3.0
4.0
1560.2
1.6
3649.8
0.
4808.0 9
2.7
new
5.0
4.0
3407.5
1
2986.2
5.3
seen in beta−n
3341.7
3
4195.0
new
6.0
seen in beta−n
2.5
5.0
542.7
542.7 100
0
0
44
Ar
7/2 −
45
Ar
Figure 1. Deduced level schemes of 44,45 Ar with intensities of transitions. Values in oval
indicate the transitions observed also in β-n decay.
2.3. 46 Cl decay
46
Energy of 2+
Ar 1552 keV is known [3,8]. Recently the structure of 45,46 Ar
1 state of
was studied using in-beam spectroscopy methods in fragmentation reaction [7]. The level
4+ was found at 3.89 MeV, (2+
2 ) was suggested at 3.49 MeV and also new 2.71 keV level
was based on coincidence measurements [7].
We have observed 2005.9 keV and 3349.5 keV transitions in coincidence with 1552.6 keV
line and also a coincidence between 2005.9 keV and 3349.5 keV transitions. The intensity
of the latter is slightly higher but still within the experimental errors, thus the order
of the transitions is uncertain. Based on this sequence of transitions we suggest the
levels 6908 keV and 3558.5 keV. Although our energy 2005.9 keV might correspond to
+
+
1919 keV forming (2+
2 ) in [7], we do not see the 3558.5 keV (22 →0 ) transition, which
was observed in the mentioned experiment. We also see 2515.5, 3953.7 and 4876 keV
transitions in coincidence with 1552.6 keV transition and thus we suggest corresponding
levels. Based on the sum of energies we tentatively place 5725.6, 1181.8 and 922.3 keV
transitions in the scheme.
Number of unassigned low-intensity transitions was also observed in the 46 Cl decay: 775,
1157, 2030, 2697, 2788, 3040, 3884, 4030, 4188, 4687, 6932 keV. The unassigned 1157 keV
transition is a candidate to match the 1140 keV transition in [7], but its intensity is too
low to exclude (from the intensity of coincidences) the location of 1157 keV transition
above 1552.6 keV level.
We can clearly identify 45 Ar gamma transitions 797, 874, 1228, 1339, 1416 keV in
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J. Mrázek et al. / Nuclear Physics A 734 (2004) E65–E68
10.0
Sn +2040
4
1181.8
922.3 3
4876 14
10
10
6428.6
5725.6
5506.3
2515.5
9
6.0
4068.1
+
2
3558.5
5725.6
28
2005.9 8.5
4.0
2.0
Sn +415
8071 Sn
6908
3953.7
7733.3
Sn +1654
Sn +1340
3349.5
8.0 7936
6410 2
6180.7
12
new
1552.6
1552.6 100
0
46
Ar
Figure 2. Proposed level scheme of
46
Ar with intensities of transitions.
gamma spectra gated by the emitted neutron in 46 Cl decay. We also observe an unplaced
1346 keV transition belonging to 45 Ar. Moreover we have found a strong coincidence of
797 keV transition with neutron energy 315 keV and thus we propose a level at 1655 keV
above Sn . We can also see 1.13 MeV neutrons in coincidence with 542 keV gamma
transition, that could deexcite the same level as above. Neutrons of 1500 and 800 keV
are seen in coincidence with 542 keV gamma transition and we could place the levels at
Sn +2040 and Sn +1340 keV into the scheme.
3. Summary and Outlook
New levels and transitions were observed in heavy isotopes of argon. Probable intruder
states were observed in 45 Ar isotope, levels above neutron separation energy Sn were also
identified using neutron detector TONNERRE. Beta feeding intensities and branching
ratios are still to be improved. Acknowledgements to the Center of Excellence of the EC
IDRANAP HH-NIPNE Bucharest-Magurele, WP16 IDRANAP 29-02/2002.
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