Gamma-ray spectroscopy of ΛBe : B(E2) …...showed a much smaller B(E2) value of E2(5/2+→1/2+)...
1
Be BeTe BeO Gamma-ray spectroscopy of 9 Λ Be : B(E2) measurement of cluster-like hypernuclei Dept. of Phys. Tohoku Univ., Japan. K. Shirotori for the Hyperball-J collaboration 8 Be is known as the α-α cluster nucleus and it has an importance of having the simplest α-cluster nuclei. The ab initio calculation predicts a well developed α-α cluster structure, and the recent experiment[2] shows that the B(E2) value of 4 + →2 + transition agrees with this calculation. 8 Be is an unbound nucleus and the direct B(E2) measurement is experimentally very difficult. The γ-ray spectroscopy experiment of 9 Λ Be, in which a Λ was added to 8 Be, was performed in BNL-E930(‘98). We obtained the upper limit for the lifetime less than 0.1 ps[3] from the measurement via DSAM of the corresponding 2 + →0 + transition in 8 Be. However, the measured lifetime is much shorter than that predicted by the cluster model[4] since the calculation resulted in a sizable reduction in B(E2) value reflecting a shrinkage in the inter α cluster distance with a presence of Λ. This inconsistency between the data and the theory shows that the theoretical prediction based on the cluster picture of 9 Λ Be fails to apply to 9 Λ Be, while a good agreement is seen for 7 Λ Li. The experimental indicates a small change in the core 8 Be structure and suggests that 8 Be has rather shell-like structure than the 2α cluster. Thus measurement of B(E2) of 9 Λ Be is important not only to understand the 8 Be structure but also to test the cluster model in light nuclei. Nuclear shrinkage effect in Nuclear shrinkage effect in 7 Λ Λ Li Li 19% shrink Tanida et al., Phys. Rev. Lett. 86 (2001) 1982 γ-ray energy spectrum of E2(5/2 + →1/2 + ) transition τ(5/2 + )=5.8± 0.9 0.7 ±0.7 ps 7 Λ Li level scheme 0 Study of Study of 8 Be structure from the Be structure from the γ γ- ray spectroscopy of ray spectroscopy of 9 Λ Λ Be Be E [MeV] <r>α-α [fm] B(E2) [e 2 fm 4 ] τ [ps] 8 Be 理 3.1 3.04 4 4 22 15 0.15 0.2 9 Λ Be 理 3.1 3.5 11 0.3 9 Λ Be 実 3.05 ? 22> <0.1 Red : T. Motoba et al., Prog. Theor. Phys. 70 (1983) 189 Blue : R.B. Wiringa et al., Phys. Rev. C62 (00) How many hyperisotope can we produce ? : 5 Be hyperisotope 9 Be(K - , π - ) 9 Λ Be : Aiming for B(E2) measurement 10 B(K - , π 0 ) 10 Λ Be : Need to construct π 0 spectrometer 11 B(K - , π 0 ) 11 Λ Be : Need to construct π 0 spectrometer 12 C(K - , π + ) 12 Λ Be : DCX (double charge exchange reaction) , small cross section (~0.1 µb/sr) 13 C(K - , π + ) 13 Λ Be : DCX, small cross section, optimum enrich carbon target Level energy : change in excitation energy B(E2) : 9 Λ Be, 11 Λ Be, 13 Λ Be Experimentally an impurity effect of Λ was observed for the first time from the γ-ray spectroscopy of 7 Λ Li. Lifetime measurement via the Doppler shift attenuation method (DSAM) showed a much smaller B(E2) value of E2(5/2 + →1/2 + ) transition than the E2(3 + →1 + ) transition in the 6 Li core. The theoretical calculation[1] attributes 19% shrinkage of the α-n-p cluster distance to reproduce the measured B(E2) value. B(E2: 3 + →1 + : 6 Li) = 9.3 ±0.5 e 2 fm 4 ⇒B(E2: 5/2 + →1/2 + : 7 Λ Li) = 3.6 ±2.1 e 2 fm 4 ＊B(E2)∝R 4 , 1/τ∝B(E2)E γ 5 The study of hypernuclei is one of the ways to understand the baryon-baryon (BB) interactions, through 1) the investigation of hyperon-nucleon interactions, 2) the properties of baryons in the nuclear matter, and 3) impurity effects of Λ on the core nucleus. The ΛN interaction is studied through the Λ hypernuclear level structure and its precise structure can only be observed from the γ-ray spectroscopy by using germanium (Ge) detectors. The method of γ-ray spectroscopy with Ge detectors has been successfully used to study structure of light p-shell Λ hypernuclei. We discuss the third subject, the effects of Λ as an impurity. Even a single Λ added to a nucleus may drastically change the properties of the nucleus, such as size, deformation, collective motions, shell /cluster structure, and etc. These effects had not been observed until the shrinkage of 7 Λ Li was experimentally confirmed. Thus the hypernuclear γ−ray spectroscopy offers a new area in nuclear physic, that is, “impurity nuclear physics”. The change in the core nucleus or the core polarization induced by a Λ arises from its “glue-like role” in a nucleus. In particular, this glue-like aspect of Λ can be used to probe light nuclei as to whether they posses cluster-like or shell-like structure. If the core structure is forming clusters, inter cluster distance can be shrunk leading to a dramatic structural change. On the other hand, the core nucleus is close to the shell structure, the inclusion of Λ is expected to have a small effect. By looking at the changes of the core of a hypernucleus, it will be possible to study the structure of a nucleus. The ΛΝ two-body potential is attractive and provides extra binding to a hypernucleus that brings an unbound normal nucleus to a bound hypernucleous. Therefore, we gain access to energy levels and the B(E2) measurement of the otherwise unbound core nucleus via γ-ray spectroscopy experiment. Introduction Introduction B(E2) measurement of B(E2) measurement of 9 Λ Λ Be by Doppler shift attenuation method (DSAM) Be by Doppler shift attenuation method (DSAM) 9 Be (K - , π - γ) 9 Λ Be 43±5 keV E γ (keV) Akikawa et al., Phys. Rev. Lett. 88 (2002) 082501 9 Λ Be level scheme 9 Λ Be : experimental result α α α α Λ Λ Cluster-like picture Shrink Shell-like picture 9 Λ Be 8 Be Which ? Charge distribution Shrink 9 Λ Be 8 Be Which is the true picture of core 8 Be ? Comparison between theory and experiment [1] E. Hiyama et al., Phys. Rev. C 59 (1999) 2351 [2] V. M. Datar et al., Phys. Rev. Lett. 94 (2005) 122502 [3] H. Tamura et al., NPA 754 (2005) 58c [4] T. Motoba et al., Prog. Theor. Phys. 70 (1983) 189 unbound +92 keV, Γ~7 eV Nucleon density distribution (g.s 0 + ) 8 Be Level scheme Intrinsic frame Lab. frame 4 fm 8 Be : level and structure R.B. Wiringa et al., Phys. Rev. C62 (00) B(E2) values are extracted from a lifetime of excited states. We use DSAM to measure the lifetime. The hypernuclei produced recoils and are slowed down and stopped by loosing the kinematic energy in a target. When the stopping time is comparable to lifetime of the excited state, the energy peak shape has two comportments, namely the shifted and stopped component from γ-ray emitted event during the slow down and after the stopping, respectively. The lifetime can be extracted from a fitting of the peak shape using a lifetime as a parameter. Summary Summary Future possibilities Future possibilities Neutron → Evolution of cluster structure π-orbital σ-orbital 9 Be (2α+1n) σ 2 config. π 2 config. πσ config. σ 2 π 2 config. • M. Seya, et al., PTP65, 204 (1981). • Y. Kanada-En’yo, et al., PRC60, 064304(1999). • N. Itagaki, et al., PRC62 034301, (2000). … 10 Be (2α+2n) 12 Be (2α+4n) 8 Be (2α) Systematic measurements of Be hyper isotopes A systematic measurement of nuclear isotopes is one of interesting studies of the nuclear clustering using Λ as a probe. The Be isotopic chain is the most suitable so that evolution of clustering nuclei as a function of neutron numbers can be studied systematically through five Be hypernuclei. In addition, by adding neutron to hypernuclei, the isospin dependent ΛN-ΣN coupling effect can be stronger. We may study a unique structure change of cluster hypernuclei due to the ΛN-ΣN coupling effect. For a production of Be isotopes for the systematic studies, three kinds of Λ hypernuclear productions must be used. These are challenging experiment pursued at J-PARC. Simulated response of γ-ray spectrum 0.1 0.4 0.9 GeV/c (0°) 1.1 GeV/c (0°) 1.1 GeV/c (5°) 1.5 GeV/c (5°) 1.1 GeV/c (10°) BeO Be BeTe BeO (Natural product) [5] M. Ukai et al., Phys. Rev. Lett. 93 (2004) 232501 Recoil velocity v.s. stopping time Stopping time vs initial beta For an ideal DSAM measurement, a stopping time should be the same or at most 4 times longer than a lifetime. Following factors are important to select a target. 1. Density : determine the stopping time 2. Crystal or uniform material : decrease the ambiguity of stopping time 3. Background of a compound : S/N ratio and the same energy γ ray In the previous BNL-E930(‘98) experiment, a pure Beryllium (1.85 g/cm 3 ) was used, and the stopping time was too short for DSAM using the K - beam of 0.93 GeV/c. At J-PARC, we can use beams of the same momentum, but a much higher density target has to be used. Beryllium oxide (BeO, 3.01 g/cm 3 ) is one of the suitable target. It is of uniform crystal and has higher density than the pure Beryllium. Oxygen in the compound target should not be a contaminant as the 16 Λ O γ-ray background because the 16 Λ O experiment was performed in BNL- E930(‘00) experiment[5]. The oxide is one of the safe to use target for the DSAM measurement. For the same reason, Li 2 O will be used as a target for the B(M1) measurement of 7 Λ Li in the J-PARC E13 experiment. The stopping time of 9 Λ Be in various targets is estimated from the SRIM code and shows that the stopping time is too short. To measure the B(E2) of 9 Λ Be, we have to use lower beam momentum (0.8 GeV/c) as well as an optimal configuration of the Ge array with high statistics. (BeTe (5.1 g/cm 3 ) is a reference of heavier target) Nuclear structure studies with an extra Λ binding as a probe Study of the cluster-like or shell-like structure Study of an unbound nucleus from a bound hypernucleous via a precise γ-ray spectroscopy measurement γ-ray spectroscopy of 9 Λ Be Study of two α cluster nucleus, 8 Be B(E2) : inconsistency between theory and experiment Possibility to measure B(E2) by using the Beryllium oxide (BeO) target Need low beam momentum, an optimal configuration of Ge detector array, and high statistics Future possibilities Systematic measurement of Be hyper-isotopes Study of 2α + neutron cluster nuclei ΛN-ΣN coupling effect via γ-ray spectroscopy Email : [email protected]
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Be
BeTeBeO
Gamma-ray spectroscopy of 9ΛBe :B(E2) measurement of cluster-like hypernuclei
Dept. of Phys. Tohoku Univ., Japan.K. Shirotori for the Hyperball-J collaboration
8Be is known as the α-α cluster nucleus and it has an importance of having the simplest α-cluster nuclei. The ab initio calculation predicts a well developed α-αcluster structure, and the recent experiment[2] shows that the B(E2) value of 4+→2+
transition agrees with this calculation. 8Be is an unbound nucleus and the direct B(E2) measurement is experimentally very difficult.
The γ-ray spectroscopy experiment of 9ΛBe, in which a Λ was added to 8Be, was performed in BNL-E930(‘98). We obtained the upper limit for the lifetime less than 0.1 ps[3] from the measurement via DSAM of the corresponding 2+→0+ transition in 8Be. However, the measured lifetime is much shorter than that predicted by the cluster model[4] since the calculation resulted in a sizable reduction in B(E2) value reflecting a shrinkage in the inter α cluster distance with a presence of Λ.
This inconsistency between the data and the theory shows that the theoretical prediction based on the cluster picture of 9ΛBe fails to apply to 9ΛBe, while a good agreement is seen for 7ΛLi. The experimental indicates a small change in the core 8Be structure and suggests that 8Be has rather shell-like structure than the 2α cluster. Thus measurement of B(E2) of 9ΛBe is important not only to understand the 8Be structure but also to test the cluster model in light nuclei.
Nuclear shrinkage effect in Nuclear shrinkage effect in 77ΛΛLiLi
19% shrinkTanida et al., Phys. Rev. Lett. 86 (2001) 1982
γ-ray energy spectrum of E2(5/2+→1/2+) transition
τ(5/2+)=5.8±0.90.7±0.7 ps
7ΛLi level scheme
0
Study of Study of 88Be structure from the Be structure from the γγ--ray spectroscopy of ray spectroscopy of 99ΛΛBeBe
E [MeV]
<r>α-α[fm]
B(E2) [e2fm4] τ [ps]
8Be理3.13.04
44
2215
0.150.2
9ΛBe理 3.1 3.5 11 0.3
9ΛBe実 3.05 ? 22> <0.1
Red : T. Motoba et al., Prog. Theor. Phys. 70 (1983) 189Blue : R.B. Wiringa et al., Phys. Rev. C62 (00)
How many hyperisotope can we produce ? : 5 Be hyperisotope9Be(K-, π-)9
ΛBe : Aiming for B(E2) measurement10B(K-, π0)10
ΛBe : Need to construct π0 spectrometer11B(K-, π0) 11
ΛBe : Need to construct π0 spectrometer12C(K-, π+)12
ΛBe : DCX (double charge exchange reaction) , small cross section (~0.1 µb/sr)13C(K-, π+)13
ΛBe : DCX, small cross section, optimum enrich carbon target
Level energy : change in excitation energyB(E2) : 9ΛBe, 11
ΛBe, 13ΛBe
Experimentally an impurity effect of Λ was observed for the first time from the γ-ray spectroscopy of 7ΛLi. Lifetime measurement via the Doppler shift attenuation method (DSAM) showed a much smaller B(E2) value of E2(5/2+→1/2+) transition than the E2(3+→1+) transition in the 6Li core. The theoretical calculation[1] attributes 19% shrinkage of the α-n-p cluster distance to reproduce the measured B(E2) value.
B(E2: 3+→1+: 6Li) = 9.3 ±0.5 e2fm4
⇒B(E2: 5/2+→1/2+: 7ΛLi) = 3.6 ±2.1 e2fm4
＊B(E2)∝R4, 1/τ∝B(E2)Eγ5
The study of hypernuclei is one of the ways to understand the baryon-baryon (BB) interactions, through 1) the investigation of hyperon-nucleon interactions, 2) the properties of baryons in the nuclear matter, and 3) impurity effects of Λ on the core nucleus. The ΛN interaction is studied through the Λ hypernuclear level structure and its precise structure can only be observed from the γ-ray spectroscopy by using germanium (Ge) detectors. The method of γ-ray spectroscopy with Ge detectors has been successfully used to study structure of light p-shell Λ hypernuclei.
We discuss the third subject, the effects of Λ as an impurity. Even a single Λ added to a nucleus may drastically change the properties of the nucleus, such as size, deformation, collective motions, shell /cluster structure, and etc. These effects had not been observed until the shrinkage of 7ΛLi was experimentally confirmed. Thus the hypernuclear γ−ray spectroscopy offers a new area in nuclear physic, that is, “impurity nuclear physics”.
The change in the core nucleus or the core polarization induced by a Λ arises from its “glue-like role” in a nucleus. In particular, this glue-like aspect of Λ can be used to probe light nuclei as to whether they posses cluster-like or shell-like structure. If the core structure is forming clusters, inter cluster distance can be shrunk leading to a dramatic structural change. On the other hand, the core nucleus is close to the shell structure, the inclusion of Λ is expected to have a small effect. By looking at the changes of the core of a hypernucleus, it will be possible to study the structure of a nucleus.
The ΛΝ two-body potential is attractive and provides extra binding to a hypernucleus that brings an unbound normal nucleus to a bound hypernucleous. Therefore, we gain access to energy levels and the B(E2) measurement of the otherwise unbound core nucleus via γ-ray spectroscopy experiment.
IntroductionIntroduction
B(E2) measurement of B(E2) measurement of 99ΛΛBe by Doppler shift attenuation method (DSAM)Be by Doppler shift attenuation method (DSAM)
9Be (K-, π- γ) 9ΛBe
43±5 keV
Eγ (keV)
Akikawa et al.,Phys. Rev. Lett. 88 (2002) 082501
9ΛBe level scheme
9ΛBe : experimental result
α α
α αΛ Λ
Cluster-like picture
Shrink
Shell-like picture
9ΛBe
8BeWhich ?
Chargedistribution
Shrink
9ΛBe
8Be
Which is the true picture of core 8Be ?Comparison between theory and experiment
[1] E. Hiyama et al., Phys. Rev. C 59 (1999) 2351
[2] V. M. Datar et al., Phys. Rev. Lett. 94 (2005) 122502[3] H. Tamura et al., NPA 754 (2005) 58c[4] T. Motoba et al., Prog. Theor. Phys. 70 (1983) 189
unbound+92 keV, Γ~7 eV
Nucleon density distribution (g.s 0+)8Be Level scheme
Intrinsic frame
Lab. frame
4 fm
8Be : level and structure
R.B. Wiringa et al., Phys. Rev. C62 (00)
B(E2) values are extracted from a lifetime of excited states. We use DSAM to measure the lifetime. The hypernuclei produced recoils and are slowed down and stopped by loosing the kinematic energy in a target. When the stopping time is comparable to lifetime of the excited state, the energy peak shape has two comportments, namely the shifted and stopped component from γ-ray emitted event during the slow down and after the stopping, respectively. The lifetime can be extracted from a fitting of the peak shape using a lifetime as a parameter.
SummarySummaryFuture possibilitiesFuture possibilitiesNeutron → Evolution of cluster structure
π-orbital
σ-orbital
9Be(2α+1n)
σ2 config.π2 config.
πσ config.
σ2 π2 config.
• M. Seya, et al., PTP65, 204 (1981).• Y. Kanada-En’yo, et al., PRC60, 064304(1999).• N. Itagaki, et al., PRC62 034301, (2000). …
10Be(2α+2n)
12Be(2α+4n)
8Be(2α)
Systematic measurements of Be hyper isotopes
A systematic measurement of nuclear isotopes is one of interesting studies of the nuclear clustering using Λ as a probe. The Be isotopic chain is the most suitable so that evolution of clustering nuclei as a function of neutron numbers can be studied systematically through five Be hypernuclei.
In addition, by adding neutron to hypernuclei, the isospin dependent ΛN-ΣN coupling effect can be stronger. We may study a unique structure change of cluster hypernuclei due to the ΛN-ΣN coupling effect.
For a production of Be isotopes for the systematic studies, three kinds of Λ hypernuclear productions must be used. These are challenging experiment pursued at J-PARC.
Simulated response of γ-ray spectrum
0.1 0.4
0.9 GeV/c (0°)
1.1 GeV/c (0°)
1.1 GeV/c (5°)
1.5 GeV/c (5°)
1.1 GeV/c (10°)
BeO
Be
BeTe
BeO (Natural product)
[5] M. Ukai et al., Phys. Rev. Lett. 93 (2004) 232501
Recoil velocity v.s. stopping time
Stopping time vs initial beta
For an ideal DSAM measurement, a stopping time should be the same or at most 4 times longer than a lifetime. Following factors are important to select a target.1. Density : determine the stopping time2. Crystal or uniform material : decrease the
ambiguity of stopping time3. Background of a compound : S/N ratio and
the same energy γ ray
In the previous BNL-E930(‘98) experiment, a pure Beryllium (1.85 g/cm3) was used, and the stopping time was too short for DSAM using the K- beam of 0.93 GeV/c. At J-PARC, we can use beams of the same momentum, but a much higher density target has to be used. Beryllium oxide (BeO, 3.01 g/cm3) is one of the suitable target. It is of uniform crystal and has higher density than the pure Beryllium. Oxygen in the compound target should not be a contaminant as the 16
ΛO γ-ray background because the 16
ΛO experiment was performed in BNL-E930(‘00) experiment[5]. The oxide is one of the safe to use target for the DSAM measurement. For the same reason, Li2O will be used as a target for the B(M1) measurement of 7ΛLi in the J-PARC E13 experiment.
The stopping time of 9ΛBe in various targets is estimated from the SRIM code and shows that the stopping time is too short. To measure the B(E2) of 9ΛBe, we have to use lower beam momentum (0.8 GeV/c) as well as an optimal configuration of the Ge array with high statistics. (BeTe (5.1 g/cm3) is a reference of heavier target)
Nuclear structure studies with an extra Λ binding as a probe Study of the cluster-like or shell-like structureStudy of an unbound nucleus from a bound hypernucleous via a
precise γ-ray spectroscopy measurement
γ-ray spectroscopy of 9ΛBe Study of two α cluster nucleus, 8BeB(E2) : inconsistency between theory and experimentPossibility to measure B(E2) by using the Beryllium oxide (BeO)
targetNeed low beam momentum, an optimal configuration of Ge detector
array, and high statistics
Future possibilitiesSystematic measurement of Be hyper-isotopesStudy of 2α + neutron cluster nucleiΛN-ΣN coupling effect via γ-ray spectroscopy
Email :[email protected]
