Transcript 1 + - 大阪大学 核物理研究センター

Study of M1 Quenching in 28Si
by a (p,p') Measurement
at zero-degrees
(0度（p,p’）測定による28SiのＭ１クエンチングに関する研究)
核物理研究センター 松原礼明
Feb. 14 2006 defense
Collaborators
阪大RCNP
民井淳、畑中吉治、酒見泰寛、伊藤正俊、新原佳弘、清水陽平、
藤田訓裕、中西康介、爲重雄司、橋本尚信、與曽井優
阪大理
Univ. of Witwatersrand
東大CNS
京大理
藤田佳孝、足立竜也
J. Carter
川畑貴裕、笹本良子
坂口治隆、銭廣十三
iThemba LABs
F.D. Smit、藤田浩彦
Gent Univ.
L.A. Popescu
九州大学
堂園昌伯
Feb. 14 2006 defense
Introduction
Gammow-Teller (GT)
ΔS=1, ΔTz=1
στ+
στ-
(n,p) type, στ+
(p,n) type, στ-
σ
στ0
M1 (1+) transition
ΔS=1, ΔTz=0
σ (T=0)
isoscalar
στ0 (T=1)
isovector
Feb. 14 2006 defense
GT quenching problem
• Less strength is observed than predicted with sum rule. (~60%)
GT sum rule : S   S   3( N  Z )
Two mechanisms were proposed to explain the quenching.
・many-particle-many-hole configurations (np-nh)
・Δ-hole excitations (Δ-h)
60 → 90% of the strength is observed up to Ex = 50 MeV.
T. Wakasa et al., PRC55(1997)2909 (p,n) reaction
K. Yako et al., PLB615(2005)193
(n,p) reaction
Feb. 14 2006 defense
How about M1 strengths ?
Quenching is observed in M1 strengths in 28Si.
N. Anantaraman et al.,PRL52(1984)1409
Almost no quenching is observed
in 24,26Mg, 28Si, 32S.
G.M. Crawley et al.,PRC39(1989)311
Improvements of the data quality are required.
ΔT=0 (IS)
ΔT=1 (IV)
np-nh
possible
possible
Δ-h
impossible
possible
T=1
T=0
Another aspect of the quenching can be found.
Feb. 14 2006 defense
Experimental condition
•
•
•
•
28Si(p,p’) at 0 deg.
Measurement
Incident energy
Ep = 295 MeV
Measured angles (lab)
0 ~18 deg
High resolution
- dispersion matching technique
- under focus mode
Experimental Setup (0-deg.)
Under focus mode
As a beam spot monitor
in the vertical direction
Transport : Dispersive mode
Intensity : 3 ~ 8 nA
Target : natSi (2.22 mg/cm2)
Feb. 14 2006 defense
Background subtraction
After calibration
Feb. 14 2006 defense
A typical spectrum of 28Si(p,p’) at 0-deg.
Background events were subtracted reasonably.
Feb. 14 2006 defense
G.M. Crawley et al, PRC39(1989)311, at Orsay
2000
1500
500
d2σ/dωdE [mb/sr/MeV]
Present data
Excitation energy [MeV]
Feb. 14 2006 defense
Flow chart of calculations
・Shell model calculation by the code OXBASH. (USD interaction within sd –shell)
.
・Distorted wave Born approximation (DWBA)
Trans. density : USD (from shell model calculation)
NN interaction. : Franey and Love, PRC31(1985)488. (325 MeV data)
Optical potential : K. Lin, M.Sc. thesis., Simon Fraser U. 1986.
Experimental result
Calculated result
Spectra
Jπ assignment
d
( )
d
DWBA
d
d q 0
Wave func.
Unit cross section
Shell-model calc.
B(σ) ; exp
Cumulated
Quenching
B( )exp
B( ) shell mod el
B(σ) ; predicted
The ambiguity of shell-model calc.
is canceled !!
Distinction between IS and IV
DWBA, T=0 ; IS
DWBA, T=1 ; IV
Ex = 11.45 MeV ; T=1
×0.35
dσ/dΩ [mb/sr]
dσ/dΩ [mb/sr]
Ex = 9.50 MeV ; T=0
×2.50
×0.11
Θcm [deg]
×0.77
Θcm [deg]
From angular distribution, isospin value is identified.
Feb. 14 2006 defense
Other states identified as 1+
1+, T=0 states
1+, T=1 states
13.04 MeV
10.60 MeV
12.24 MeV
15.15 MeV
9.50 MeV
10.73 MeV
dσ/dΩ [mb/sr]
dσ/dΩ [mb/sr]
13.19 MeV
10.90 MeV
13.23 MeV
15.94 MeV
12.33 MeV
13.32 MeV
15.50 MeV
15.76 MeV
2+ : 9.48
MeV
11.95 MeV
14.03 MeV
The flat distribution is in nature of the isoscalar excitation.
T=0 : IS
T=1 : IV
ΘCM [deg]
ΘCM [deg]
Feb. 14 2006 defense
Formula of unit cross section
d
(q,  ) = σσ F(q,ω) B(σ)
d
q : momentum transfer
ω : energy transfer
σσ
: unit cross section for B(σ)
F(q,ω) : kinematical factor
B(σ) : spin-flip excitation strength
σσ =
d
(q, /) B(σ)
d
S.M.
DWBA
ˆT 0  3.23 0.26[mb/ sr / n2 ]
T=0 ; IS
[μn2]
← Obtained by calculations.
[mb/srμn2]
[mb/srμn2]
At F(q,ω) = 1 :
ˆT 1  1.07  0.06[mb/ sr / n2 ]
T=1 ; IV
[μn2]
Feb. 14 2006 defense
Strength fragmentation
Feb. 14 2006 defense
Total sum of the strengths
T=1 ; IV
Quenching factor
∑ B(σ) [μn2]
∑ B(σ) [μn2]
T=0 ; IS
(preliminary)
Quenching
The present result is consistent with the previous one.
B( )exp
B( ) shell mod el
The uncertainty from
shell-model cal. is canceled.
Feb. 14 2006 defense
Summary
• We have realized a 28Si(p,p’) measurement at 0o with high resolution .
• The present study has found three new 1+, T=0 states and the flat angular
distribution of the isoscalar excitation.
• Unit cross section is determined by calculations.
• The B(σ) strength is quenched.
• The Δ-h mixing seems to have little role in the M1 quenching.
Future
• Comparison with (e,e’) and (γ,γ’) experiments.
• Systematic study in other nuclei.
Feb. 14 2006 defense
0+ states
dσ/dΩ [mb/sr]
9.72 MeV
0+
1+
10.80 MeV
11.14 MeV
12.98 MeV
13.79 MeV
Θcm [deg]
Feb. 14 2006 defense
Kinematical factor F(q,ω)
Calculated by using DWBA
Ambiguity of wave func.
Calculated by using shell-model cal.
T=1 ; IV
dσ/dΩ / B(σ) [mb/sr/μn2]
dσ/dΩ / B(σ) [mb/sr/μn2]
T=0 ; IS
Θ[deg]
Θ[deg]