e 12 a,g 16o reaction at ecm 1.4 mev using pulsed abeams
Post on 12-May-2022
2 Views
Preview:
TRANSCRIPT
E1 and E2 cross sections of the 12C(a,g)16O reactionat Ecm ~ 1.4 MeV using pulsed a beams
Hiroyuki MAKIIa
Y. Nagaia,b, K. Mishimac, M. Segagwaa, T. Shimab, H. Uedab and M. Igashirad
a Japan Atomic Energy Agencyb Research Center for Nuclear Physics, Osaka Universityc RIKENd Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology
Contents:
1. Introduction
2. Experimental Setup
3. Result and Analysis
4. Summary
1. Introduction
Reaction rate of the 12C(a,g)16O at He burning stage determines
• Mass fraction of 12C and 16O
• Abundance distribution O ~ Fe
• Iron-core mass before the super-nova explosion
Accurate cross section value at the stellar temperature(Ecm = 300 keV)is required as an adequate input of stellar models
The reaction rate at Ecm = 300 keV (10-17b) istoo small to be measured directly.
2
The cross section at Ecm = 300 keVis derived by extrapolating measured cross sections at Ecm > 1 MeV
• Electric dipole (E1)Jp = 1- resonance at 7.12 MeV and
9.59 MeV• Electric quadrupole (E2)
Jp = 2+ resonance at 6.92 MeV andDirect capture Process
• Cascade (C.S. → 6.05, 6.92, 7.12 MeV)Should be included but theircontribution is not so large.
→ stotal ≈ sE1 + sE2
sE1 and sE2 have difference E dependence
3
O16
0.00+
2+1-
E + 7.16C.S. α
C+α12
E2 E1
1-
6.92
7.12
9.59
0E
In order to derive the total cross section, stotal, it is essential• Measure the g-ray angular distribution of 12C(a,g)16O, and• Determine the energy dependence of E1 and E2 cross sections.
Problem in the 12C(a,g)16O studySerious background due to neutrons from 13C(a,n)16O
(s(a,n) ≈ s(a,g) × 107、En ≈ 4 MeV )
4
Cro
ss s
ecti
on
[b
arn
]
Ecm [MeV]
12C(a,g)
16O sE1
13C(a,n)
16O
0.5 1 1.5 2 2.5 3 3.5
10-10
10-8
10-6
10-4
10-2
12C(a,g)16O v.s. 13C(a,n)16O Calculated spectrum for Ge
Neutron induced reactions in the g-ray detectorbecome significant background in the 12C(a,g)16O study
[H. Makii et al., Phys. Rev. C 76, 022801(R) (2007).]
Co
un
tsE (MeV)
(n,n'g)
(n,n'g)
(n,a)
(n,g)
12C(a,g)
16O
0 2 4 6 8 10 12
101
102
103
104
105
0.5 1.0
1
2
2. Experimental Setup
Pulsed a beams → obtain 12C(a,g)16O events with a large S/N by reducing neutron background with a time of flight (TOF) method
5
• Beam Intensity: 7 ~ 8 mA• Time resolution : ~ 2 ns (FWHM)
H. Makii et al., NIMA 547 (2005) 411.
TDC Channel
Co
un
ts (
arb
. u
nit
)
1.89 ns
(FWHM)
5.13 ns
(FWTM)
800 850 900 950 1000
2. Experimental Setup Three large anti-Compton NaI(Tl) Spectrometers
→ high statistics
6
90 deg
α bea
Central NaI(Tl)
: Lead
40 deg
130 deg
Annular NaI(Tl)
: B-doped Polyethylene
Powerful shield against neutrons and external g rays
→ reduce neutron background
Enriched 12C target→ reduce neutron yield from
the 13C(a,n)16O
Monitor target thickness→ determine the target thickness
free from systematic error
H. Makii et al., NIMA 547 (2005) 411.
3. Result and AnalysisObserved g-ray spectrum (Ecm ≈ 1.6 MeV, q = 90o)
7
Most of the observed event → neutron induced backgroundEnergy [MeV]
127I(nthermal,g)
127I(nMeV,g)(n,n'g)
12C(a,g)
16O region
Co
un
ts /
25
keV
2 4 6 8 10 1210
0
101
102
103
104
105
106
TOF spectrum taken by NaI(Tl) spectrometer (Ecm ≈ 1.6 MeV, q = 90o)
Clearly discriminate true event from neutron background 8
TotalEg = 8 - 9 MeV (X 100)
Background
Foreground
127I(n,g)
27Al(n,n'g)
Pb(n,n'g)
TOF [ns]
X 104
Co
un
ts /
ns
12C(a,g)
16O
-40 -30 -20 -10 0 10 20 30 400
0.5
1
1.5
2
Foreground and background spectra (Ecm ≈ 1.6 MeV, q = 90o)
True (Net) event = (foreground) – (background)9
Energy [MeV]
127I(nthermal,g)
27Al(n,n'g)
12C(a,g)
16O region
Co
un
ts /
10
0 k
eV
2 4 6 8 10 1210
0
101
102
103
104
TOF [ns]
X 104
Co
un
ts /
ns
-40 -30 -20 -10 0 10 20
0
0.25
0.5
0.75
Net g ray from 12C(a,g)16O (C.S. → g.s.)Ecm ≈ 1.6 MeV
10
Ecm ≈ 1.4 MeV
Calculated peak positions and shapes are in good agreement with the experimental data !!
→ Observed peaks were due to 12C(a,g)16O
Eg [MeV]
130o
6 7 8 9 10 11
0
20
40
90o
0
20
40
60
40o
020406080
130o
Eg [MeV]6 7 8 9 10 11
0
20
40
60
90o
0
50
100
150
40o
Cou
nts
/ 1
00 k
eV 0
50
100
150
Absolute differential cross sections
Considering
• Energy loss of a-beams and
• Energy dependence of the 12C(a,g)16O cross section in the targets
→ Effective beam energy Eeff, and Effective cross section s(Eeff)
• Energy spectrum of alpha particles scattered from the Au Backing
→ Target thickness and number of incident particle
11
Experimental data Target Thickness
2 E
Carbon Target + Au Backing
Au Backing Only
Co
un
ts /
keV
/ m
C
Ea [MeV]0.5 1 1.5 2
0
50
100 348 mg/cm2
Target Thickness [mg/cm2]
FWHM: 43 mg/cm2
Ch
arg
e [
mC
]
200 250 300 350 400 450
0
10
20
Analysis of g-ray angular distributions
Fitting the data to the angular distribution formula
12
qqs
s
qqs
s
qp
sqs
coscoscos5
56
cos7
12cos
7
51
cos14
,
3311
21
1
2
44221
2
221
PQPQEE
E
PQPQE
E
PQE
d
Ed
E
E
E
E
E
→ calculated from phase shift analysis
2
tan 1
12
EEEE
→ Determine sE1
, sE2
/sE1
Ec.m. = 1.6 MeV
Ec.m. = 1.4 MeV
q [deg]
Co
un
ts /
eff
icie
ncy
25 50 75 100 125 150 1750
0.5
1
1.5
2
Result (total cross sections)
E1 : agree with Dyer and Barnes, Redder, Asummção, Kunz , AzumaE2 : agree with Redder, Asummção 13
Dyer and Barnes(‘74)Redder et al. (‘87)Kremer et al. (‘88) Ouellet et al. (‘96)Roters et al. (‘99)Kunz et al. (’01)Gialanella et al. (‘01)Asummção et al. (‘06)PresentAzuma et al. (‘94)(R-matrix calculation)
sE
1 [
nb
]
0.5
1
1.5
sE
2 [
nb
]
Ecm [MeV]1.3 1.4 1.5 1.6 1.7
0.2
0.4
0.6
4. Summary
• Intense pulsed a beam
• High efficiency anti-Compton NaI(Tl) spectrometers
→ Angular distributions of 12C(a,g)16O reaction
With a large S/N and high statistics
• Absolute differential cross sections
→ Taking account of energy dependence of the cross section
Relative to the 197Au(a,a)197Au elastic scattering
• Total E1 cross section and E2 / E1 ratio
→ Determined with a high accuracy
E1 ~ 8 %, E2/E1 ratio ~ 20 % → E2 ≤ 23 %
• Measurement of the sE1 and sE2/sE1 ratio down to ~ 1.2 MeV
→ Already done.
Analysis is now in progress…
14
top related