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