Neutron-induced reactions

Michael HeilGSI Darmstadt

Or

How can one measure neutron capture cross sections in the keV range on small scale facilities?

• Summary of s-process nucleosynthesis and neutron capture data needs

• Production of neutrons (small vs. large scale facilities)

• Experimental methods and techniques• Time-of-flight method with illustrative examples from FZK • Activation method with illustrative examples from FZK

• Current challenges and possible contributions/solutions from FRANZ

Outline

How can one measure neutron capture cross sections in the keV range on small scale facilities?

Introduction: The s process

s process:• responsible for nucleosynthesis of about half of the heavy elements• best understood nucleosynthesis process• stellar sites are known• advanced stellar models

constantNσ AA

For the s process, neutron capture cross section measurements

are mainly needed.

Branchings

(n,)

Z+1

AA-1 A+1

A+1Z+1

(n,)

(n,)

(-) (-)

1A1,Z

1Z

nβ

ββ Nσ

Nσ

λλ

λf

Branchings can be used to determine • neutron density• temperature• mass density• convection time scales

in the interior of stars

One needs the cross section of involved stable and the branch point nuclei.

Experimental challenge: Measure (n,) of unstable isotopes

Ann σvnλ

Classical analysis:

Nuclear data need for the s-process• reliable neutron capture cross section measurements• stellar enhancement factors (SEF) and• stellar -decay rates are important

Nuclear data needs for the main s-process

Terrestrial -decay rates or cross sections are “easy” to measure but in stellar plasma additional effects have to be considered:

• nuclei are ionized • equilibrium of ground state and excited states due to hot photon bath

This can lead to drastically modified stellar -decay rates. Theoretical support needed!

gs

faster -decay

SEF

Energy range of neutron capture cross section measurements for the s process

03/2 dE

kT

EexpEσ(E)

kT

1

μπ

8σvStellar neutron capture rate

In stars, the neutron energy distribution can be described by a Maxwell-Boltzmann distribution:

Typical neutron energy distribution for kT=25 keV

We need to measure the cross sections in the range 1 keV – 500 keV

s-process sites

Two components were identified and connected to stellar sites:

Main s-process 90<A<210 Weak s-process A<90

TP-AGB stars 1-3 M⊙ massive stars > 8 M⊙

core He-burning shell C-burning 3-3.5·108 K ~1·109 K kT=25 keV kT=90 keV 106 cm-3 1011-1012 cm-3

22Ne(,n)

shell H-burning He-flash 0.9·108 K 3-3.5·108 K kT=8 keV kT=25 keV 107-108 cm-3 1010-1011 cm-3

13C(,n) 22Ne(,n)

How to measure neutron capture cross sections?

Neutron production:

• e- linear accelerators (Geel, Oak Ridge)

• Spallation neutron sources (Los Alamos, CERN)

• Van de Graaff / Tandem / RFQ (Karlsruhe, Demokritos, Frankfurt ...)

Methods:

• Direct measurements (n,)

- ToF method

- Activation method

• Indirect methods

- Inverse measurements (,n)

- Coulomb dissociation

- Transfer reactions, e.g. (d,p)

The Time-of-Flight (ToF) method

start signal stop signal

neutron production

target

detector

flight path length s

t

sv Energy of neutron which caused the event: 2mv

2

1E

pulsed beam,short pulse

good timing properties

ToF-experiments in Karlsruhe

Neutron production: 7Li(p,n) reaction at energies above threshold (>1881 keV)

6LiCO3

samplesample

7 Li-Target

Collimatedneutron beam

10B + araldite

n

nn

nPulsed proton

Beam

n

lead

lea

d

77 cm flight path

42 BaF2 scintillators form a closed shell withinner diameter of 20cm and thickness of 15cmDetector efficiency > 95% for capture events

Pulse width: ~0.7 ns

Average current: 2 μA

Frequency: 250 kHz

Time resolution: ~ 600 ps Energy resolution: 14% at 662 keV, 7% at 2.5 MeV

Detection principle

AX + n A+1X + Q

iQ if detector has 100% efficiencyCharacteristic line at

Detection of prompt -rays after neutron capture.We need to measure -rays after neutron capture

Sum energy spectra and corrections

Example 143Nd

143Nd

143Nd

Measured background with C sample

Background from scattered neutrons

and isotopic impurities!

Example 143Nd 143Nd

143Nd

Measure background from isotopes by using samples with different

enrichment.

144Nd

sample ladder

142Nd208Pb/C143Nd145Nd197Au146Nd148NdEmpty144Nd

ToF spectra

No background for early times

Cross section results

• Cross sections in the energy

range from 1 to 200 keV• Cross sections with an

accuracy of ~2%

180Tam: the world rarest isotope

Wisshak et al., Phys. Rev. Lett. 87 (2001) 251102

Sample: world supply of enriched tantalum, consisting of

150 mg oxide powder with a 180Tam content of only 5.5%.

Result: 1465 mb at kT=30keV,Much smaller than theoretical predictions.

180Tam can be produced in the s process!

Activation experiments

Induced activity can be measured after irradiation with HPGe detectors.

Gold foils for flux determination.

Neutron production: 7Li(p,n) reaction at a proton energy of 1911 keV

HPGe

H. Beer, F. Käppeler et al., Phys. Rev. C21, 534 (1980)

Activation sources

18O(p,n) reaction

At Ep=2582 keV

Käppeler et al.Phys. Rev. C35,936–941 (1987)

Heil et al. Phys. Rev. C 71, 025803 (2005)

18O(p,n)3H(p,n)

Only possible when product nucleus is radioactive

High sensitivity -> small sample masses [e.g. 28 ng for 147Pm(n,)]

Use of natural samples possible, no enriched sample necessary

Direct capture component included

Measurement of radioactive samples possible due to excellent energy resolution of HPGe detectors

So far only MACS at a thermal energy of kT=25, 5, and 52 keV possible

Advantages and disadvantages of the activation technique

The production of 60Fe in core collapse supernovae depends strongly on the uncertain 59Fe(n,) and 60Fe(n,) cross section.

Detection of 60Fe with INTEGRAL or RHESSI

Example: 60Fe(n,) by activation

The detection of the ratio 60Fe/26Al in our galaxy can be used to test stellar models

60Fe: t1/2= 1.5(3) Ma

60Fe/26Al = 0.11 ± 0.03

Harris et al, A&A 433 (2005) L49

Activation of 60Fe

Sample: 7.8·1015 atoms ~ 800 ng

70 mm

sample

1205

61Co

1325

1205

61Fe6 min298 38 %

27 %

1027

1027

60Fe sample irradiated 40 times for 15 min, then activity

counted for 10 min

Result: <>=10.2 (2.9sys) (1.4stat) mb

Example – 147Pm

150Sm

148Sm

nβ

ββ Nσ

Nσ

λλ

λf

Ann σvnλ

solve for n to obtain neutron density

147Pm sample mass: 28 ng

Analyze combined branching

147Pm activation results

147Nd

mbarn

147Pm

mbarn

148Pm

mbarn

nn

108 cm-3

550±150 985±250 1410±350 4.1±0.6 Wisshak et al. 1993

544±90 1290±470 2970±500 Bao et al. 2000

544±90 709±100 1014±175 Reifarth et al. 2003

38.030.07.2

60.050.094.4

measured with 28 ng

Reifarth et al., Astrophysical Journal, 582 (2003) 1251

Summary: neutron capture cross sections

• Light elements have small cross sections and are difficult to measure,but they are very abundant in stars. Therefore, they can change the neutron balance.Most important neutron poisons: 12C(n,)13C, 16O(n,)17O, 22Ne(n,)23Ne, 23Na(n,)24Na, ….

• Neutron capture on medium mass nuclei are important for the s-process in massive stars. Since these are the progenitors of supernovae explosions the s-process determines the composition before the explosion.

• The reaction path around neutron magic nuclei is especially sensitive to model parameters. Therefore, the neutron capture cross section of neutron magic nuclei can constrain stellar models.

• Neutron capture measurements on unstable branch points are most challenging.

The Frankfurt neutron source at the Stern-Gerlach-Zentrum (FRANZ)

Design by Prof. Ratzinger, Prof. Schempp, O. Meusel and P. C. Chau

Neutron beam for activation

2 mA proton beam250 kHz< 1ns pulse widthneutron flux: 4·107 s-1 cm-2

neutron flux:1·1012 s-1

Factor of ~1000 higher than at FZK!!!

The Frankfurt neutron source at the Stern-Gerlach-Zentrum (FRANZ)

Design by Prof. Ratzinger, Prof. Schempp, O. Meusel and P. C. Chau

Neutron beam for activation

2 mA proton beam250 kHz< 1ns pulse widthneutron flux: 4·107 s-1 cm-2

neutron flux:1·1012 s-1

Factor of ~1000 higher than at FZK!!!

Experimental program at FRANZ

The Frankfurt neutron source will provide the highest neutron flux in the astrophysically relevant keV region (1 – 500 keV) worldwide.

Neutron capture measurements of small cross sections:• Big Bang nucleosynthesis: 1H(n,)• Neutron poisons for the s-process: 12C(n,), 16O(n,), 22Ne(n,).• ToF measurements of medium mass nuclei for the

weak s-process. Neutron capture measurements with small sample masses:• Radio-isotopes for -ray astronomy 59Fe(n,) and 60Fe(n,)• Branch point nuclei, e.g. 85Kr(n,), 95Zr(n,), 147Pm(n,),

154Eu(n,), 155Eu(n,), 153Gd(n,), 185W(n,)

63Ni79Se81Kr85Kr147Nd147Pm148Pm151Sm154Eu155Eu153Gd160Tb163Ho170Tm171Tm 179Ta185W204Tl

Production of radioactive samples

So far, milli-gram samples are necessary to perform neutron capture experiments on radioactive isotopes.

Problems:• Activity of the samples:

Assume 500 mg 85Kr: I=0.43 %, E = 514 keV: 30 GBq

• Availability of the samples

We need an experimental setup which allows to measure neutron capture cross sections of nano-gram samples

We need a possibility to produce isotopically “pure” nano-gram samples

Possible future experimental setup

4 BaF2

Proton beam Neutronbeam

TOF (ns)

En (keV)

100 39

100 5.5

(n,)on sample

other reactions

prompt flash

Sample by ion implantation of radioactive beams

Proton accelerator

Neutron production via 7Li(p,n)

4 cm flight pathfor high neutron flux

4 BaF2 detector for efficient -ray detection

Reifarth et al. NIM A 524 (2004) 215–226

Sample production

To perform neutron capture experiments on radioactive isotopes one needs samples with about 1015 atoms: With FAIR and other upcoming RIB facilities (Spiral2, RIA, Eurisol) intensities of >1010 ions/s are reached for a wide variety of isotopes.

Implantation of selected isotopes in thin carbon foils:• beam intensity ≥ 1010 1/s (8.64·1014 1/day)• beam size Ø < 2 cm • high purity (<10% contaminant beam)• thin backings (<1 mg/cm2 carbon backings) -> low energy radioactive beam (< 5 MeV/u)

5 MeV/u 59Fe ions in carbon

Expected production intensities: • 6·109 for 59Fe• 3·1010 for 85Kr

radioactive

ions

Production rates at FAIR

r-process pa th

Z

N

know n nucle i

K.-H. Schmidt

Example 85Kr

• No experimental data available, theoretical calculations at 30 keV:

123 mb, 67 mb, 25 mb, 150 mb: Uncertain by a factor of 6• Beam time of 2 days:

– 85Kr beam of 3.25·1010 1/s (> 5.6·1015 atoms in two days, 800 ng)– Neutron flux of 1·108 neutrons/s/cm2

– Neutron capture cross section of 100 mb

collection of > 35 000 counts in 1 week

background from backing: 125 000

Activity of target:50 kBqI=0.43 %, E = 514 keV

carbon

85Kr

This setup would also allow measurements of very small (n,) cross sections (weak s-process, neutron poisons)

Summary

• Although the s-process is the best known nucleosynthesis process it is still an exciting research field– Many accurate cross section measurements allow to test advanced

stellar models in detail– New neutron capture processes such as LEPP are discussed

• FRANZ and other neutron sources (e.g. short flight path at n_ToF) with increased neutron fluxes will open completely new possibilities.

• There are many exciting experiments waiting to be performed and many problems to be solved!