Low-energy neutrons and the prospects for neutron capture nucleosynthesis measurements using NIF

Presentation to the NIF Users Group Meeting

February 15, 2012

Lee Bernstein LLNL

LLNL-PRES-530513

Collaborators – We need lots of them! D.H.G. Schneider, W. Stoeffl, D.L. Bleuel, C. Cerjan, J.A. Caggiano, R. Fortner,

A. Kritcher, L. Dauffy, H. Khater, R. Hatarik, E. Hartouni, K. Moody, J. Gostic, D. Shaughnessy, D. Sayre – LLNL

H. Herrmann, Y.H. Kim, C.S. Young, J. Mack, D. Wilson, S. Batha, N. Hoffman, J. Langenbrunner, S. Evans - LANL

G. Gosselin, P. Morel, V. Meot - CEA-DAM (BIII) S. Siem, A. Gorgen, T. Renstrøm- U. of Oslo

M. Wiedeking - iThemba Labs

C. Brune, T. Massey, A. Schiller – Ohio U.

C.J. Horsefeld, M. Rubery – AWE

U. Greife, E. Grafil – CSM

J. Knauer - LLE

R.B. Firestone - LBNL

M. Wiescher - Notre Dame

K.-H. Langanke - GSI

NIF allows studies of nuclear physics in a plasma environment

3

Hot kT=1-100 keV

Dense 1-1000 g/cm3

1040

1020

100 LANSCE/WNR Reactor SNS NIF

1027-33{

Flux

(n/s

/cm

2 )

High Neutron Flux ≈1027-33 cm-2 s-1

(fluence=1017-22cm-2)

Neutron Capture Nucleosynthesis

We have results from two different diagnostics showing that neutron capture experiments can be done at NIF right now

Charged-particle reactions (McNabb, Petrasso)

Nuclear Physics @ NIF Philosophy “Only do things at NIF that can’t be done anywhere

else”

“Experiments that complement NIC and WCI program goals are more likely to get shot time

and diagnostic support sooner”

Neutron Capture Nucleosynthesis

(…asking to be ride-along means you probably won’t be turned down…)

Step #1: Are stellar-like (< 1 MeV) neutrons present at NIF? Predictions are that shots with ρRfuel > 1 g/cm2 make many

MCNP (Hagmann) HYDRA (Cerjan/Sepke)

251 J HT(0.5%D) ρR≈1.26 g/cm2

But these are only predictions…

This huge predicted neutron fluence (1018 cm-2) suggests neutron capture nucleosynthesis could be studied at NIF

(n,γ) cross sections on branch-point nuclei in a thermal distribution of excited states cannot be measured with even the most intense neutron beams

*Busso, Gallino and Wasserburg, Annu. Rev. Astron. Astrophys. 1999. 37:239–309

NIF crams 2800 years* of neutron capture into every shot

AGB stars è disperse elements

Pre-solar grains

170Yb

171Yb

172Yb

173Yb

169Tm

170Tm

171Tm

172Tm

5.04 keV 4 ns

3/2+

1/2+ 171Tm

= Prediction from 6 leading modeling groups prior to measurement = Measured value

F. Kappeler et al., has shown that modeled (n,γ) cross sections have large uncertainties

x6

x3

167Tm 168Tm 169Tm 170Tm 1/2+ stable

1- 3 keV

3+ 93 d 1/2+ 9.25 d

7/2+ 180 keV

1- 265 d

7/2- 293 keV

7/2- 316 keV

7/2- 379 keV

171Tm 1/2+ 1.9 y

3- 183 keV

7/2- 424 keV

“Normal” Thulium Reaction Network

The Thulium reaction network is important for both Astrophysics and Stockpile Radchem

Astro

Radchem 3/2+ 10 keV

? 80 keV 4+ 64 keV ? 47 keV 2- 41 keV

? 17 keV 3/2+ 8.4 keV

11/2 368 keV

9/2- 430 keV

2- 39 keV

2- 220 keV

3/2- 5 keV

HEDP-populated states

•  These states have been left out of network calculations •  Their population depends

plasma conditions (T and ρ)

Thulium was the most fielded radchem tracers in UGTs

We have two ways to simultaneously measure sub-MeV (n,γ) on samples loaded into a NIF capsule

Collect “debris” & count Gostic, Shaughnessy,

Moody, Greife

Measure prompt γ-rays following capture

(Stoeffl, Herrmann)

I will show data from both approaches

≤1017 atoms can be implanted in a NIF capsule (Kucheyev, Hamza)

RBS Yield (Xe)

7x1016 Ge atoms in layers 2 & 3

Diagnostic #1: γ-rays from (n,xγ) are measured using a Gas Cherenkov-based system: GRH (Gamma Reaction History)

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 5 10 15 20

Gamma Energy (MeV)

Co

mp

ute

d R

es

po

ns

e

(ph

oto

ns

/in

cid

en

t g

am

ma

)

Transition Radiation

Gas pressure determines Eth

(H. Herrmann & W. Stoeffl)

LLE data

DTγ

Snout n-γ Chernenkov

NIF shot N101212

Mach Zehnder

Short pulse 40 ps cal-laser (PiLas)_

PMT

Al-Converter

Fiber light insertions

Pressure Window

W Shield

Off-axis Parabolic

Mirror

6 m from TCC

NIF Fidu

γ-rays →  e- → UV/Vis CO2 or SF6

Response to 14 MeV only (MCNP)

There were hints of the existence of sub-MeV neutrons about 1 year ago

Interpreting GRH data requires good knowledge of the (n14,xγ) cross sections on materials near TCC (Al, Si, Au etc.)

~ ½ ns

T/H/D: 74/24/2, t0±¼ ns

1.E -03

1.E -02

1.E -01

1.E+00

1.E -07

1.E -06

1.E -05

1.E -04

1.E -03

1.E -02

0 5 10 15 20

Gamma -ray energy (MeV)

1.E-13

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

0 5 10 15 20

Gamma -ray energy (MeV)

DT HT

≤108 gammas

Our plan (Dan Sayre) is to perform a multi-shot analysis of NIF GRH data

Model 4th order poly. IRF for 2.9/4.5 chan.: 8 Gamma source terms for 2.9/4.5: 2 ρRablator for 2.9 MeV channel: 5 1 gaussian RH (X0, W, H):15

(30 variables)

Input 4 Iγ(t) for lowest 2 lowest GRH channels (2.9 and 4.5 MeV)

for 5 shots (≈40 observables)

Analysis Engine

Simulate Fit GRH data

Repeat

Best fit (preferable χ2)

Hohlraum γ’s

All we need is a mass model (MCNP) for the 575 hohlraum

Diagnostic #2: Solid radchem (SRC) Dawn Shaughnessy, Julie Gostic, Ken Moody

12

Chamber diameter: 10 meters (Drawing not to scale)

50 cm

Polar Diagnostic Insertion Module (DIM)

Equatorial DIM

Disposable Debris Shield (DDS) + Final

Optics Assembly

25-50 cm 8-10 cm

Equatorial DIM

DDS

DDS

DDS

7 m

Al Shields: covered Neutron Activation Diagnostic Ta Collimators: part of x-ray diagnostic assembly Glass DDS: covered the final optics assembly

Passive Particle Detector

Blast Shield removed post-shot & counted

Gold debris, mainly from the Hohlraum/TMP was identified via SEM/EDS & neutron activation

USGS Federal Center TRIGA Reactor

Au Cu Si

Secondary Electron Microscopy & Energy Dispersive Spectroscopy

≈120(25)% of Solid Angle collection efficiency obtained Thanks to Julie Gostic, Dawn Shaughnessy and Ken Moody!

196,198Au from the hohlraum has been observed on Ta plates 50 cm from TCC at NIF*

197Au(n,γ)198gAu

197Au(n,2n)196gAu

Ratio of Au, Ta (n,γ) to (n,2n) activity determined to ≈±20%

*N111103 DT cryo with Y14=5x1014

Thanks to Julie Gostic, Dawn Shaughnessy and Ken Moody!

Collector Schematic

Hohlraum Ta collector

50 cm

A Gold foil was then behind the Ta collector @ 50 cm to see the (n,γ) activation from scattered neutrons

N120126 DT cryo with Y14=3x1014

10

100

1000

10000

100000

1000000

320 330 340 350 360 370 380 390 400 410 420 430

Cou

nts

Energy (keV)

Au foil 0.1 mm

Au debris on Ta collector 198Au

196Au

196Au

The foil shows that low-energy from neutrons scattered off of material in the target chamber do not contribute to (n,γ) from the hohlraum

Collector Schematic

Hohlraum Au on Ta collector

50 cm

Au Foil

Thanks to Julie Gostic, Dawn Shaughnessy and Ken Moody!

The N111103 post-shot simulated neutron spectrum* is consistent with measured Gold ratio

Predicted ratio of Gold (n,γ) to (n,2n): 4.46 x 10-3

Observed ratio: 4.28±1.1 x 10-3 *Thanks to C. Cerjan & O. Jones

0-225 keV ≈45%

13.5-14.5 ≈13% 0-1 MeV

≈65%

Cross S

ection (b)

197Au(n,γ)198Au

197Au(n,2n)196Au

We not only saw γ-rays from the decay of the 196Au ground state but also from a 9.7 hour isomer (Jπ=12-)

197Au(n,2n)196gAu 197Au(n,2n)196mAu

Isomer-to-ground state ratio previously measured as being 8(2)%

N111103 DT cryo with Y14=5x1014

125 175 225 275 325 375

1000 100

Energy (keV)

Nuclear-plasma interactions on E<6 MeV states could change the isomer-to-ground ratio in 196Au

197Au 196Au

Sn

Here there be dragons… G

round S

tate

Isomer

Ground S

tate

Isomer

2- 12-

This experiment requires a “thinned Gold” hohlraum (2-3 µm)

{ρ≈1 eV-1 Vacant

electron orbital

Γ≈1 eV {ρ≈1 eV-1

Tri-doped capsules (Si, Ge and Cu) (16 February+) offer the first opportunity to observe γ-rays/debris from in situ capsule dopants

Layer   Dopants  (at%)  1  (inside)   No  Si    (<0.05)  

No  Ge  (<0.02)  Cu  0.10  ±  0.02  

2   Si  0.7±0.2  Ge  0.15±0.02  No  Cu  (<0.02)  

3   Si  1.7±0.2  Ge  0.15±0.02  No  Cu  (<0.02)  

4   Si  1.0±0.2  No  Ge  (<0.02)  No  Cu  (<0.02)  

5  (outside)   No  Si  (<0.05)  No  Ge  (<0.02)  No  Cu  (<0.02)  

…and Oslo just measured the γ-ray spectrum for Ge, Al and Si (last week)…

mm_pxEntries 190586Mean 1590.6422RMS 1528.9638Underflow 0Overflow 7Integral 186811

E(NaI) [keV]2000 4000 6000 8000 10000 12000

1

10

210

310

410

mm_pxEntries 190586Mean 1590.6422RMS 1528.9638Underflow 0Overflow 7Integral 186811

xE(NaI) : E

Thanks to Therese Renstrøm and everyone at the Univ. of Oslo

Thanks to Hye-Sook Park!

External “targets” can now be loaded on the Hohlraum for cross section measurements (NIC ride-along)

10-50% of 4π 200-1000 mg

(≈1.5-7.5 x current MAu)

Prompt γ-rays using GRH

Modeling by Dan Sayre

Decay γ-rays using RAGS

Future Improvements •  Improve the existing/add new diagnostics

•  γ-rays: Super-GCD (increased solid angle) •  Highly-segmented detector “Furlong” •  Compton Spectrometer/Bent Crystal •  Low-energy neutron spectrometer (LENS) •  Increased/faster solid radchem (SRC)

•  Including capsule debris! •  “Ride-along” nuclear physics experiment

•  External hohlraum “targets” •  Thinned-out Gold hohlraums (treasure hunt!)

Next Step for Diagnostics – Greater Efficiency •  Solid Radchem: Simple - More real estate near TCC

•  Chemical separation of debris prior to counting would help w/bkgrnd. •  Prompt Gammas: “Super GCD”

•  Real γ-ray spectroscopy (Energy resolved)

Charge integrating detector Source

θBragg

Sagittally Bent Ge/Si crystals

M. Moran, RSI 56, 1066 (1985)

Compton Spect. Pixilated Single-Hit at a “Furlong”

Bent Crystal (<1.5 MeV)

e-

Only 20 cm from TCC

A spectrally resolved measurement of the low-energy neutron spectrum is now essential

•  Approach #1: CVD diamond + a layer of 235U or 6Li (En=250 keV resonant break-up to T+α)to enhance low-energy response

CVD (13C-based)

≈1 mg/cm2

•  Approach #2: 6Li scintillator in the 20 m alcove

GEANT Models by D. Sayre

Conclusions

We are poised to perform unique neutron capture measurements on material on the hohlraum, and possibly in the capsule itself

•  Simulations have predicted large numbers of sub-MeV neutrons at NIF from multiple scatter in the cold, dense fuel

•  These neutrons allow studies of the (n,γ) reactions responsible for heavy element formation in stellar cores (s-process)

•  Recent results from both GRH and Solid Radchem suggest the presence of a large NIF-thermal neutron component in DT and THD capsules.

•  The role of nuclear-plasma interactions on neutron capture rates can now be explored using NIF (196mAu/196gAu)

•  NIC ride-along nuclear physics experiments can now be considered

•  Often these experiments provide valuable info for NIC •  Example: ρRfuel from the 198Au/196Au ratio

This work has many different parts – Lots of room for collaboration!

Thanks for your attention