C.W. Beausang, Physics Department, University of Richmond

Surrogate Ratio Method Measurements in the Actinide Region

The University of Richmond

The Surrogate Reaction Technique … a brief tour …

The Problem  Neutron induced reactions on unstable nuclei are difficult or impossible to measure directly.

 E.g. Half life of 237U is only about one week.

 Tough to make a target.

A solution … (?)  Pioneered by Chip Britt, Jerry Wilhelmy etc., in the 70’s

 Produce the same (compound) system at similar excitation energies and spins using light charged particle induced reactions on stable targets.

 Measure the decay probabilities, using particle and / or gamma-rays as tags.

Charged particle tag identifies

a)  The final nucleus

b)  The excitation energy

237U n

Desired reaction 238U*

Fission “(n,γ)”

238U*

(d, d’γ)

Pγ

“(n,n’)”

237U*

(d, d’nγ) Pn

The Surrogate Reaction Technique

“Surrogate” reactions

238U d d

238U α

α

σ(n,f) = Σ(states) σabsorption Pfission

237U n

Desired reaction

“Surrogate” reactions

238U d d

238U*

σ(n,f) = Σ(states) σabsorption Pfission

Fission

∝ N(d,d’)(238U)

N(d,d’f)(238U)

Hauser-Feshbach σα = ΣJ,π σαCN (E,J,π) . GCN(E,J,π)

Weisskopf-Ewing limit: Decay probabilities are independent of J,π: GCN(E,J,π) ------> GCN(E)

Target contaminants (C, N and O) make extraction of absolute cross sections difficult.

P(d,d’f)(238U) ∝ N(d,d’)(238U)

N(d,d’f)(238U)

The External Ratio Method

σ (d, df) (236U) ∝

N (d, df) (236U) N (d, d) (236U) σ(d, df) (238U)

N (d, d) (238U) N (d, df) (238U)

σ formation(238U) σ(n,f) (236U) σ formation(236U) σ(n,f) (238U)

σ (n, f) (236U) N (d, df) (236U) σ (n, f) (238U) N (d, df) (238U) =

Challenges (Theory and Experimental):

2. Accounting for the J π population mismatch:

1. The role of pre-equilibrium reactions - or:

Is the intermediate nucleus truly in a compound state?: Bohr Model 1936

3. Choice of the reaction:

Benchmarking

What do you need to make this work? 1.  An efficient charged particle detector array

  … to tag / select the excitation energy in the compound system and to select a decay path (gamma-decay, fission).

  STARS 2.  An efficient gamma-ray detector array

  …to i.d. & to measure the decay products.

  Liberace (LBNL) 3.  Help from theorists and modelers

  LLNL Nuclear Theory Group

“clover” Ge g-ray detectors

1000 µm E detectors

Al δ-electron & fission fragment shield

55 MeV a’s

140 µm ΔE detector

Fission Fragments

γ

γ

140 µm fission detector

238U target

Fron

t En

ergy

Back Energy

α

p,d,t

3He

STARS: Silicon Telescope Array for Reaction Studies

STARS-LIBERACE Can achieve relatively high particle-γ coincidence statistics!!!

•  Targets: 238U and 236U (300 µg/cm2), each on a carbon backing of 200 µg/cm2

•  Beam: Deuterium at 24 and 32 MeV

•  Reactions of interest: Surrogate: – 237U(n, f) 238U(d, d’f) – 235U(n, f) 236U(d, d’f) – 238U(n, f) 238U(d, pf) – 236U(n, f) 236U(d, pf)

1st Experiment: STARS at Yale (2004)

Benchmark

New

C. Plettner, et. al., Phys. Rev. C 71, 051602 (2005).

Benchmark

New

236U(d,pf) / 238U(d,pf)

238U(d,d’f) / 236U(d,d’f)

1st Experiment: STARS at Yale (2004)

SETUP STARS at Berkeley:

•  Advantages:  Self supporting 238U target, which removes contaminations for

the 238U measurement.  Alpha beam, which reduces activation (dead-time).

Targets: Self Supporting 238U, and 236U on a natural Tantalum backing. Beam: Alpha particles at 55 MeV. Reaction of interest:

238U(α, α’f) 237U(n, f) 236U(α, α’f) 235U(n, f)

Particle I.D. Spectrum

… much cleaner than the previous Yale data.

Gate on alpha’s and select fission in coincidence …

Fission Fragment and α Particle Spectra

Energy (channels)

238U

Blank target frame

Energy (channels)

Total α energy

Cross Section Ratios 236U / 238U

(a, a’f) (d, d’f)

Jason Burke et al., Phys. Rev. C73 054604 (2006).

236U(a,a’f) / 238U(a,a’f)

235U(n,n’f) / 237U(n,n’f)

S.R. Lesher et al. Phys. Rev. C79 044609 (2009)

Benchmark:

Experimental cross section ratio agrees with evaluation to ~5% over 20 MeV energy range. 233U(n,f)/235U(n,f)

vs. 234U(α,α’f) / 236U(α, α’f)

Alpha’s in coincidence with fission

Internal Surrogate Ratio Method

Measured

Measured

ENDF

ENDF

Internal Surrogate Ratio Method

ENDF Cross Sections for 235U(n,x)

Ex(236U) = 6.55 MeV = Sn

γ-channel probabilitiy drops dramatically > Sn; hard to measure with neutrons!!!

Energy-Spin Distributions

STARS-LIBERACE

d Beam @

21 MeV 235U

ΔE E1 E2

Clover (5 used)

Clover (5 used)

p

Fission-E

Si Det"

fis

Si Det"

γ .5mm 1mm 1mm .14mm

+ 12C**

** 450 µg/cm2 target with 12C backing

“raytrace”"

Can achieve relatively high particle-γ coincidence statistics!!!

Identification of 235U(d,pxn)

Particle ID:

Banana gate on p

p-γ

p-f

Fission and γ Spectra: 235U(d,p)236U 642 keV γ-ray has the highest statsitics and is well “isolated” from doublets!!!

Partial Level Scheme: 235U(d,pγ) 236U

N235U-γ = Inttot(0+) Effγ -ch “γ-ch yield”

The γ-channel yield is defined as the total “parallel” yield to the ground state

=“γ-ch yield”

γ-Channel Yield (by 642 keV)

Inttot(0+) = Int(687) + Int(958) + Int(966) + Inttot(2+) + Γunseen

642 keV Fraction for interval (2.6 MeV<E*<6.5 MeV) = Int(642) / Inttot(0+) = 36.3 +/- 1.2 % for Γunseen =0

γ-channel yield =

Can only measure 642 keV γ-ray for E*>Sn; determine fraction of γ-channel < Sn

γ-Channel Yield (by 642 keV)

Inttot(0+) = Int(687) + Int(958) + Int(966) + Inttot(2+) + Γunseen 642 keV Fraction for interval (2.6 MeV<E*<6.5 MeV) = Int(642) / Inttot(0+) = 36.3 +/- 1.2 % for Γunseen =0

γ-channel yield =

Is “Unobserved” side feeding a BIG contribution to γ-channel yield, Inttot(0+) ???

Can only measure 642 keV γ-ray for E*>Sn; determine fraction of γ-channel < Sn

“Unobserved” Feeding Distribution

Intout

Intin-discrete

U(J)=Intout- Intin-discrete

Unobserved Feeding to 0+ and 2+ (Γ) small?

“Unobserved” Side Feeding

Continuum

J

Can we infer unobserved side feeding to J=0,2 yrast?

Determined for gate on 2.6 MeV<E*<6.5 MeV

“Unobserved” Feeding Distribution

Intout

Intin-discrete

U(J)=Intout- Intin-discrete Unobserved Feeding to 0+ and 2+ (Γ) looks small

“Unobserved” Side Feeding

Continuum

J

Unobserved side feeding to J=0,1,2,3 is less than that to J=4!!!

Determined for gate on 2.6 MeV<E*<6.5 MeV

“Unobserved” Feeding Distribution

Intout

Intin-discrete

U(J)=Intout- Intin-discrete

“Unobserved” Side Feeding

Continuum

J

100% error

Unobserved Feeding = 4.6% of γ-channel yield, Inttot(0+)

Infer unobserved side feeding to J=0,2 yrast by extrapolation.

Determined for gate on 2.6 MeV<E*<6.5 MeV

N235U-γ / Effγ-ch ---> Inttot(0+) ---> Int(642) / Fraction(642) γ-ch-yield:

642 keV Fraction of γ-channel Yield

642 keV fraction of γ-channel is independent of excitation energy within error!!!

Internal Ratio Result for 235U(n,γ)

ZOOM IN

γ-channel probabilitiy drops dramatically > Sn

Internal Ratio Result for 235U(n,γ)

ENDF

Stars-Liberace

Within error, the surrogate γ-to-f ratio is similar to the neutron induced reaction

J.M. Allmond et al., Phys. Rev. C79, 054610 (2009).

236U

232Th

238U(p, t) 235U(d, p)

236U(p, p’)

236U(a, a’)

237Np(d, 3He)

232Th(6Li, d)

Benchmarking: Choice of Reaction Populate the same nucleus via several different surrogate reactions.

Compare the resulting cross section (ratios)

238U(p, t)236U

Multiple surrogate measurements in a single experiment

Approved to run at the 88-Inch this fall.

157Gd(n,g) 155Gd(n,g)

Absolute cross section

Investigating the N = 90 Region via (p, t), (p, d) & (p, p’)Reactions

N = 90

Gd 152 154 156 158

2.19 3.01 3.24 3.30

(p, t)

(p, d)

R(4/2)

Spherical Deformed Abundance of low

lying 0+ states

D.A. Meyer, et al., Phys. Rev. C 74, 044309 (2006)

Transitional

0+ 0+

0+

Identification of 156Gd(p,t)154Gd

t-γ Example: Triton Spectra

# Tritons

Gate on γ

Gamma gated triton spectra show the ensemble of states which are directly populated and subsequently gamma decay to the level selected.

154Gd

156Gd 152Gd

Total Proj

Gate: 2+ 0+

Gate: 02 2+

0+

2+

-1.0000000000

0.0000000000

1.0000000000

2.0000000000

3.0000000000

4.0000000000

5.0000000000

6.0000000000

2 --> 0 4 --> 2 6 --> 4 8 --> 6 0 --> 2 0.0

1.0

2.0

3.0

4.0

5.0 N

orm

aliz

ed S

lope

(a

rb. U

nits

) 154Gd

152Gd

156Gd

Slope: Measures the continuum population which feeds the selected level Larger slope for spherical and transitional nuclei => less feeding from very high in the continuum?

Surrogate Reactions Summary •  Fission: Seems to work within a few percent over large energy range.

•  (n,γ) and (n,2nγ) … initial results show agreement to ~20%. Now working on other benchmarking experiments in mass 90 and mass 150 regions.

•  Benchmarking using different entrance channels!

•  Looking at structure/surrogates for (n, γ) in mass 150 (and other regions)

Conclusions and future outlook: Need systematics!!!