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The (n, ) cross section of 135 Cs

S. Dababneh, M. Heil, F. Kppeler, R. Plag Karlsruhe

N. Patronis, P.Assimakopoulos, D.Karamanis, Ioannina

R. Gallino , Torino

P.E. Koehler , Oak Ridge

A. Mengoni , CERN

and the n_TOF Collaboration

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Motivation

Nuclear Astrophysics

Branching points on the s-process path.Improve calculation for the 134 Cs(n,)cross section abundance ratio of

s-only134

Ba/136

Ba Physicalconditions during s-process (neutrondensity and temperature).

Recent models kT = 8 - 26 keV normalize the theoretical cross section.

Nuclear Transmutation

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Experimental MethodThe activation technique

At 30 and 500 keV, the Karlsruhe 3.7 MVVan de Graaff was used. Neutron source: 7Li(p,n) 7Be.

Irradiation with thermal neutrons atTRIGA, Mainz number of 135 Cs nuclei.

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Reference: 197 Au (n,) 198 Au1/2 = 2.7 d185

Re (n,)186

Re 1/2 = 3.7 d187 Re (n,) 188 Re

1/2 = 17.0 hr

Sample: ~ 1.810 15 135 Cs atoms(~ 400 ng)

1/2 = 2.3 106

yno gamma rays

Product: 136 Cs

1/2 = 13.2 dgamma rays:818 keV, 340 keV,1048 keV, 1235 keV

and others

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Correction factors:decay during irradiation, waiting andmeasuring

flux variationself absorptionsumming effectgold foil position

C = tot N f b I f m f wk Activated nuclei

Time (days)

N e u t r o

n s

Flux history

2 4 6 8 10

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Neutron energy distribution inthe sample

Measured TOFgives proton

beam energy

Calculateddifferential

neutron yield

Simulated neutron

beam atsample

position

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Induced activity measurementAu samples measured with HPGe detector.

Cs sample measured with 2 Clover detectorsn close geometry.Each Clover consists of 4 HPGe crystals.Nearly 4 geometry High efficiency.

Event by event Flexibility.Direct vs. Addback mode.Singles vs. Coincidence mode.

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Addback vs. Direct

Coincidences vs. Singles

13 days

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Coincidence mode Correction for summing effects in the

singles spectrum.16% in Direct ModeConfirmed by Monte-Carlo

139 La (n,) 140 La

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Experimental Results

30 keV

500 keV

E (keV) This work ENDF /B-VI

Ratio

30 164

10 204 0.804500 34.8 3.0 43.6 0.798

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Lower rate of incineration of 135Csthan previously assumed.

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