ObservedChannel181Hf180Hf(n,γ)181Hf180mHf180Hf(n,n')180mHf179m2Hf180Hf(n,2n)179m2Hf179Hf(n,n')179m2Hf175Hf176Hf(n,2n)175Hf173Hf174Hf(n,2n)173Hf

The Astrophysical 187Re/187Os Ratio: Measurement of the 187Re(n,2n)186mRe Destruction Cross Section

Experimental Setup

The REU program is funded by National Science Foundation (NSF) NSF-05-52723. Other support from the NNSA under the Stewardship Science Academic Alliances Program DE-FG52-06NA26155, and Office of Science of the US Department of Energy DE-FG02-97ER41033 (Duke), DE-FG02-97ER41042 (NC-State), DE-FG02-97ER41041 (UNC), DE-AC02-06CH11357 (ANL) is acknowledged.

Future plans: At present the decay spectrum is dominated by the Compton spectrum from 185Re(n,2n)184Re/184mRe decay -rays. We are actively seeking solutions that can lower the impact of the Compton spectrum, with active suppression shields on a HPGe planar detector. We aim to obtain 187Re destruction cross sections with an emphasis on the production of 186mRe.

We have continued a program to measure (n,2n) reaction cross sections on 187Re with an emphasis on population of the 186mRe isomer with half-life 2x105 y. We produced 12 MeV quasi-monoenergetic neutrons with the TUNL Tandem Van de Graff accelerator and a deuterium gas cell, and we measured the 187Re(n,b)Y cross sections using activation techniques. While

various reaction channels were populated in the activation, our emphasis is on the (n,2n) reaction channels, where 186Re is populated. Of specific interest is the population of 186mRe, which has an extremely long lifetime that complicates the measurement. This data is intended to reduce uncertainties in the 187Re/187Os cosmochronometer.

The neutron profile was carefully measured at each energy and convoluted with the cross section curves for the monitor foils in order to correct the 241Am(n,2n) cross section for any possible contribution from low-energy breakup neutrons.

]

Irradiation

TUNL Low Background Counting Facility. After irradiation with neutrons, the activity of the Re samples and the Au and Al monitor foils were measured at the TUNL Low Background Counting Area using lead shielded 60% efficient HPGe detectors.

The neutron beam was produced via the 2H(d,n)3He reaction using a deuterium gas cell pressurized to 3.0 atm. The 0.2 g natRe samples were placed together with a set of gold and aluminum monitor foils encased within an Cd screen to reduce the interaction of thermal neutrons. The holder was placed 2.5 cm from the front face of a deuterium gas cell. The neutron flux on target at this distance was approximately 3107 n (cm2 s)-1.

Typical neutron energy distribution spectra measured at TUNL showing the monoenergetic peak and low energy breakup neutrons.

The activation setup consisted of a beam monitor NE-213 liquid scintillator detector positioned at 0˚ relative to the incident deuterium beam. The detector was located at 4.7 m from the deuterium gas cell.

Eric Pooser1, J. Kelley2, A. Hutcheson3, H. Karwowski4, E. Kwan3, C. Huibregtse2, A. Tonchev3, W. Tornow3, F. Kondev5, S. Zhu5

1North Georgia College and State University, 2Triangle Universities Nuclear Laboratory and North Carolina State University, 3TUNL and Duke University, 4TUNL and UNC Chapel Hill,

5Argonne National Laboratory

s-process (produces 187Os)

Yamamuro, Nuclear Science and Engineering, 118 (1994)249. INDC(NDS)-288

Id

=1A7.8 atm

beam

pickoff

2H(d,n)3He

For long-term measurements the irradiated samples were arranged in a 3 x 3 matrix and will be placed at fixed distances from our HPGe detector.

We have observed transitions from several reactions (see Table) and we are tracking the evolution of their decays so that reaction cross sections for the various channels can be determined.

Experimental Results

Measurements

Motivation

187Re(n,g)188Re, Eg=478.7 keV

y = -0.0401x + 8.7757

7.9

8

8.1

8.2

8.3

8.4

8.5

8.6

8.7

8.8

0 5 10 15 20 25

Time (hours)

185Re(n,2n)184Re, Eg=903.232 keV

y = -0.0007x + 11.065

11

11.01

11.02

11.03

11.04

11.05

11.06

11.07

0 10 20 30 40 50 60 70

Time (hours)

ENDF/B-VII.0 Library, Druzhinin 1967, Khurana 1961Evaluated Nuclear Data File, www.nndc.bnl.gov

187187Re(n,2n)Re(n,2n)186186Re Cross sectionsRe Cross sections

We intend to decrease the systematic uncertainties concerning the 187Re/187Os cosmochronometer. Since the s-process abundance from 187Os can be deduced from the observed abundance of 186Os. Excess in the abundance of 187Os, implies contribution from 187Re decay. Using the life time of 187Re ( 4.35 x 1010 y) the ratio of 187Os (from 187Re decay) to the abundance of 187Re dates the age of formation in a given sample. Thus we are able to determine the age of formation of galaxies

3.7d

8.7 h

17 h

23.7 h

38.0 d

187Re(n,2n)186Re

187Re(n,α)184Ta

187Re(n,γ)188Re(thermals)

187Re(n,p)187W

185Re(n,2n)184Re

Reaction T1/2

Observed Reaction Channels

0 200 400 600 800 1000

103

104

105

106

187Re(n,2n)186Re

y = -0.0076x + 11.384

11.22

11.24

11.26

11.28

11.3

11.32

11.34

11.36

11.38

11.4

0 5 10 15 20 25

Time (hours)

Ln

(Act

ivit

y)

One foil: t~3 days: t=24 hrs

Nine foils: t~week 10: t=24 hrs

184Re #

186Re @

188Re %

#

#

#

#

#

#

#

#

#

#

@

@

@

@

%

%

%

Nucleosynthesis of elements in the 187Re/187Os region occurs by the r-process and s-process. r-process:High neutron flux environment: supernovae Time between captures is less than decay time s-process: Low neutron flux environment

Time between captures is greater than decay time

186,187Os are shielded from r-process by 186W &187R creating the cosmochronometer.