1. Planetary Geochronology

2. Neutron source modelling (MCNP)

3. Shielding

Instrumentation Development for Planetary in situ 40Ar/39Ar Geochronology

Leah E. Morgan1, Brett Davidheiser-Kroll1, Madicken Munk2, Karl van Bibber2, Patrick Harkness3, Martin Lee4, Ian Wright5, Sanjeev Gupta6, Darren Mark1

1Scottish Universities Environmental Research Centre, East Kilbride, G75 0QF, UK, 2Department of Nuclear Engineering, UC Berkeley, 3School of Engineering, University of Glasgow, 4School of Geographical and Earth Sciences, University of Glasgow, 5Department of

Physical Sciences, Open University, 6Department of Earth Science & Engineering, Imperial College London, UK

Point source with spherical geometry Cylindrical source with fissionable booster and cylindrical geometry

252Cf source ± 9Be booster

sample chamber

reflector material

absorber material

booster material

A key to understanding the history of planetary and asteroidal bodies is the accurate and precise determination of the timescale over which they developed. Although absolute dating of planetary materials remains a primary goal of planetary research, sample return missions from key Solar System sites remain a distant prospect. Given the success of recent unmanned missions to Mars (e.g., Spirit, Opportunity, Curiosity), development of an in situ absolute dating instrument packages for future robotic missions is a logical next step. Although several ongoing programs of research are seeking to develop in situ packages for in situ application of the K-Ar tech-nique (e.g., Farley et al., 2013), these approaches could potentially deliver ages with questionable geologic meaning due to disurbed thermal histories (see Figure) and excess 40Ar. The 40Ar/39Ar method is the most promising geochronometer for obtaining accurate ages and thermal histories for rocks on the Martian surface but relies on the 39K(n,p)39Ar reaction so that 39Ar can be measured as a proxy for the parent element K. This work explores the possibility of developing a passive neutron source for space flight and in situ implementation of the 40Ar/39Ar method (e.g. Li et al., 2011).

Neutron flux requirements (1010 - 1011 n/cm2/s) Neutron source selection

0.01

0.1

1

10

100

1000

10000

100000

1000000

109

irrra

diat

ion

dura

tion

(day

s)

required source flux (n/cm2s)

100:1 1000:1 10,000:1

1010 1011 1012 1013 1014

40Ar/ 39Ar = 10040Ar/ 39Ar = 1000

40Ar/ 39Ar = 10000

~ max duration?

OSU

4Ga irradiation

PremadexPolyethylene 0.9 g/ccPolyethylene 0.99 g/ccPolyethylene 1.8 g/cc

Shield effectiveness of various materials

Frac

tion

of re

mai

ning

neu

trons

Depth into shield (cm)

Figure 1. Shows required irradiation durations for various source flux values. Based on OSU TRIGA neu-tron flux, which approximates 252Cf flux. The following are assumed here:

(a) OSU “fast” flux = 2.5 x 1013 n/cm2s

(b) typical OSU irradiation, 4 Ga sample, requires 330 hours for 40Ar/39Ar = 100

(c) if we accept loss of precision, require 33 hours for 40Ar/39Ar = 1000

(d) somewhat arbitrary max. irradia-tion of 200 days

Figure 2. Passive neutron source options for space-flight. Note that many require large quantities (and masses) of mate-rial, and others have very short half-lives and/or large heat outputs. 252Cf does have a short half-life but in general is the best option.

Figure 4. Possible source geometries. (a) Point source with Cf source surrounded by spherical sample chamber and shielding. MCNP modeling presented here is based on this design. (b) Cylindrical source surrounded by sample chamber and shielding, and rotatable (or removable) rods allow for variable amounts of fissionable booster material (e.g. 235U) to exposed to the source.

0.0E+00

2.0E-02

4.0E-02

6.0E-02

8.0E-02

1.0E-01

1.2E-01

0 1 2 3 4 5 6 7 8 9

Nor

mal

ized

Neu

tron

Pro

duct

ion

(n/s

ourc

e_n)

Be Booster Thickness (cm)

Be Booster Effectiveness

Be booster

Figure 3. (a) Effectiveness of the addition of 9Be to neutron source, which induces (n,2n) reactions. Unfortunately a large mass (6 kg) of Be is required to boost the source by even 10%. (b) Mass of Be for each thick-ness shown above.

Shield mass limitations, MSL specs Shield effectiveness, polyethylene Shield effectiveness, other materials Shield effectiveness, composite shields

a

b

H-chondrite age spectrum, modified from Swindle et al., 2009

Figure 5. Mass specifications for the MSL mission. Total mass for neutron source needs to be reasonable con-sidering total mass of SAM and MSL instrument payload. The possibility that the source could provide power to the rover may allow for higher mass and volume.

Shield effectiveness of composite shields

Frac

tion

of re

mai

ning

neu

trons

Depth into shield (cm)

Shield effectiveness of various materials

Frac

tion

of re

mai

ning

neu

trons

Depth into shield (cm)

PremadexB4CCdGd

Paraffin

Premadex + CdPremadex + GdPremadex + B4C (a)

Premadex + B4C (c)Premadex + B4C (b)

Figure 6. Shield effectiveness for polyethylene of various densities. Figure 7. Shield effectiveness for various materials. Figure 8. Shield effectiveness for composite shields. Premadex + Cd, Gd, and B4C (a) have 20 cm of Premadex surrounded by 14 cm of other material. (b) alternates B4C with Premadex every 2 cm in the shield. (c) has two B4C layers: one 8 cm and one 6 cm thick.