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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
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10
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109
irrra
diat
ion
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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
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8.0E-02
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0 1 2 3 4 5 6 7 8 9
Nor
mal
ized
Neu
tron
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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.