346 | Nature | Vol 577 | 16 January 2020
Article
Direct thermal neutron detection by the 2D
semiconductor
6
LiInP 2 Se 6Daniel G. Chica1,5, Yihui He1,5, Kyle M. McCall1,2, Duck Young Chung^3 , Rahmi O. Pak^3 ,
Giancarlo Trimarchi^1 , Zhifu Liu^2 , Patrick M. De Lurgio^4 , Bruce W. Wessels^2 &
Mercouri G. Kanatzidis1,3*Highly efficient neutron detectors are critical in many sectors, including national
security^1 ,^2 , medicine^3 , crystallography^4 and astronomy^5. The main neutron detection
technologies currently used involve^3 He-gas-filled proportional counters^6 and light
scintillators^7 for thermalized neutrons. Semiconductors could provide the next
generation of neutron detectors because their advantages could make them
competitive with or superior to existing detectors. In particular, solids with a high
concentration of high-neutron-capture nuclides (such as^6 Li,^10 B) could be used to
develop smaller detectors with high intrinsic efficiencies. However, no promising
materials have been reported so far for the construction of direct-conversion
semiconductor detectors. Here we report on the semiconductor LiInP 2 Se 6 and
demonstrate its potential as a candidate material for the direct detection of thermal
neutrons at room temperature. This compound has a good thermal-neutron-capture
cross-section, a suitable bandgap (2.06 electronvolts) and a favourable electronic
band structure for efficient electron charge transport. We used α particles from an(^241) Am source as a proxy for the neutron-capture reaction and determined that the
compact two-dimensional (2D) LiInP 2 Se 6 detectors resolved the full-energy peak with
an energy resolution of 13.9 per cent. Direct neutron detection from a moderated Pu–
Be source was achieved using^6 Li-enriched (95 per cent) LiInP 2 Se 6 detectors with full-
peak resolution. We anticipate that these results will spark interest in this field and
enable the replacement of^3 He counters by semiconductor-based neutron detectors.
Direct neutron detection presents a tremendous challenge because
neutrons interact weakly with most matter^8. Neutron detectors exploit
the properties of certain nuclides that decay into highly energetic
charged fragments upon capture of a neutron, providing easily detect-
able signals through ionization products^9. In practice, only^3 He,^10 B and
(^6) Li combine high neutron-capture cross-sections with detectable decay
products, and these isotopes have been incorporated into various
detector architectures. The detector of choice for several decades has
been the^3 He-gas-filled proportional counter. However, in the past two
decades, the stockpile of^3 He has been greatly depleted with no viable
means for increasing production to meet demand^6. Therefore, many
alternative technologies have attracted interest, including^10 BF 3 -filled
tubes,^10 B-lined tubes,^6 LiF-based scintillators, lithium-loaded plastic
scintillators^7 ,^6 LiF-filled micro-structured semiconductor detectors^10
and neutron-sensitive semiconductors. These technologies have advan-
tages and disadvantages^11 , but so far no widespread replacement for
(^3) He tubes has been found.
(^6) Li- and (^10) B-containing semiconductors are emerging technolo-
gies that promise highly efficient detectors because the concentra-
tion of neutron-absorbing isotopes is much greater in solids. There
are two classes of semiconductor neutron detectors: indirect- and
direct-conversion detectors^12. The thermal-neutron detection effi-
ciency for indirect-conversion semiconductors reaches a maximum of
about 40%^13. Direct-conversion semiconductors use a single material
for both neutron capture and charge collection, enabling a simpler
detector geometry, with intrinsic thermal-neutron detection effi-
ciencies approaching 100%. The semiconductors, however, must be
extremely pure and with low carrier trapping because neutron fluxes
can be very low, creating very small numbers of excited charges. Mate-
rials used for direct conversion include LiInSe 214 –^16 and hexagonal-BN
(h-BN)^17 , both of which suffer from severe drawbacks that limit their
performance. Despite these challenges, we demonstrate here the devel-
opment and outstanding neutron detection capabilities of the layered
semiconductor LiInP 2 Se 6.
The selenophosphate compound LiInP 2 Se 6 is a 2D layered semicon-
ductor that offers the requisite properties needed to achieve direct
thermal neutron detection. Bulk LiInP 2 Se 6 was synthesized phase pure
(see Extended Data Fig. 1a–c) via a slightly off-stoichiometric solid-state
reaction at 750 °C (see Methods). LiInP 2 Se 6 crystallizes in the trigonal
space group Pc 31 with lattice parameters a = b = 6.3975(9) Å,
c = 13.351(3) Å (estimated standard deviations in parentheses),
α = β = 90° and γ = 120° (details of the refinement and structure can be
https://doi.org/10.1038/s41586-019-1886-8
Received: 28 April 2019
Accepted: 8 October 2019
Published online: 15 January 2020
(^1) Department of Chemistry, Northwestern University, Evanston, IL, USA. (^2) Department of Materials Science and Engineering, Northwestern University, Evanston, IL, USA. (^3) Materials Science
Division, Argonne National Laboratory, Argonne, IL, USA.^4 Strategic Security Sciences Division, Argonne National Laboratory, Argonne, IL, USA.^5 These authors contributed equally: Daniel G.
Chica, Yihui He. *e-mail: [email protected]