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Next Generation Neutron Detection

A.N. Caruso, M.B. Kruger and W.Y. ChingDepartment of Physics, UMKCcarusoan@umkc.edu

MO Energy Summit Columbia, MO April 23rd, 2009

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Outline – Applied / Basic Motivation

I Applied / Basic Motivation

II Solid State 1n Detection Challenges

III Work Underway at UMKC

IV Summary

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Spent Fuel Reprocessing

It makes sense to recycle (reprocess) – growth expected with next generation reactors

→ more plutonium available worldwide

Active Facilities Spent Fuel Quantity (Tons/yr) Pu Extracted (kg/yr)La Hague, France1600 14700Sellafield, UK 1500 13800THORP, UK 1200 11000Tarapur, India 260 2400Rokkasho, Japan 1010 9200Mayak, Russia 400 3700Krasnojarsk, Russia 1500 13800Total 7470 68600

Let’s assume only 1% of this plutonium is recoverable toward weapons grade (low 240Pu)→ 686 kg/yr

Let’s further assume that 1% of this refined Pu falls into the wrong hands→ 6.8 kg/yr

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Applied Motivation – Pu Proliferation

239Pu Smuggling Scenario:

-2.47 kg sphere 239Pu reflected by 5.2 cm Be at 19.25 g/cm3 → rc=3.12 cm

-SF + (α,n) give 21.6 n/sec/kg → 3201 1n into 4π for 60 sec portal scan

-at 10 ft (3.1 m) number of neutrons reduces to 84

-for a shielded source, drops to 8.4 neutrons

-energy range from 370 keV to 4.9 MeV

►state of the art gas detectors (<2% efficient) just don’t cut it

rc=3.12 cm (239Pu)

rreflector=5.2 cm (9Be)

Thermo Electron Corporation with efficiencies of 0.01% at 0.7 MeV, 0.1% at 3.4 MeV and 1.6% at 19 MeV

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Motivation – High Efficiency Over Large Energy Range

>16`

Radiation Portal Monitor at Newark, NJ

<6`

Moderation – >40 cm of PE

Distance – >8` from center

Time – scanning a >20` container

8`

J. Physics Conf. Series 41 (2006) 514

Intensity Reduction

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Basic / Applied Motivations – Other Applications1n Capture Therapy Water/Oil Probe High Energy Interactions

Nature Struct. Mol. Bio. 14 (2007) 1056

Physical Structure Magnetic Structure

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Outline – Neutron Properties / Interactions

I Applied / Basic Motivation

II Solid State 1n Detection Challenges

III Work Underway at UMKC

IV Summary

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Charge particle detector:Detection Type: separation of e-h pairs (like a solar cell)Mechanism: e-h pairs from fast charged particles (direct, recoil or γ)

Calorimeter:Detection - heat production creates temperature gradientMechanism - energy loss of charged particles

Scintillator: Detection Type: light, photons Mechanism: charged particles cause radioluminescence

Defect Chip:Detection Type: leakage current in CMOS (sense defects)Mechanism: displacement damage

Detector Classes – Solid State Summary

++++

++-

-

---

-

Q

Q

Q

++++

++-

-

---

-

Q

Q

Q

++++

++-

-

---

-

X++

Y--

X++

Y--

++++

++-

-

---

-

γ`

γ

γ`` X++

Y--

++++

++-

-

---

-

γ`

γ

γ`` X++

Y--

X++

Y--

Incident particle

Scattered particle

Primary knock-on atom (PKA)

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Detector Class – e-h pair production → charge separation

e-h pairs are generated by fast charged particles inelastically losing their energy.

I(t) is then proportional to the incident 1n flux

but…need a way to separate and collect the charge…..

++++

++-

-

---

-

X++

Y--

Solid Form Geometry(Direct Conversion)

Conversion Layer Geometry(Indirect Conversion)

++++

++-

-

---

-

X++

++++

++-

-

---

-

X++

Y--

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Nuclearvoltaic Operation

1 α,β → 1E4 e-h pair

p nDR

I,V

+-

+-

+-

+-

+-+-

+-

+-

+-

+-+-+-+-

+-

p nDR

I,V

+-

+-

+-

+-

+-

+- +-

+-+-

+- +-

+-+-

+- +-+- +-

+-+-

+- +-

+-+-

+- +-+-+-+-+-+-+-

+-+-

Ev

EF

Ec U

+

- -

+

exciton ≡ electron-hole pair

I

V

rad on

rad off

Jsc

Voc

Pm

Fourth Quadrant Conductivity

W. Ehrenberg, Chi-Shi Lang and R. West “The Electron Voltaic Effect”Proc. Roy. Soc. 64 (1951) 424

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Materials Challenges – Energy Dependent Cross Section

Cross Section for Interaction (capture or scattering) is given by:

where microscopic cross section is given in units of barns, where 1 barn = 10-28 m2

Nmacroscopiccross section nuclei per

unit volume

microscopiccross section

[m-3*m2=m-1]

The neutron mean free path,

1 is on the order of cm’s for most materials.

So, the challenge (for direct conversion devices) is to come up with materials (solids) that are semiconducting AND have a large neutron cross section (preferably for capture that releases high energy particles).

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Materials Challenges – Energy Dependent Cross Section

And, the cross sections are:- very energy dependent- very element specific (even isotope)- reaction products differ wildly

non-proliferation application239Pu evaporation spectrum390 keV to 4.9 MeVcarrier limited…..at cm’s

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)72(

%]6)[01.1()78.1(

%]94)[480()839.0()47.1(

)75.2()73.2(

01

15764

10

42

73

42

7310

510

42

31

63

10

11

10

11

10

keVeGdn

MeVHeMeVLi

keVMeVHeMeVLiBn

MeVHeMeVHLin

HnHn

Interaction Products of High Cross Section Isotopes

Let’s put them to work! Efficiently produce and detect:- light (scintillation) or

- e-h pairs (separable charge) or- defects (leakage current)

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material must have large neutron capture or scatter cross section→ moderation can help increase cross sectionbut, with moderation comes attenuation

→ the material could be made thicker to effectively increase the cross sectionbut, the photons or charges may not make it out in the conversion layernor, the e-h pairs in the solid form detectors

charged particle interaction products must yield a detectable signal→ materials with great luminescence can workbut, they can’t reabsorb in that wavelengthor, they should emit in a wavelength that works for modern PMTs/Ads

→ 157Gd has a two-order of magnitude higher cross section than 10Bbut, the reaction product is two orders of magnitude lower in energy(it costs 3.8 eV/e-h pair……so, 104 e-’s per 1n with 157Gd vs. 106 with 10B)

operation in mixed radiation fields (insensitivity to gammas to reject false positives)typically, the higher Z a material is, the larger its cross section for γ

Summary of Materials Challenges – The “but” Balance

must find the right balance for the application

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Outline – Neutron Work Underway at UMKC

I Applied / Basic Motivation

II Solid State 1n Detection Challenges

III Work Underway at UMKC

IV Summary

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Example Interactions in Boron10B + 1n 7Li (0.84MeV) + 4He (1.47MeV) + (0.48MeV) (94%)

10B + 1n 7Li (1.02MeV) + 4He (1.78MeV) (6%)

1n (420 keV)

7Li (840 keV)

4He (1.47 MeV)

γ (480 keV)

β (100 keV)

β (208 keV)

1n (3.4 MeV)

1H (1.9 MeV)

1n (1.5 MeV)

1n (1 keV)

7Li (1.02 MeV)

4He (1.78 MeV)

1n (2.3 MeV)a-B5C:Hx

1n (420 keV)

7Li (840 keV)

4He (1.47 MeV)

γ (480 keV)

β (100 keV)

β (208 keV)

1n (3.4 MeV)

1H (1.9 MeV)

1n (1.5 MeV)

1n (1 keV)

7Li (1.02 MeV)

4He (1.78 MeV)

1n (2.3 MeV)a-B5C:Hx

17J Phys Cond Matt 16 (2004) L139

Aluminum

(ortho) p-type B5C

(meta) n-type B5C

orthocarboranecloso 1,2 dicarbadodecaborane

metacarboranecloso 1,7 dicarbadodecaborane

Semiconducting Boron Carbide: heteroisomer

9 hr exposure to 1015 neutrons/cm2/s

PECVD

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BC/Si Solid State Neutron Detector

J Phys D: Appl Phys 39 (2006) 2920

Voltaic Response-zero bias / zero power

-zero bias / variable power

Effect of bias:

-increase depletion width

-increase drift velocity

-increasing bias allows for increased counts, but decreased resolution

performed at the KSU TRIGA II reactor15 n/cm2/sec/W thermal fluxspectra collected for 600 sec

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All Boron Carbide Solid State Neutron Detector

Mat Sci Engr B 135 (2006) 129

-3V Reverse Bias - Unshielded

-3V Reverse Bias – Cd Shielded

A - 2.8 MeV sum peak = 7Li (0.84 MeV) + 4He (1.47MeV)

B - 2.3 MeV sum peak =7Li (1.02 MeV) + 4He (1.78MeV)

C - 4He (1.47MeV)

D - 7Li (1.02 MeV)

E - 7Li (0.84 MeV)

A

B

CD

E

paraffin moderated 5 Ci PuBe source

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Photovoltaic Response – Optimized Heterojunction

We can make excellent heterojunction diodes with thermally treated boron carbide that are decently photovoltaic.

As deposited vs. thermally treated with light on only

Thermally treatedwith light on/off

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Hydrogen Profiling: NRA/FTIR – Carborane Flow

Nuclear Reaction Analysis (NRA): 15N + 1H 12C + + (4.965MeV)

It was speculated previously that the lower H content leads to better voltaic properties. The question now, is what happens to the hydrogen when the films are thermally treated: does it leave the film or passivate dangling bonds?

Both NRA and FTIR show that higher flow rates lead to greater hydrogen content

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Bonner Sphere - IntroductionIntention: detect incident neutrons and crudely resolve incident kinetic energy

Top Side

Moderator: slow neutrons by scatteringH is most efficient scattering center

moderator

cylindrical gas detector

Thickness dependence

2” 4” 6” 8” 10”

Technology of the early 1960’sModeling still today

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250 μm DSP n-Si(111) 1-10 ohm-cm

3 μm p-B5C:H (B-10 enriched)

No Mask - Complete Metallization

SiN encap w/ lithographic metallization

Moderator 1mm to 10 mm

250 μm DSP n-Si(111) 1-10 ohm-cm

3 μm p-B5C:H (B-10 enriched)

No Mask - Complete Metallization

SiN encap w/ lithographic metallization

Moderator 1mm to 10 mm

Proposed Stack – Bonner Replacement

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Initial MCNPX Calculation of Detector Efficiency

Assumptions: (1) 5 um thick boron carbide diodes; (2) 10 total diodes placed at 0.5, 1.0, 1.5, 2.0, 3.0, 5.0, 7.5, 10, 15, and 20 cm; (3) every captured neutron is a detected event; and (4) 100% isotopically pure 10B.

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Outline – Summary

I Applied / Basic Motivation

II Solid State 1n Detection Challenges

III Work Underway at UMKC

IV Summary

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Summary

Applied Motivation- physical and magnetic structure- dosimetry for BNCT, nuclear energy production and waste- nuclear non-proliferation

Basic Motivation- high energy physics interactions / reactions / formations

Neutron Properties and Interaction- neutral fermion (no charge with a magnetic moment)- possibility for electric dipole moment but unverified- creation and subsequent detection of charged particle products is efficient

Classes of Solid State Neutron Detector- calorimeter - defect creation- separated e-h pairs - radioluminescence

Materials Challenges- finding a material that has a large cross section for neutron interaction, but still allows for the efficient transport of charge, heat or other

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Sergeant Herb Lehr carrying a subcritical mass of

plutonium for the first test in 1945.

All for Nuclear Power & Reprocessing

Please balance the spending on renewable energies with more rigorous means by which to detect illicitly transported Pu.