# Presentation: 949202

##  BORON CARBIDE BASED 

**NEUTRON DETECTORS**

**Ellen Day****	****	****	****Manuel Diaz Andrew Harken****	****	****Carl Lundstedt**
**Brian Robertson**
**Shireen Adenwalla**
**University of Nebraska-Lincoln**

## Funded by the ONR, State of Nebraska and NASA.

## NEUTRON DETECTORS 

- •Enforce nuclear safeguard agreements  •Monitor reactors, nuclear stockpiles
- •Homeland security applications
- • Neutron scattering science at the new
- Spallation Neutron Source
- • Planetary and space exploration

**ALL SOLID STATE NEUTRON DETECTOR**
**•**Compact
- •Sturdy
- • Low power operation (even 0V!)

## PROBLEMS

- Most materials have a tiny capture cross section for neutrons** **
**ENTER BORON CARBIDE**
- •SEMICONDUCTING
- •LARGE capture cross section

## SEMICONDUCTING FORM OF BORON CARBIDE

**1990S PETER DOWBEN’S GROUP **

**Boron Carbide **
**on n-type Si**

**Nickel doped boron carbide**
**on p-type Si**

## Source Molecule

- C2B10H12 (ortho-carborane) :
- Closo-1,2-dicarbadodecaborane

**Source Molecule**

**DEOPSITION USING PLASMA-ENHANCED CHEMICAL **
**VAPOR DEPOSITION (PECVD) **

## Film Deposition

- with Ar gas (90 °C)

*Ellen Day *

- Chamber Temp : 90 C
- Pressure : 200 mTorr
- Gas Flow Rate : 10 sccm
- Deposition rate :
-                    80 nm/10 min

**Film Deposition**

##    Capture c.s. for thermal neutrons  =3850 barns

- 		  7Li (1.02 MeV )+ 4He ( 1.78 MeV ) 		6%

** ********  **Capture c.s. for thermal neutrons  =3850 barns
- 				    1/√E

**•**** **Abundance of  10B in naturally occurring B ~ 20%

**FACT SHEET**** **

- •   **All neutron data at KSU TRIGA tangential**
**beam port ~ 10.6 n/cm****2****secW**

## 10B Capture Cross Section

| Neutron energy (eV) | Capture cross section (barns) |
| --- | --- |
| 0.025 | 3850 |
| 1 | 607 |
| 100 | 60 |
| 1000 | 18.1 |
| 100000 | 1.9 |
| 1000000 | 0.2 |

****** ****~ 1/√E**

## EFFICIENCY=[1-exp(-Nx)]

**EFFICIENCY=[1-exp(-N********x)]**

## IS IT A TRUE SOLID STATE DEVICE?

**CONVERSION LAYER**
**1. B RICH LAYER CAPTURES NEUTRONS**
**2. CHARGE IS CAPTURED BY SEMICONDUCTING LAYER**
**(DISTINCT FROM B RICH LAYER) **
**TRUE SOLID STATE**
**BOTH NEUTRON CAPTURE AND **
**CHARGE CAPTURE OCCUR IN THE SAME MATERIAL**
**IN THIS CASE BORON CARBIDE**

**IMPLICATIONS FOR HIGHEST ATTAINABLE EFFICIENCY**

## Distinction between Different Devices

- Conversion layer

- Diode device

-  Path 1
-  Path 2

## Simplified Model Comparisons with GEANT4 for Planar Conversion and All-B5C Detectors (Lundstedt et al, NIMPR, 2005)

- Simplified Model Comparisons with GEANT4 for Planar Conversion and All-B5C Detectors (Lundstedt et al, NIMPR, 2005)

## Initial 1 mm B5C / Si heterojunction diodes 

**(from 3” wafers with 276nm B****5****C)**

**Up to > 3 x 10****5 ****electrons / neutron**

## Irradiation       by ~  1015 n/cm2

**Irradiation****       ****by ~  10****15**** n/cm****2**

## All BC device

**BC on Sapphire**
**Cr/Au contacts**
**Purely ohmic I-V**

_**All BC device**_

## =1.8 X 106 -cm 

**ALL BC RESISTIVE DEVICE**

**V****app****=7.5 V**
**E field= 3 X 10****4**** V/cm**
**d****BC****= 250 nm**

******=1.8 X 10****6 ********-cm**

## MONTE CARLO SIMULATION FOR A 500 nm

**MONTE CARLO SIMULATION FOR A 500 nm**
**ALL BC DEVICE USING GEANT 4**

**NO SMEARING**

**30 KEV + 50 KEV SMEARING **

## EXPERIMENTAL EFFICIENCY  5 X 10-5

**CALCULATED   EFFICIENCY     2 X 10****-3**

**1**
**40**

**=**

**Incomplete charge collection-DISMAL FAILURE! **

**Calc. efficiency≈N********x**

## Capacitance measurements indicate effective area only

**Capacitance measurements indicate effective area only**
** ****10%of what we thought it was!  **
**Brings down incident neutron flux by factor of 10**
**NOW....**** **

**EXPERIMENTAL EFFICIENCY = 5 X 10****-4**
**CALCULATED   EFFICIENCY      2 X 10****-3**

**=0.25 !!**

## BC/Si DIODE 

**BC/Si DIODE**

**V****app****= 0 V**

**d=230 nm**

- E.E. Day et al
- J. Phys. D, **39, **2920 (2006)

## WITH INCREASING Vapp, PEAK POSITIONS CHANGE

**WITH INCREASING V****app****, PEAK POSITIONS CHANGE**

## Increase of ~ 60%

**PEAK POSITION vs. APPLIED BIAS**

**Increase of ~ 60%**
**in going from 0V to**
**3.15V **

**INCREASED charge **
**collection **
**•****higher E field,**
**•**** ****larger depletion region**
**(in Si)**** **

## EFFICIENCY

| REVERSE BIAS  | EFFICIENCY |
| --- | --- |
| 0 V | 3.05 X 10-3 |
| 1.57 V | 3.24 X 10-3 |
| 3.15 V | 3.35 X 10-3 |

**10%**
**increase **

**1. Operation at 0V bias is both feasible** **AND efficient**
**2. Increased bias DOES increase the charge collection significantly**
**but **_***not ***_**the efficiency.  **
**i.e. such a LARGE amount of charge is liberated per neutron capture event that even if only small fractions are collected they still provide a large enough signal above detector  noise**

## SCOPE TRACES AND CHARGE COLLECTION TIMES

**SCOPE TRACES AND CHARGE COLLECTION TIMES**

**Notes:**
Oscilloscope traces of the voltage pulses resulting from neutron detection (and the subsequent production of electron hole pairs) are shown in figure 4. Eighteen scope traces were captured at zero bias and 50 kW reactor power. The rise time of the pulse is governed by the characteristics of the semiconducting material, whereas the decay time is solely due to the preamplifier time constant of 50s. Shown in figure 4 are scope traces corresponding to two different peak voltages at zero bias. The height of the voltage spikes is related to the energy deposited by (and hence the path lengths of) the He and Li ions in both the Si and the BC and the subsequent production and collection of electron hole pairs. The small thickness of the BC layer results in a geometrical constraint on the amount of charge capture in that layer; most of the energy is deposited in the Si layer and hence most scope traces are dominated by the fast rise time of the Si (which is shorter than the resolution of our scope traces, but is < 20 ns). However, the presence of the electrically active BC layer implies that a small percentage of neutron capture events will lead to significant energy deposition in the BC. 
In a small fraction of cases, the total energy of both the He and Li ions will be deposited into the device, the so-called sum peak . A simple geometrical calculation  shows that the probability of observing the sum peak in the current device with 232nm of BC is 6% of all neutron capture events. Such a sum peak charge collection event can be seen in the lower scope trace in figure 4. The outlined area shows a second peak with a much longer rise time (~20 s) riding on the decay of the first short rise time peak. Previous electrical measurements indicate a low carrier concentration and mobility in BC  compared to Si, leading to a longer rise time for any charge collection occurring in the BC. Hence we interpret this scope trace as a neutron charge capture event, leading to the production of both a He and Li ion, both of which have deposited significant fractions of their energy into the device. Since the ions fly apart at 180o, one ion deposits most of its energy into the Si with a short rise time, the other deposits most (or a significant fraction) of its energy into the BC with much slower rise time. The relative heights of the peaks depend on a myriad collection of factors - the angle the ions make with the film, the mobility and charge recombination times in the two semiconductors, the precise position of the charge capture event in the BC layer - and hence the relative peak heights may be quite disparate. 
This second pulse was observed in 3 of the 18 scope traces collected, a 16% rate of occurrence. Although somewhat higher than the calculated 6%, this is  within the margin of error (>20%) of the small sample size of 18 traces. Moreover, it is possible that some of the double peak scope traces are not sum peaks, but rather events in which a significant fraction (but less than 100%) of the total energy is deposited. 

## CONCLUSIONS

**Semiconducting boron carbide is a promising and versatile **
**material for neutron detection**.
**All BC device WORKS! (albeit inefficiently)**
**3. Detected signal varies considerably depending on thickness**
** ****of material, device types, device parameters**
**4. In order to understand the neutron generated signal, **
**comparisons  with modeling and/or simulations proves**
** ****insightful.**
_**Future developments:  **_
**Higher resistivity material****	****	**** Homojunction diodes**
**Measurements of relevant semiconducting parameters**
**Enriched boron carbide**

## Monte Carlo 

- Monte Carlo simulation toolkit used was GEANT 4.5.2. and GEANT 4.8.0.
- GEANT handles the entire physical simulation.
**Detector construction (Materials and geometry), neutron transport, capture cross sections etc...**
- GEANT tracks the ion energy loss as it travels through the material.

## Simulation Assumptions

- GEANT simulation assumptions
**All the energy deposited is converted into electron hole pairs**
**All charges generated are collected by the electrodes**
**Boron carbide layer (B****5****C) constructed out of 19% ****10****B**** and 81% ****11****B**** natural occurrence**
**No electronic noise incorporated**

## Base Device

- Device Construction
**Thickness of Boron Carbide layer varied **
**381****μ****m**** Si layer.**
- Thermal neutron beam is incident normal to the Boron Carbide surface.
- Capture cross section for 0.025eV neutron is 3840 barns.

## Diode

- Incident

- Conversion Layer

## 500nm plus moderator

- Device Construction
**500nm Boron Carbide layer  **
**381****μ****m**** Si layer.**
- Moderator
**H****2****O**
**Thickness varied**
- Neutron Energy
**100eV **

## Slide 32

| Moderator Thickness (cm) | Capture % |
| --- | --- |
| 0 | 0.006945 |
| 0.5 | 0.02291 |
| 1.0 | 0.03421 |
| 1.5 | 0.03904 |
| 4.0 | 0.01812 |
| 7.0 | 0.002389 |

**Notes:**
Energy 	Thickness (cm)
1eV		0.8
100eV	1.4
1MeV		1.8

## Calorimeter

- Measures the energy of particles by discrimination.
- Neutrons with energy greater than their surroundings will lose energy by elastic scattering in the moderator.

## Simplified Calorimeter

- Device construction
**500nm Boron Carbide layer**
**381****μ****m Si layer**
- Moderator
**H****2****O **
- 1.5cm
- 4.0cm
- Neutron energy
**100eV**

## Calorimeter

| Top % captures | Moderator Thickness (cm) | Bottom % captures |
| --- | --- | --- |
| 0.006945 | 0.0 |  |
| 0.0314 | 1.5 | 0.039 |
| 0.0396 | 4.0 | 0.0183 |

## Calorimeter Modification

- Graphite Reflector added
**1.0 cm thick**

## Graphite Reflector

- Top Detector

-  Bottom Detector

**____ **no moderator no reflector
**____ **graphite reflector above moderator
**____** moderator no reflector

## Slide 38

| Top % captures | Moderator Thickness (cm) | Bottom % captures |
| --- | --- | --- |
| 0.006945 | 0.0 |  |
| 0.0314 | 1.5 cm H2O | 0.039 |
| 0.016 | 1.0 cm 12C
1.5 cm H2O | 0.031 |

## Conclusions

- Monte Carlo modeling has proven successful in determining the expected spectra for novel neutron detectors.
- Modeling has demonstrated that using a calorimeter incorporating boron carbide detectors can be effective as an energy discriminator.

## ACKNOWLEDGEMENTS

_**FUNDING **_
**Office of Naval Research N00014-04-1-0605**
**Nebraska Research Initiative**
**NASA**

**MIKE WHALEY **
**TROY UNRUH  **
**For technical support in the TRIGA **
**MARK II Nuclear Reactor Facility **
**at the Kansas State University.**

## BC/Si Diode     d=650 nm

**BC/Si Diode     d=650 nm**

## Slide 42

## 500 nm Conversion Layer BC with 

- 500 nm Conversion Layer BC with
- Additional smearing (eg incomplete charge collection)

## MONTE CARLO SIMULATION (GEANT)

## Substrate Cleaning 

- Acetone, Methanol, DN water

**Ar Plasma Etching**

*Ellen Day *

- Base Pressure : < 10-7 Torr
- Working Pressure : 200 mTorr
- RF Power : 30 Watt
- Ar Gas Flow Rate : 8 sccm
- Ar Gas Temperature :