n-Source Needs

Jason Hodges, Yacouba Diawara, Cai-Lin Wang, Rick Riedel

Instrument and Source Design Division, Oak Ridge National Laboratory

August 1st, 2012

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He3-Shortage

• Major Uses

– neutron scattering instruments

– portal monitors (US & overseas)

• ‘Gold Standard’ Characteristics

– high detection efficiency

– good gamma discrimination

– nontoxicity (cf. alt. BF3)

– detector stability

– low cost prior to shortage

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He3 Production & Demand

• US Production

– Decay 6% / yr of H3 in US NNSA tritium stockpile (1980-95, 2003-08)

– Reduction in nuclear weapons has reduced supply of He3

– Estimated annual production 2011-18 is 8,000-10,000 liters

• US Demand

– Large SNS instrument requires ~ 7,000 liters

– DOE has a modest stockpile for neutron detectors

– Portal monitor ~45 liters each

– Policy committee eliminated allocation of He3 for domestic portal monitors FY2010

– US annual deficit expected to be ~5,000 liters, yrs 2011-18

• Extremes in Pricing, World [May 2011, Physics Today]

– Prior to shortage: US ~$100 liter

– Current: US $600 government, $1000 commercial

– March 2011: UK supplier ~$4500 liter

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Large Scale Themes for US n-Source

Detector Development

NB: Spallation source instruments – are all Time-of-Flight (ToF) instruments

Reactor source instruments – mainly nonToF instruments but some are ToF

• He3-alternative technologies whenever possible

• 1 microsecond time resolution \ event based detection mode

– Required for all ToF instruments

– Vast majority of detector modules will be for Spallation sources, therefore ToF

– Event time stamping detectors can be used at non-TOF instruments

– Event detection leads to highest possible dynamic range and background suppression

– Event based corrections are applicable to all instruments

• Changing conditions at sample, source

• Stroboscopic experiments

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Detector Characteristics?

- Brief Survey of ORNL Instruments

Pixel dimension (mm)

Array size (m^2) High Eff.

Beamline Name < 1.0 ~ 5.0 ~ 10.0 ~ 0.3 ~ 1.0 > 2.0 l < 1Å

1B NOMAD X X X

2 BASIS X X

3 SNAP X X

4A MagRef X X

4B LiqRef X X

5 EQ-SANS X X X

6 CNCS X X

7 VULCAN X X X

8 VENUS X X

9 CORELLI X X X

11A POWGEN X X X

11B MaNDi X X X

12 TOPAZ X X X

14B HYSPEC X X

15 NSE X X

16B VISION X X X

17 SEQUOIA X X

18 ARCS X X X

CG2 Gen-SANS X X

CG3 Bio-SANS X X

HB-1 Pol-TAS X X

HB-1A TAS X X

HB-2A NPD X X X

HB-2B NRSF2 X X

HB-3 TAS X X X

HB-3A 4C-SCD X X

CG-4C Cold-TAS X X

CG-4D IMAGINE X X

HB-2C WAND X X

29 instruments

when sorted 90% of this table

collapses to just four types of

detector

6 out of 7 large arrays

are 10 – 40 m2

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Detector Characteristics?

- Brief Survey of ORNL Instruments

• Almost equal instrument split between 10, 5 & 1 mm pixel resolution requirements

• Just 4 detector types are used for all except 2 instruments

– 1-D He3 tube detectors, these can be arrayed close to form pseudo 2D detector

– 2-D wire He3 detectors

– Scintillator WLSF and Anger camera detectors

• Exceptions:

– Imaging [ inc.Tomography] req. 50 um pixel and very high count rate

– Laue SCD gained $ advantage by using 100 mm pixel resolution of flat plate technology

IMPROVEMENTS / ALTERNATIVES

NEEDED FOR ALMOST ALL

INSTRUMENT TYPES

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1-D He3 Detectors:

• Backscattering & Triple –Axis Spectrometers

– Array size range 0.02 - 0.4 m2

– 1-D He3 still the most appropriate technology

– Total He3 demand anticipated is small

• TOF Chopper Spectrometers

– Array size range 20 - 40 m2

– Reserve of He3 is inadequate

– Alternative has to be developed

• WLSF is a possibility

- Increased pixels = increased PMT anodes per module

- Multianode PMT modules

- Gamma sensitivity?

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1x1-D He3 Detectors

• SANS, Neutron Spin Echo, SESANS

– Array size range 1 - 3 m2

– Reserve of He3 is adequate at ORNL

– Flux near beamstop saturates detector

• Very high count rate 2-D detector required for central area

-- see next slide

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2-D He3 Detectors

• Reflectometers, GISANS, NSE, SCD

– Detector size range ~ 1 m2

– Efficiency limited due to He3 pressure

– SCD: expensive to tile reciprocal space

– Refl: Count rate required is x100 current technology

• Parallel Plate Avalanche Chamber Technology

- 1 Mcps in 1 cm2

- 100 mm resolution capability

- ~0.5 Mcps measured in HFIR beam (He3)

- He3-free version under development

• Anger camera a possibility for Reflectometers

- new high throughput FPGA needs to be developed

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Li6F:ZnS Wavelength Shifting Fiber Detectors

• Powder Diffractometers

– Array size range 4 - 40 m2

– Efficiency cf. 10 atm He3 1” tubes

– Current pixel size = 5 x 55 mm2

– Ghosting at ~ 0.5% level

– Pixel resolution limited by number of PMTs

– More PMTs = more HV supplies + interconnects + weight = $$/ module

0.1 0.2 0.3 0.4 0.5 0.6 0.7

0

10000

20000

30000

40000

50000

60000

70000

80000

co

un

ts

d-spacing (Å)

V2 Module on POWGEN

1g diamond powder

x 25

Each module

– 0.3 m2

– 40 kg

– ~ $110,000

POWGEN:

– 23 of 148 possible modules currently installed

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WLSF: Detector Development

Component Current Pro Con Alternate Improvement Requirement Scintillator Single

corrugated

easily

exchanged expensive Sandwich of

two flats

Cost saving =

$15,000

PMTs 32 single

ended

can be

individually

balanced

Requires 32

HV power

supplies and

magnetic

shields

4 Multianode

PMTs

Cost saving =

$20,000

New

processing

algorithm

Algorithm 154 x 7 pixels Doesn’t meet

high angle

requirement

308 x 13 pixels Spatial

resolution

Algorithm No individual

fiber efficiency

correction

responsible for

some ghosting

Flat field to

fiber efficiency

weighting

scheme

Decreases

ghosting and

permits use

of MA PMTs

Further development for Chopper Spectrometer instruments is anticipated

- symmetric pixels

- lower background

Total savings: ~ $35,000 per module & weight reduced by 50%

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Anger Camera Detectors

• Single Crystal Diffractometers

– Array size range ~ 1 m2

– Efficiency is 90% at 1 Å

– Max. count rate > 100 kcps

– Current spatial resolution avg. 1.0 mm

– Pixel resolution limited by scintillator brightness

– greater spatial resolution means larger crystal unit cells; more complex proteins

More details on Anger Cameras & WLSF will be given by Rick Riedel

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VENUS: Versatile Neutron Imaging

Instrument at the SNS

• Detector Requirements

– Size: min. 0.08 m2

– Efficiency: 50% at 1 Å

– Temporal: 100 ms

– Resolution: acceptable 200 mm

desired 20 mm

– Count rate: 10 Mcps

• Candidate Technologies

– Anger Camera: 3 mm anodes but brighter scintillator needed => 200 mm

– Parallel Plate: in development; size and tiling may be limited

– MCP (Nova Sci.): highest resolution ~ 20 mm plate is size limited

• May need to use a combination of two detectors

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Conclusions

• Anticipated user demand for neutron detectors esp. He3 alternatives is very high

• All current ORNL alt-He3 detector technologies are 1st generation

– they can meet many ‘acceptable level’ requirements for certain instruments

– however, current cost per module (inc. engineering for stands) is too high for the largest instruments

– both Anger Camera and WLSF types need significant improvements for 2nd generation

• reduce cost and reduce weight

• increase spatial resolution, increased count rate capability and reduced background:

This will allow use in

larger arrays > 100 modules: powder & disordered material diffractometers,

expanded suite of instrument types: imaging, SANS, reflectometers, chopper spectrometers

• Biggest deficiency exists for TOF Imaging Instrument: VENUS

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Opportunities

• Polarized Beam Instruments: Mag. Refl, NSE, Chopper Spec., SCD

– All could benefit (x2) from detectors that measure the neutron polarization

• Photo-detectors

– Solid-State PMTs: requires noise suppression

– Multi-multi anode PMTs: alleviates gaps => large size format