AbstractA time resolved radial profile neutron diagnostic is being designed for the National Spherical Torus Experiment (NSTX). The design goal is to achieve 5-7 cm radial resolution while minimizing the mass of the shielding. Experiments with a calibration neutron source have been performed to determine the dimensions and material composition of a collimating device needed to reduce cross-talk between channels and contributions from stray particles to acceptable levels. The well established MCNP transport code has been used to simulate attenuation and scattering. The laboratory experiment measuring attenuation through borated polyethylene, lead, and stainless steel has been simulated to determine optimal shielding around the detector. A model of a test collimator was produced, and the most effective dimensions for apertures was examined. Experimentally, the e-folding distance in borated polyethylene, the primary shielding candidate, was found to be 12 cm, but computer simulation found it to be 20 cm. Better agreement was found in the attenuation study where computer simulation correctly approximated the slope of the curve within a few percent. Best results were obtained from the simulation of the collimator when MCNP exactly mimicked experimental results. This result gives confidence in MCNP for future use. Much has been learned about materials and dimensions, so design of a neutron collimator can begin with more working knowledge as a guide.

Supported by US DoE contract DE-AC02-76CH03073.

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Goals

• Design of a neutron profile detector– Seven channels– 5-7 cm radial resolution– At most 1ms time resolution

• Determine if signal will be sufficient

• Analyze possibility of back scattering and detector cross-talk

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Methodology

• Two pronged approach– Experiment

• Calibration neutron source• Scintillation detector• Nuclear counting electronics

– See A.L. Roquemore, this conference

– Modeling• MCNP code to simulate neutron transport• Simulated experimental setup

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Proposed location

• Looking through 7 chords on midplane

• Sight lines into neutral beam port and pumping duct, to reduce backscatter

• Collimator behind RF antenna

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Simulation

• MCNP: Monte Carlo N-Particle transport code

• Goal: predict neutron transport through collimator designs

• Achieved: reasonable agreement between simulation and experiment

• Modeled experimental setups

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Shielding measurement

• Objective: – experimentally determine e-folding length of fast neutrons

through polyethylene and lead

• Set Up:

Source

Polyethylene

Detector + PMT in shield

Lead

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Materials

• Why choose polyethylene and lead?– Polyethylene is hydrogen rich

• Elastic scattering from light nuclei reduces neutron energy drastically

– Lead is a high Z material• Lots of electrons to absorb gamma rays

• Both will be used to create an effective collimator

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Shielding results

• Found a 12cm e-folding length in borated polyethylene

• Lead affects different detectors differently, see Detectors section

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MCNP shielding

• Problem models experimental setup

• Source modeled with 239PuBe spectrum

• Current through detector tallied

Source

Polyethylene wall

Detector

• Assumed 1% efficient detector above 1 MeV, insensitive to gammas and slow neutrons, like ZnS

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Comparison of shielding results

• MCNP e-folding length 20 cm vs. experimental 12 cm

• Possible causes of disagreement:– Model lacks floors and

walls– Exact dimensions are

missing– Other slight

inaccuracies of model

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Detectors

ZnS

• Naturally selects fast neutrons

• Opaque scintillator limits size

• No response <1MeV (scattered counts reduced)

Plastic Scintillator

•Counts everything

•Easy to get light out

•Large volume detectors

•Lower efficiency

What type of detector will detect particles we want

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Detector comparison

• ZnS detector not affected by lead

• Lead removes gammas from plastic count

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Test collimator

• Created to test ability to collimate

• Determine resolution of different size collimators

• Find conditions to eliminate cross talk

• 0.5”x2”x36” tube through polyethylene

• Used ZnS detector

• Positioned 33 cm away and scanned across source to create profile

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Collimator

Aperture

Source position when raisedBorated polyethylene

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Why so wide?

• Full width half max ~3.5 cm, twice the width of collimator

• Either resolution is terrible, or this is not a point source

Manufacturer:

This is not a point source!

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MCNP collimator

• Accurate model of experiment

• Tally current through detector

• Assume 1% efficiency

SourceBorated polyethylene

Hole through polyethylene

Detector

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MCNP collimator results

• Originally assumed point source, result was nothing like experiment

• Tried a larger spherical source, still bad

• Manufacturer gave dimensions yielding much better agreement

• Added stainless capsule and aluminum guide tube

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Agreement

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Experiment and simulation

• MCNP result multiplied by experimental source strength and counting efficiency

• Found good agreement between simulation and experiment

• Proves ability to get realistic data from our MCNP models. Builds confidence in our methods

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Scattering and attenutation

• What will happen to neutron rate through RF antenna?

• Composed of:– Stainless steel 304– Copper– Molybdenum

• Collimator used to measure attenuation and scattering from SS and Cu

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MCNP attenuation and scattering

• Beam source• Tally through rings around beam at assorted

distances from plate• Gives percentage of total beam scattered

Source

Test material

Concentric detector surfaces

θ

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Attenuation results

• Beam given PuBe source spectrum

• Percentage multiplied by experimental full-scale for comparison

• Fall-off comparable

• Unscattered beam compared to experimental attenuation

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MCNP scattering results

Material Backscatter (%) Transmitted (%) Captured (%) Forward scatter (%)

5/8" SS 13.2 62.8 0.186 23.184

2 mm SS 1.88 94.3 ~0 3.77

2" PE 17.43 53.2 1.6 27.78

5/8" PE 6.89 82.1 0.324 10.686

1/2" Cu 10.45 71.34 0.31 17.9

1/2" BN 8.21 78.4 3.38 10.01

1/8" Mo 2.895 91.5 ~0 5.605

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MCNP scattering results

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MCNP scattering results

• Predict intensity of scattered neutrons through various materials at different angles

• Important when dealing with vessel walls and RF antenna

• Help predict current to collimator

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Conclusions

• Expanding knowledge base for design of neutron collimator– Shielding data– Detector comparison– Attenuation data

• Developed skills using MCNP to model NSTX realistically– Predicts good response from test collimator

• These are steps toward effective collimator design