a scintillation detector for neutrons below 1 mev with gamma-ray rejection scintillators are 3 mm...

A scintillation detector for neutrons below 1 MeV with gamma-ray rejection

Scintillators are 3 mm BC408, 10 layers totalAdjacent layers are optically isolatedActive scint. area approx. 10 cm x 10 cm in this prototypeEach PMT discriminator triggered near top of 1 photoelectron distributionL-R and T-B thresholds approx. 10 keVee; Coincidence requirement removes noise

The light response of BC-418 plastic scintillator to protons with energies from 60 keV to 5 MeV

Motivation

• primary interest in n+p=>d+γ for Big-Bang Nucleosynthesis models

• Important neutron kinetic energies: 50 – 500 keV=> produced deuteron kinetic energy 25 – 250 keV

• we used fast plastic scintillator BC-418 as active target (AT)

• AT in coincidence with NE-213 liquid scintillator used to test limits of the detector and observe low energy proton recoils from n-elastic scattering in BC-418

Motivation

• data on relative light response of plastic scintillators to heavy charged particles scarce and/or non-existent below 350 keV

=> Determination of neutron detector efficiency depends on the threshold, big systematic errors for detection of low energy neutrons

=> Cross calibration with γ-sources difficult

• general applications: - (not so) fast neutron, proton detection - nuclear safeguards (search for 3He substitute(s)) - modeling of detector response - etc…

light response• The only information from Bicron (aka Saint-Gobain) is for BC-400, or semi-generic for high energy only. => what about BC-418 ?

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Smith et al., NIM64(1968)

300 keV

Same as BC-400

• light response data scarce and/or nonexistent for Eproton< 300 keV

Experiments at WNR

• proton beam (Ep=800 MeV) impinges on bare tungsten spallation target

• 4FP15R – neutron beam line 15° on the right of the proton beam axis – neutron energies ~450 keV – 800 MeV (with 1.8 μs beam pulses) – neutron energies ~120 keV – 800 MeV (with 3.6 μs beam pulses)

Experimental setup

• neutron beam impinges on the active target (BC-418; 2mm thick)

• energy of beam particles is determined from their time-of-flight • when neutron is elastically scattered in the active target (AT) the recoil proton (Ep = f Ebeam) is detected in AT in coincidence with elastically scattered neutron detected in neutron detector (NE-213 2x2 inch cylinder) (En= (1-f) Ebeam )

• f is function of scattering angle (=0.11 for Θ=20°; =0.5 for Θ=45°; )• analog signal from AT integrated by LeCroy 4300B FERA QDC

• most of the beam neutrons with energies ~ 1-5 MeV

• time-of-flight to AT for 1 MeV neutron is ~ 1.2 us

• time resolution ~ 2ns => high energy-resolution

• events of neutron elastic scattering in AT selected from 2D-plot of ToF(AT=>ND) vs. Ebeam

=> defined by complete kinematics

Ebeam [MeV]

Ebeam [MeV]

To

F(A

T=

>N

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ns

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nts

elastic scattering

Experimental results

Ep-recoil [MeV] Ep-recoil [MeV]

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res

po

ns

[A

.U.]

lig

ht

res

po

ns

[A

.U.]

high gain low gain

Ep-recoil = 100 ±10 keV Ep-recoil = 250 ±25 keV

light respons [A.U.]light respons [A.U.]

241Am (59.54 keV)

133Ba (~31 keV)

Smith et al. (68)


• measurement of the BC-418 light response to both protons and electrons reaches new low energy limits for plastic scintillators


• BC-418 light response data seem to confirm that light response to protons increases with respect to light response to electrons below ~ 300-500 keV

p/β ratios:

November 4, 2010DNP Fall Meeting, 2010

Brian DaubMassachusetts Institute of Technology

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Measurement of Neutron-Proton Total Scattering Cross Section by

Neutron Transmission

Brian Daub, Vladimir HenzlMassachusetts Institute of Technology

Michael Kovash, Khayrullo ShoniyozovUniversity of Kentucky



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Motivation

There are numerous fields which would benefit from precise n-p total cross section data.

Two Body Nucleon Interactions Nuclear Reactors Detector Efficiencies Particle Astrophysics



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MotivationHowever, there are few measurements of the n-p total

cross section below 500 keV.



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Transmission Measurement

Data taken using the Van de Graaff Accelerator at the University of Kentucky.

Neutrons produced by pulsed protons on a LiF target, through the 7Li(p,n)7Be reaction.

Detector is placed directly in the beam.

Sample is placed in the beam-line upstream of the detector.

Neutrons are scattered out of the beam by the sample.

Determine total cross section from number of neutrons scattered out.



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Setup for Transmission Measurement

287 cm from LiF to Neutron Detector

85 cm from LiF to Sample



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2.25 MeV protons pulsed at 1.8 MHz to produce neutrons up to 450 keV. Minimum energy was 200 keV.

Neutron detector was 5-inch diameter BC501 liquid scintillator.

287 cm flight path from LiF target to neutron detector.



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Four Samples

1/2 Inch Carbon

1/2 Inch CH2

1/4 Inch CH2

Wax Blocker Ratios of normalized

target-in to target-out yields give cross section independent of dead time and efficiency.



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γ-flash from LiF target

neutrons producedfrom LiF target

Neutron time of flight spectra, showing deficit of neutrons.



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Transmission MeasurementCorrelated band in neutron energy (from time of flight) vs. neutron

detector pulse height, used to exclude non-neutron background.



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First Results - Carbon Total n-C scattering cross sections with Endf Tabulation.

Data matches Endf within ~2%.



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First Results - Carbon Our results are consistent with previous measurements.



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First Results - Hydrogen Total n-p scattering cross sections with Endf tabulation and

other data in range. Most results ~10-15% difference with Endf.



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Results Tabulations match with higher and lower energy range, but

deviates in region with our results.



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Future Measurements

γ-ray background-rejecting detector

Discriminates between neutrons and γ-rays Tested at LANSCE in August 2010

Extending results to

Lower Energies: Lower repetition rate beam at UKY allows for longer times of flight; tested in March 2010.

Higher Energies: Increased proton energy yields higher incident neutron energies.

Planned run in January 2011 at UKY with these additions.



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Cross Section Calculation

Intensity as a function of Thickness Yield is Intensity times efficiency timeslivetime.

Yield as a function of thickness. Efficiency cancels in ratio.



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Cross Section Calculation

Intensity proportional to beam current. T x J = Q, livetime integrated current.

Cross Section is now independent of efficiency and deadtime.

a scintillation detector for neutrons below 1 mev with gamma-ray rejection scintillators are 3 mm...

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