Experimental Nuclear PhysicsSome Recent Activities

1. Development of a detector for low-energy neutrons

a. Hardware -- A Novel Design Ideab. Measure the light response at low energyc. Measure the neutron-proton scattering cross section

2. Measure the electromagnetic polarizability of the neutron

Compton scattering of 100 MeV gamma rays

Experimental Nuclear Physics - Some Recent Activities

1. Development of a detector for low-energy neutrons

a. Hardware -- A Novel Design IdeaDeveloped in Lexington

b. Measure the light response at low energyUKy accelerator/Los Alamos accelerator

c. Measure the neutron-proton scattering cross sectionUKy accelerator

2. Measure the electromagnetic polarizability of the neutron

Compton scattering of 100 MeV gamma rays MAX-lab accelerator in Lund, Sweden

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

Low energy neutrons produce recoil protons of very small range, unlike theelectrons created by gamma rays. For a 3 mm scintillator thickness, no recoil protons from np scattering cross into adjacent cells. But, some low energy scattered neutrons do rescatter in other cells -- usually nottriggering the discriminators.

Therefore,

NEUTRON TRIGGER: (T and B) or (L and R)Most gamma rays fire all 4 PMTs.

Detector Construction

Top Left: The assembled detector. (The bottom PMT is hidden by the table.)Bottom Left: An inside view of five of the scintillators mounted in one light guide.Above: The assembled scintillator box, with five horizontal and five vertical scintillators. Each set is attached to two PMTs operated in coincidence.

Gamma-Ray Rejection

These 60Co spectra were gated by all 4 PMTs firing (“Gamma Rays Selected”), and by only 1 pair of PMTs firing (“Gamma Rays Rejected”).

In-Beam Tests @ WNR/15R August, 2010

“Low Energy Neutrons”: Cut on TOF for E<1.4 MeV; Cut on single-plane events

Pulse height

Measure the Light Produced by Recoiling Protons

• 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

D) [

ns

]c

ou

nts

elastic scattering

Experimental results

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

co

un

ts

co

un

ts

lig

ht

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)

Experimental results

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

NEW!

November 4, 2010DNP Fall Meeting, 2010

Brian DaubMassachusetts Institute of Technology

12 Of 16

Measuring n-p Scattering at Low Energy

There are few measurements of the n-p total cross section below 500 keV.

November 4, 2010DNP Fall Meeting, 2010

Brian DaubMassachusetts Institute of Technology

13 Of 16

Transmission Measurement

Setup for Transmission Measurement at UKy

287 cm from LiF to Neutron Detector

85 cm from LiF to Sample

November 4, 2010DNP Fall Meeting, 2010

Brian DaubMassachusetts Institute of Technology

14 Of 16

Transmission Measurement

γ-flash from LiF target

neutrons producedfrom LiF target

Neutron time of flight spectra, showing deficit of neutrons.

November 4, 2010DNP Fall Meeting, 2010

Brian DaubMassachusetts Institute of Technology

15 Of 16

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

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