Neutron Beam Intensity for the Spallation Neutron Source Beamline 13: The NPDGamma Experiment Analysis and Results Jeremy Stewart University of Tennessee at Chattanooga

1

Background
The NPDGamma
Experiment, n p ? d ? , is a precision
experiment designed to test the weak nuclear
force in a proton/neutron interaction by
examining parity violating asymmetry in produced
gamma rays. As such, this experiment will test
the predictions of the Standard Model concerning
interactions of the weak nuclear force. To
analyze the data that will be obtained in the
experiment, the neutron flux at SNS Beamline 13
must be known. The flux is required for modeling
the beam correctly, correcting the experimental
data, and estimating the needed statistics for
the measurement. Accordingly, the purpose of this
project was to measure the actual flux, and to
compare it with the theoretically predicted
value.
Acknowledgments I would like to thank my FaST
Program colleague Daniel Parsons, faculty mentor
Joshua Hamblen, and our ORNL mentor Seppo
Penttila. I would also like to extend my thanks
to David Bowman, Paul Mueller, Mark McCrea,
Septimu Balascuta, and Zhaowen Tang.
Neutron Beam Intensity for the Spallation Neutron
Source Beamline 13The NPDGamma
ExperimentAnalysis and ResultsJeremy
StewartUniversity of Tennessee at Chattanooga
Apparatus The apparatus consisted of two cadmium
apertures, a 6Li scintillator, and a
photomultiplier tube (PMT). The signals from the
PMT were conditioned and counted using Nuclear
Instrumentation Modules (NIM).
Neutron Flux Calculation
The neutron flux is found by integrating the
horizontal and vertical scans. This value is then
multiplied by the step increment in x and y. It
is then divided by the peak neutron flux and the
area of Aperture 2. Finally, this value is
multiplied by the ratio of the area of the guide
and the area of Aperture 1.
Experimental Setup
Cross Section Calculation
Horizontal Scan, Sx
Source Effect
Detector Efficiency 0.1
Air Attenuation 24 - 26
Aluminum Windows in NPDGamma Apparatus 9.2 - 9.8
6Li Detector Edge Effect negligible
Discriminator Threshold Level 7.9
NPDGamma beam Monitor 1 Monitor 2 2.2-2.7, 1.5-1.9
Total Beam Loss 39
Divergent beam distributions should be
Gaussian. As seen in the above graph, this is
what we get in horizontal data.
Vertical Scan Data, Sy

• Results/Conclusions
• The calculated raw neutron flux was (2.78
  0.18) x 109 neutron/s at 800kW power.
• The calculated intensity normalized to 1 MW and
  corrected for losses was (5.70 0.37) x109
  neutrons/s/MW. This flux measurement is
  comparable to the expected value.
• The structure seen in the Y distribution is
  likely due to Aperture 1 being installed 3.81
  0.16 cm below beam center, and the neutron
  moderator being displaced approximately 1 cm in
  the vertical direction. However, more study is
  needed.
• The ratio of the flux measured before the
  polarizer over the flux measured after (Polarizer
  transmission ratio) was 28. This result is what
  we expect, and is consistent with other
  independent measurements.

The graph shows that the beam is NOT a Gaussian
distribution in the vertical direction. This was
not expected! What could be the cause?