Neutron Beam Intensity Measurement For The NPDGamma Experiment
by William Parsons
University of Tennessee at Chattanooga
Faculty Mentor: Joshua Hamblen; Team member: Jeremy Stewart;
Lab Mentor: Seppo Penttila; and help from David Bowman, Paul Mueller, Mark McCrea, Septimiu Balascuta, and Zhaowen Tang
Experimental Setup
NPDGamma Experiment Setup with Aperture Placement
Housing Scanner For P2 and PMT
PMT
Shielding
The NPDGamma Experiment
The NPDGamma experiment is a high-precision measurement of the
parity-violating asymmetry in polarized cold neutron capture on
hydrogen. This will be used to observe the weak interaction between
nucleons which will lead to a better understanding of the interactions as
a whole on the subatomic level.
P1
What This Has To Do With That
P2
In order for NPDGamma to be as precise as it needs to be, it is
important to know the nature of the neutron beam. Therefore knowing
the intensity of the neutron beam is a must. Problem is the beam is too
big (1010 neutrons/s) to count effectively.
Method
Counting Logic System
PMT
The photomultiplier tube is a extremely sensitive
detector of light in the range from ultraviolet to
near-infrared of the electromagnetic spectrum. It
was set up to detect neutrons by placing a 6Li doped
glass scintillator onto the front of the PMT
t0 Output
(npdg DAQ start)
Output from
PMT
Discriminator
Logic Gate
Low
Pass
The Logic system was set up
to count signals greater then
the discriminator threshold
in a window of 5ms. This cut
out signals from cosmic rays
and gamma background.
High
Pass
Filter Construction
Low
Pass
Counter
Data
Logic System
The raw signal for the PMT was run through a set of low and high pass filters
and recorded with counts taken in intervals of 999 t0’s using a logic system
(see left). Each t0 is 1/60 of a second and represents the start of a new
neutron pulse from the SNS. Data was taken with a 21.1ms delay after the t0
from the NPDGamma electronics. This was done to allow for the time of
flight for the neutrons in our desired energy range. The count data was
graphed in a x-scan and a y-scan which we used to find the flux.
Signal
Before Filter
Low Pass
To measure the beam intensity a sampling method was used where two
cadmium apertures (P1 and P2) are placed to cut the beam down so the
neutron counts/sec can be measured by a photomultiplier tube (PMT). This
will also allow the probability distribution of the beam to be mapped by
graphing the counts. This can been used to find the flux of the beam and as
such the intensity. P1 was placed to cut down the size of the neutron beam.
P2 was attached to the PMT which was mounted on a scanner capable of
moving in a xy-plane perpendicular to the beam so that different parts of the
beam can be measured.
After Filter
High Pass
The filters where constructed with R=50Ω, L=4μH, and C=1500ρF
so the signal width would be at 80ns
Calculations
To calculate the flux the following
equation was used:
Attenuation of Beam Through
Matter and other
Experimental Losses
Source
m0 = counts/sec at the peak of the xy-scan
F0 = flux of the neutron beam
Sx = total counts/sec of x-scan
Sy = total counts/sec of y-scan
Ag = area of the beam guide
Δx = step size of x-scan
Δy = step size of y-scan
A1 = area of P1
A2 = area of P2
% Beam Lost
Detector Efficiency
0.10%
Air Attenuation
24-26%
Aluminum Windows in
NPDG apparatus
9.2-9.8%
6Li
P1
Detector Edge Effect
negligible
P1= (3.03 ± .0042) x 107 pixels2
= 0.082 ± 0.0011 in2
= 0.529 ± 0.0071 cm2
P2= (4.5 ± .023) x 104 pixels2
= 0.0012 ± 0.000062 in2
= 0.0077 ± 0.00040 cm2
P2
Both Apertures with ruler
Discriminator Threshold Level
7.9%
Beam Monitor 1
2.44%
Beam Monitor 2
1.70%
Total Beam Lost
39.9%
The area of P1 and P2 was determined by scanning the apertures into a digital image containing
19200 pixels per inch. The number of pixels in the image was counted which yielded the area of P1
and P2. The error for the areas was found by making a small and large polygon around the unclear
region of the edge of the apertures and splitting the difference between the polygons.
Thanks to David Bowman for the derivation
Beam guide
P1
What was Found
moderator
By graphing the different points in the x and y positions we get a general look at how the distribution of the neutrons in the beam.
Because of the reflections in the beam guide this distribution should be a Gaussian shape with the center of the beam at the peak
but as seen in the y-scan this is not the case. This abnormality can only be seen by using the sampling method of this experiment
The Y-Scan of the Neutron Beam
The X-Scan of the Neutron Beam
After correcting for the different sources of beam loss it was found that
the neutron intensity of the beam was (5.97 ± 0.39) x 109
neutrons/sec/megawatt. After the data was taken it was found that P1 was
placed below the beam’s center which could account for some of the
unexpected structure in the y-scan buy we still don’t fully understand the
reason why the y-scan is shaped that way.
What All This Means
The polarized neutron transmission ratio (neutron intensity after polarizer
compared to unpolarized intensity at the end of the beam guide) was 28%
which was expected and also coincides with other measurements of the
beam intensity. This can be taken into account when the primary data is
taken for the NPDGamma experiment this Fall.
The Y-scan shows unexpected asymmetry
The X-scan looks like what was expected