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.

Data

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 t 0’s using a logic system

(see left). Each t 0 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 t 0 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.

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.

Calculations

To calculate the flux the following equation was used:

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

The Y-Scan of the Neutron Beam The X-Scan of the Neutron Beam

Method

What was Found

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

LowPass

HighPass

LowPass

Output from PMT

Counting Logic System

The Y-scan shows unexpected asymmetry The X-scan looks like what was expected

P1= (3.03 ± .0042) x 10 7 pixels2

= 0.082 ± 0.0011 in 2

= 0.529 ± 0.0071 cm 2

P2= (4.5 ± .023) x 10 4 pixels2

= 0.0012 ± 0.000062 in 2

= 0.0077 ± 0.00040 cm 2

Both Apertures with ruler

P1

P2

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.

P1 P2

Housing Scanner For P2 and PMT

NPDGamma Experiment Setup with Aperture Placement

ShieldingPMT

Experimental Setup

Thanks to David Bowman for the derivation

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

P1Beam guide

moderator

Filter Construction

Low Pass High Pass

The filters where constructed with R=50Ω, L=4μH, and C=1500ρF so the signal width would be at 80ns

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 Discriminator Logic Gate Counter

t0 Output(npdg DAQ start)

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.

SignalBefore Filter After Filter

Logic System

Neutron Beam Intensity Measurement For The NPDGamma Experiment

by William ParsonsUniversity of Tennessee at Chattanooga