Normalization of the NPDGamma Experimental Data

F. Simmons, C. Crawford University of Kentucky, for the NPDGamma collaborationBackground: NPDγ Experiment

In the NPDGamma experiment, polarized neutrons strike a parahydrogen target to form a deuteron and release a photon (gamma ray). The correlation between the spin of the neutron and the direction of the emitted photon can be used to measure fπ, the long-range coupling constant of the force between protons and neutrons (the hadronic weak interaction).

This reaction is parity violating, which means that it does not look the same in a mirror reflection. That is because the reflection of the spinning neutron does not look the same as the reflection of the gamma ray. Parity violation reactions are very small (one part in 107), and so we need 1017 events to resolve this reaction to 10%.

Normalization

The gamma rays detected during a pulse need to be normalized with the number of neutrons incident on the hydrogen target during that pulse. There are two ways to do this. The first method is by using a neutron monitor above the target, which would give the neutron flux out from the neutron guide. The second method is to measure the proton current into the target for each pulse. The current can be used to determine the number of protons in the pulse, which corresponds to the number of neutrons splayed during the pulse.

My summer project was to write computer drivers to access the proton current information from a specialized electronics crate, which obtained information from the SNS accelerator.

Result of Summer Project

The proton current is sent to a VME crate via a cable. A program was written to read the proton current from a module in the VME crate once per pulse (60 Hz). We were successful in writing the driver and reading out the data. The data collected was bimodal. This indicates that there is a problem with the data, which we are still working to understand.

Experimental SetupTo measure the correlation between the neutron spin and the direction of emitted gammas, three components are needed: a) an intense source of polarized neutrons (~1017 neutrons), b) a liquid hydrogen target; and c) gamma detectors capable of measuring A ~10-8.

An intense neutron beam is produced by pulsing a high energy proton beam on a mercury target.The 60 Hz pulse structure gives timing information to determine the neutron energy. The neutrons are moderated in liquid hydrogen before being guided to the NPDGammaexperiment andother neutroninstruments atthe SNS.

Spallation Neutron Source

Oak Ridge National Laboratory

CsI Gamma Detectors

48 CsI scintillators, grouped in 4 rings around the target, detect 72% of the ’s produced. The light is collected in vacuum photo-diodes, which are insensitive to stray RF fields from the spin flipper. They are read out in current mode (with counting statistics sensitivity) to handle high count rate of 50 MHz.

Neutron GuideA rectangular guide preserves the intensity, reflecting neutrons by the repulsive nuclear potential. Neutrons with small transverse velocity (8 m/s) have a large enough wave packet to feel the effective repulsion of many nuclei near the point of reflection.

3He Neutron Polarizer

n + n p pn pn

+p

Neutrons with spin anti-parallel to the polarization of the 3He nuclei are absorbed when passing through an optically pumped cell, yielding 65% neutron polarization.

J=0 = 5333 b

/0 J=1 = 0

Neutron Beam Monitors

The polarization is monitored as a function of neutron energy by measuring transmission through the 3He cell via 3He ion chambers before and after the cell.

LH2 Target

The proton target is 16 liters of LH2 cooled to 17 K. The liquid hydrogen is circulated through a catalyst which converts ortho-H2 to para-H2. Para-hydrogen preserves the polarization of cold neutrons (En < 15 meV).

holding field

sn

BRF

RF Spin Rotator

Instrumental drifts must be controlled to the level of A~ 10-8. This is accomplished by alternating the neutron spin on a pulse by pulse basis. The spin is flipped using an NMR technique: it precesses around the rotating B-field of an RF coil, tuned to the Larmor frequency of neutrons precessing in the holding field.