Measurement of the parity-odd angular distribution of $\gamma$ rays from polarized neutron capture on ${}^{35}\mathrm{Cl}$

We report a measurement of two energy-weighted $\gamma$ cascade angular distributions from polarized slow neutron capture on the ${}^{35}\mathrm{Cl}$ nucleus, one parity-odd correlation proportional to $\overrightarrow{s_n}·\overrightarrow{k_{\gamma }}$ and one parity-even correlation proportional to $\overrightarrow{s_n}·\overrightarrow{k_n}\times \overrightarrow{k_{\gamma }}$. A parity-violating asymmetry can appear in this reaction due to the weak nucleon-nucleon interaction, which mixes opposite parity S- and P-wave levels in the excited compound ${}^{36}\mathrm{Cl}$ nucleus formed upon slow neutron capture. If parity-violating (PV) and parity-conserving (PC) terms both exist, the measured differential cross section can be related to them via $\frac{d\sigma }{d\mathrm{\Omega }}\propto 1+A_{\gamma ,\mathrm{PV}}cos\theta +A_{\gamma ,\mathrm{PC}}sin\theta$. The PV and PC asymmetries for energy-weighted $\gamma$ cascade angular distributions for polarized slow neutron capture on ${}^{35}\mathrm{Cl}$ averaged over the neutron energies from 2.27–9.53 meV were measured to be $A_{\gamma ,\mathrm{PV}}=(-23.9\pm 0.7)\times {10}^{-6}$ and $A_{\gamma ,\mathrm{PC}}=(0.1\pm 0.7)\times {10}^{-6}$. These results are consistent with previous experimental results. Systematic errors were quantified and shown to be small compared to the statistical error. These asymmetries in the angular distributions of the $\gamma$ rays emitted from the capture of polarized neutrons in ${}^{35}\mathrm{Cl}$ were used to verify the operation and data analysis procedures for the NPDGamma experiment, which measured the parity-odd asymmetry in the angular distribution of $\gamma$s from polarized slow neutron capture on protons.

Figure 1

NPDGamma apparatus, with chlorine target shown inside the detector array (one of the experimental geometries).

Figure 2

(a) Drawing of the first liquid carbon tetrachloride target used in the experiment (CONF1), inside an aluminum container and (b) the improved target made of teflon (CONF2).

Figure 3

A section of a typical cold spectrum corresponding to 2.5 neutron pulses is shown. Regions V1 and V2 are indicated corresponding to average voltages in a low and high part of the spectrum. Dotted line represents the constant background that is the sum of the electronic pedestal and $\beta$-delayed signals.

Figure 4

Top-down view of the detector array. Beam direction is $+\widehat{z}$, beam left is $+\widehat{x}$, and $+\widehat{y}$ corresponds to the spin-up neutron direction.

Figure 5

A beam's eye view of a ring of detectors. Detectors $i$ and $j$ are an example of a pair that in the ideal case would have geometrical factors of equal magnitude but opposite sign.

Figure 6

In the above figure, the $PV$ geometrical factors for ideal detectors are shown. Detectors are numbered starting at 0 with the upstream, beam-right detector, and continuing clockwise. For example, detectors 12 and 23 are located in the second ring, top, beam-right and beam-left, respectively.

Figure 7

Raw geometrical asymmetry determined for each detector pair for the CONF1 data set.

Figure 8

Raw asymmetry calculated for each detector. The lines correspond to the calculated geometrical factors, $G_{\mathrm{PV}}^i$, scaled for comparison, to illustrate that the measured raw asymmetries are primarily the result of an PV asymmetry. The grid is to show the highlight the offset in the vertical direction.

Figure 9

Transient signal measured in a monitor ADC channel, with a 9.48 V battery signal as the input.

Figure 10

The additive (a) and multiplicative (b) asymmetries measured after the sources of false asymmetries were eliminated. Both instrumental asymmetries are consistent with zero at the ${10}^{-9}$ level.

Figure 11

Raw chlorine asymmetries for data set designated CHL3. The line represents the PV geometrical factors scaled by $-21.6\times {10}^{-6}$. They reproduce the shape of the data well, signaling that the raw asymmetry is due to the large PV component.

Figure 12

Raw PV chlorine asymmetries are shown from measurements done in three different geometrical configurations. The inner error bar is statistical only, whereas the outer error bar is total, once the geometrical factor uncertainty has been added.