Noise improvement using a pF-only amplitude quantity

Data used in this note is four series from June 19, after 150min PuBe activation, 180V→400V drifted bias ~ 90 minutes each, trigger thresholds 8/8/8

Some drift in gain was seen over time but no time-correction is applied.

Though not shown in this note, standard calibrations (quadratic to Am 14keV and 18keV lines) based on balanced sums of the channel OF quantities were done to similar data.

These balanced pSum calibrations produce around 40 eVee noise sigma.

Here we do a calibration based only on the amplitude of the center channel pF.

Figure 1

Here we see the fraction of amplitude in pF versus pT. We can see the Am lines at high pF-fraction and the 10keV and 1keV activation lines extending from low to high pF fraction. (Note that PTOF is not balanced, which causes the slope in the activation lines.)

Figure 2

The figure above is a spectrum in pF OF only with a high-pF-fraction selection. The four rightmost lines are Am - also seen are the activation lines.

Note that the poor resolution in these lines is mostly due to not applying a time correction.

This spectrum was used as the basis of an energy calibration using only pF. For consistency we again use the 14keV and 18keV lines. Of course this calibration will only be good at high-pF fraction.

Figure 3

The figure above shows a radial quantity - pF divided by the sum of the outer rings pA+pB - plotted versus the pF-calibrated energy. We see of course the 10 keV line has incorrect energies at high radius (bottom of the plot).

Using this plot we can apply an empirical radial correction to the pF-only energy:

Figure 4

Although we have radially-corrected this pF-only quantity we don't expect it to perform well for high radius. We can check the noise width with no fiducial selection:

pF-only calibration, no fiducial selection: 51.4 eVee noise sigma.

This is worse than the pSum case. But we can apply a fiducial cut. For example looking at Figure 4 we can require PF/(PA+PB) > 4. If we look at how much of the 10keV peak is retained in this cut we get 10.1%.

pF-only calibration, 10.1%-efficient fiducial cut: 11.2 eVee noise sigma.

If we are willing to apply a strict fiducial cut (effectively reducing our detector diameter from 100mm to 32 mm) we can get a significant reduction in threshold, by a factor of 4, with sigma below 15 eVee so probably low enough to see an n,gamma signal.