Event Rates

The total event rate for each liquid is presented, with cuts made only to remove “failed pulses”, namely noise triggering, (for a full explanation, see section 3.9) but with no further pulse shape discrimination. The number of pulses present gives an indication of the dark current as a function of drift field. The values of electric field quoted are for the region of the detector for which anode pulses are detected: the anode-grid region.

In this instance DIN was the only solvent for which samples were available with and without fluors present in the liquid. The event rates are shown in figure 4.21.

In both of the liquids, the event rate seems to plateau around an electric field of 4.5 kVcm−1, however there is a difference of roughly a factor of two between the

Figure 4.21: Event rate as a function of electric field in DIN solvent (black) and Ultima Gold F cocktail (red).

event rates. For fields below 3.5 kVcm−1, the rates are very similar, however. For higher electric fields, it appears that the presence of fluors in the liquid is detrimental to charge transport. The difference between the two liquids is further examined and discussed in section 4.2.3, where the current has been measured in both liquids.

The event rates for the solvents MIPN and MIBP are shown in figures 4.22 and 4.23 respectively. MIPN has comparatively little pulse activity at drift fields below 6 kVcm−1. The shoulder at the higher electric fields measured may be leading up to a plateau at around an event rate of 3500 pulses. Alternatively, the rate may not plateau, and the shoulder may be leading up to the breakdown of the liquid. The electric field was limited to this level by the output range of the high voltage power supply.

The event rate for MIBP is likewise at lower event rates than DIN for the electric fields lower than 5 kVcm−1, although the rate sharply increases between 5 and 6 kVcm−1, after which point the event rate is higher than all of the other liquids measured.

LAB was only obtained as a scintillation cocktail, Optifluor O. It was found that the liquid could not sustain a high voltage, and the lead-up to breakdown at voltages above 5 kV can be seen in the total event rate, shown in figure 4.24. The dark current in LAB has also been measured, and is discussed in section 4.2.3.

The dielectric strength of this liquid is too poor for it to be considered as the active medium of any detector requiring high electric fields. Any characterisation of the mobility of the liquid was impossible, since the electric fields required to

Figure 4.22: Event rate as a function of electric field in MIPN solvent.

Figure 4.24: Total event rate as a function of electric field in LAB scintillator cock- tail.

Figure 4.25: The event rate as a function of electric field in Ecoscint O.

work on charge transport in hydrocarbons [78], the shape of the LAB molecules (as shown in table 2.5) is a long molecular chain, which is the shape synonymous with low mobility liquids. The sampling for the dataset presented in figure 4.24 could be considered to be triggering in noise, as the data samples present show no consistent pulse structure, unlike the other data samples.

At the time of data-taking, PXE was obtained only as a scintillation cocktail, Ecoscint O. This liquid also showed signs of breakdown at high voltages, at fields of 7 kVcm−1 and above. The event rate is shown in figure 4.25, and shows a low initial rate, with a large increase in event rate above 6.5 kVcm−1.

current, followed by a decrease over time. The pulses observed at high electric fields in this instance considered to be the precursors to breakdown could in fact be the high initial dark current. This is further discussed in section 4.2.3.

The liquid PXE was subsequently sourced as a solvent, without scintillation fluors, and subsequent measurements were taken with this liquid, using much higher electric fields, without signs of breakdown. This suggests that the presence of fluors may be lowering the dielectric strength of the liquid.

Drift speed measurements

In reference [2], electron drift speed measurements were presented for the liquids DIN, MIPN and MIBP. The drift speed was calculated by selecting a population of pulses, fitting the rise time distribution with a Landau function, and calculating the speed across the 7 mm anode-grid distance in the gridded ionisation chamber. The selected population of pulses is shown in figure 4.20(b). Pulses belonging to this population were found to be present in all of the data. Using cut-based pulse selection required a large number of cuts, however, and produced an inefficient, but pure sample.

In order to select a pure sample more efficiently, and thereby make improved measurements, the data taken in DIN has been returned to using a pulse fitter (as described in section 3.9). The cut-based analysis is presented first, followed by the pulse fitter analysis.

Despite this data being presented as a drift speed measurement, it is, in ret- rospect a flawed measurement, in light of the dark current present in the detector. A proportion of the pulses at least must belong to the dark current, and therefore have no well-defined start point. Although there is clearly charge moving in the detector, the signal induced by said charge cannot be used to make measurements of the drift speed. The data and cuts are nevertheless presented as in [2] for completeness.

The drift speed in MIBP as a function of electric field is shown in figure 4.26. The cuts made to select the desired pulse population are the following;failflag<1, amplitude>0.02, position<325µs, onset>100µs, decaytime<175µs. These cuts are applied to the rise time data histogram at each electric field value. For reference, all of the pulse parameters are summarised in table 3.5.

The first cut gets rid of data samples which contain nothing with a pulse structure, the second of any baseline triggered pulses with low amplitude. The cuts on onset, position and decay time are those which decrease the efficiency of the pulse selection. They ensure that the pulse falls with the true onset within the data window, and the fast decay time characteristic of the pulse shape is measurable,

Figure 4.26: Drift speed as a function of electric field, in MIBP.

rather than an artificially short decay time from a pulse at a late position in the data sample.

The drift speed increases with field strength up to an electric field of around 6.2 kVcm−1, where it levels off, to a drift speed of around 180 ms−1.

The drift speed in MIPN as a function of electric field is shown in figure 4.27. The cuts made for this pulse selection were the following; failflag<1, ampli- tude>0.04, position<625µs, onset>100µs, decaytime<375µs. The justification for these cuts is identical to the MIBP case, as described above.

The drift speed increases as a function of electric field, although the rate of increase is nonlinear, with the largest increase at fields above 6 kVcm−1. At an electric field of 6.93 kVcm−1_{, the drift speed has been measured to be 143±9 ms}−1_.

The drift speed in DIN as a function of electric field is shown in figure 4.28. The cuts applied to select this pulse population were the following;failflag<1, am- plitude>0.06, position<700µs, decaytime<300µs. The justifications for the cuts are identical to the cases of the other two liquids.

The drift speed in DIN appears to increase roughly linearly over the range of 5-7 kVcm−1. In an electric field of 6.93 kVcm−1, the drift speed is 98±2 ms−1.

In all of the liquids the absolute rate of the pulse selection is much lower than the expected rate of the alpha source present in the detector. The activity of the source is 37 kBq, and for an example liquid of DIN, the total pulse rate at the highest electric fields corresponding to roughly 17 Hz, without accounting for pulse pileup due to the sampling window. With this full event rate, the efficiency of the detector would be around 0.05%, and with a pulse selection cut, the rates were

Figure 4.27: Drift speed as a function of electric field, in MIPN.

Figure 4.29: A scatter plot of two pulse fitter parameters, showing two distinct populations in a data sample taken with 3.2 kVcm−1 drift field.

In light of further experiments, discussed in section 4.2.3, it is suggested that the charge being measured with this detector was in fact, not the alpha source, but was due to a dark current in the liquid itself.

The DIN data was revisited using a pulse fitter analysis, to try to improve on the efficiency of the cut, and examine the different pulses present in the distribution. Two prevalent pulse populations observed in the liquid were selected, examples of which are shown in figures 4.20b and c. A pulse template was created from a small, hand selected sub-group of each these types of pulses, as described in section 3.9. These pulse templates were then each applied to the full data set in turn.

Thescaleresult and thelogms parameters, fully described in section 3.9 were plotted against each other, to give a visible representation of how the different scale factors grouped against the deviation from the template. For the pulse type shown in 4.20b, there are two consistently distinguishable populations in the fit. For the lower drift field data sets, these populations are visually distinguishable in the scatter plots, as shown in figure 4.29. For the higher voltages, the populations are less distinguishable from the scatter plot, but the difference in density of points is clearly evident from contour plots, as is shown in figure 4.30.

These plots show that a cut at a value of 15.2 units on the log mean square deviation scale would separate the populations. Pulses below this value, namely pulses more similar to the template, are selected. Using this cut, combined with a cut to exclude failed pulses (as described above), the efficiency is much better than

Figure 4.30: A contour plot of two pulse fitter parameters, showing two populations which are not very well separated, in a data sample taken with 5.1 kVcm−1 drift field.

4.31, for a drift field of 3.2 kVcm−1. The blue line represents the full, uncut sample (4213 pulses), the green shows the pulsefitter selected pulses (920), and the red shows the pulses selected using the previous data cuts (46). The larger selection of pulses allows for a much better fit, whilst discriminating against other pulse populations.

This cut,failflag<1 , logms<15.2 was applied to each of the data sets, and the rise time data was fitted with a Landau distribution. This is shown in figure 4.32, as a function of electric field. The drift speed measurements using a parameter-based fit are also shown, as well as the Landau distribution fit to an uncut sample, are shown in black. For electric fields up to 5.4 kVcm−1 the fitter-based pulse selection has similar mean values to the parameter-based pulse selection, but smaller errors. This means it can be considered to be selecting the same pulses, with a high purity. For drift fields above 5.4 kVcm−1, the fitter-based pulses are much closer to the uncut sample. This supports the visual evidence that the pulse distributions are not well separated by the pulse fitter at higher electric field values, and explains why the selection cut made little difference to the pulse parameters, namely risetime. In order to select the correct pulse population with high purity for the entire range of electric fields, more stringent cuts would have to be applied. Since it transpires that there is ultimately little we can learn from a clean spectrum of this data, these extra cuts have not been implemented.

The second filter pulse class (as shown in figure 4.20c) yields much less in terms of selection of pulse population. The filter response was much more homo-

Figure 4.31: A histogram of the rise time pulse parameter at a drift field of 3.2 kVcm−1, showing histograms of the uncut sample (blue), the pulse fitter-based cut selection (green), and the parameter-based cut selection (red). There are 5 ns per timing bin.

Figure 4.32: The drift speed in DIN as a function of electric field, showing the uncut sample (blue), the pulse fitter-based cut selection (green), and the parameter-based cut selection (red).

Figure 4.33: A scatter plot of two pulse fitter parameters, which shows a concentra- tion of pulses, but no distinct populations, in a sample taken with 4.6 kVcm−1 drift field.

geneous throughout the range of voltages, and therefore no additional cutting has been made using this tool. An example of the scatter plot from this pulse filter is shown in figure 4.33.

The “drift speed” measurements in this section are presented for complete- ness, and in spite of not having value in terms of determining the charge transport speed of the medium, they do show the variation in risetime of background pulses with electric field. The changes present in the sample provide strong supporting evidence for the dark current discussed in the following section. A combination of these data gives more of an insight into how the liquids behave in detectors of dif- ferent types, namely the pulse shapes in the uniform fields of an ionisation chamber compared to the high field regions of a MWPC.