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ADC dead time correction

Various off-line systematic checks of scaler values, as well as comparison of the fusion yield amongst different runs were done to make sure that the system was functioning properly. During one such check, it was discovered that some runs had fewer counts in the Si spectra compared to other runs. After some investigation, it was realized that a change made to the germanium detector circuit had caused the problem in the Si detectors, and a method was devised to correct for the loss.

The runs in questions are Runs 1683 - 1690 taken with Target II-9 (Table 4.3, page [*]). A few initial observations included:

Attention then was drawn to a circuit modification which took place during Run 1683. Before this run, a germanium detector had been disabled simply by unplugging the fast timing signal from the circuit, because its high trigger rate was causing a significant dead time for the data acquisition. (Removing the germanium detector from the trigger reduced the dead time from about 30% to 20%). In the middle of Run 1683, the circuit was modified with intention of allowing the germanium data to be recorded when other sub-detector systems fired while keeping the germanium sub-trigger out of the master trigger so that there would be no trigger if only the germanium fired. This was implemented by reviving the germanium sub-detector circuit, but with its sub-trigger (c.f. TRGn in Fig. 3.19 in page [*]) not connected into logical OR for the master trigger (TRGF in Fig. 3.19).

The problem was, as it turned out, the spectroscopic ADC (AD413A, Ortec), which was shared among the germanium detector and two silicon detectors, and which was being ``blocked'' when only the germanium sub-trigger fired, but nothing else. AD413A, unlike other camac ADCs and TDCs, had no ``fast clear'' function which would ensure in the event of no master trigger, the ADC/TDC buffer was cleared by a fast pulse signal in order to be ready for the next event. Of course, when there was a master trigger, the ADC buffer was cleared (by a CAMAC command) after the data was read into the computer. Even when no sub-trigger but germanium was activated, however, the ADC gate[*] was opened by the sub-trigger, hence the germanium energy data was still accepted by the ADC. The problem is that AD413A worked by design in such a way that when the master gate closes, no pulses were accepted until a CLEAR command was given, even if the master gate opened again. Therefore, if the germanium fired without any other triggers, and then either one of the Si detectors triggered on the following event, the Si signal was not accepted by the ADC, hence only zero was recorded. Yields of Si events were thus affected by the change in the germanium circuit.

In order to obtain a correct normalization, we derived a procedure to determine the ratio of ``good'' ADC events to ``bad'' ones. We first define the following conditions in order to identify the different events:

Firing of Si sub-trigger (Si1 or Si2)
Recorded Si energy = 0
Recorded Ge energy > 0 (i.e., not 0)
Recorded Ge time is an over flow
We used a combination of above conditions to select the events. Several different types of events can give a similar signature, and they are summarized in Table 8.1.
Table 8.1: Comparison of different types of events and their signatures. Event (i-1) is the event preceding a silicon detector event (Event i). LLD stands for low level discrimination. The conditions are defined in the text.
Type Event (i-1) Event i Condition ADC
  Ge Si Ge Si   blocking
I yes no no yes yes
II yes no yes yes yes
III no no no LLD no
IV no no yes LLD no

From the discussion above, the conditions 1 and 2 select obvious candidates for the ``blocked'' events. These candidate events could have zero silicon energy for two reasons; [a] the ADC blocking which is our concern (Event type I, II in Table 8.1), or [b] a real silicon event, but with its energy lower than the low level discrimination[*] (LLD) of the ADC (Event type III, IV)[*].

Ignoring for the moment the small chance of coincidence between an LLD event and germanium trigger in Event i (Type IV in Table 8.1), the above [a] and [b] can be distinguished by Condition 3. That is, if the event was due to blocking (case [a]), there should be non-zero germanium energy in the ADC which came from the previous Ge event (Type I, II). On the other hand, if it was due to LLD (case [b]), there is no germanium energy information left over from the previous event, hence the germanium energy is zero (Type III), unless it is a Ge-LLD coincidence event (Type IV).

The condition 4 provides information as to whether there was a germanium hit in Event i. If there was no germanium trigger, then the TDC clock would not be stopped and an overflow would be recorded. This could be used to discriminate between event type I and II, as well as between III and IV. However, in the end only Condition (123) was used to identify the blocking events. This condition inevitably included event type IV as well, which was not a blocking event, but the analysis of normal runs suggested that this type of event occurs in less than 0.5% of the total Si events. The alternative Condition (1234) would have been much worse, since it would miss selecting event type II which has a higher probability than type IV. The problem here was that when there was Ge hit in Event i, there is no way of knowing where a Ge hit also occurred (and blocked the ADC) in Event (i-1).

An independent check of the ADC problem was conducted using a quite different effect. It took advantage of an unwanted background for our main measurements, namely, the signal from the scattered beam muon directly stopping in the Si detectors[*]. When looking at the fusion signals, this background was reduced to a negligible level by rejecting the signals at prompt times, but here we used the intensity of this prompt beam peak to check to see if the silicon detector system was properly functioning. When the ADC was blocked, not only fusion events but also the beam stopping background events would be lost. Hence, by normalizing the beam background intensity to GMU, we could effectively measure the ADC live-to-dead time ratio. The limitation was that we had to assume that the beam was stable; a small change in the position of the beam could change the number of muons that could go through a tiny gap in the cryostat to reach the Si detectors. The ADC dead times derived from this method and the above method using scalars agree reasonably well, giving us some confidence in correcting for the ADC blocking events.

next up previous contents
Next: Energy spectrum features Up: Si detector data Previous: Si detector energy calibration