Документ взят из кэша поисковой машины. Адрес оригинального документа : http://rtm-cs.sinp.msu.ru/docs/borexino/laben_neutron_report.pdf
Дата изменения: Thu Jan 24 13:53:55 2008
Дата индексирования: Mon Oct 1 21:17:34 2012
Кодировка:
Preliminary results from the test aimed at improving the µ -induced neutron detection with Laben electronics
A. Razeto, G. Korga, L. Papp, M. Pallavicini This letter reports the tests done on October 20th and 21st to setup the detector for acquiring high multiplicity neutron events. The run 6165 was took with this setup for 15 hours: in the data one event with about 50 neutron was found. Introduction As already reported during the last general meetings and throughly discussed in several occasions, the current configuration of the DAQ system and of the Trigger system does not allow to readout completely events with more than one neutron following a muon in the scintillator. This fact limits our capability to tag C11 events. The effort done during the summer to arrange a group of people dedicated to the maintenance of the Laben boards have suggested a possible solution to the problem. In this document we briefly describe the proposed solution and show the results obtained in a very short test done during last weekend. Description of the problem and proposed solution The picture on the right shows a simplified schematics of the Laben board channel : all the operations are managed by the FPGA which triggers the double sample and the I/O operations on the FIFO. During normal operations the FPGA cleans the FIFO (checking the Not Empty line) every 6.4 s: only one hit is deleted each time. This cleaning procedure is stopped by the Gate input which is generated by the Borexino Trigger Board together with the Trigger line; the Gate is internally reset by the DSP, after the readout procedure is ended and all data are copied. The end of data for each channel is signaled by a fake sample (with the F bit) in the FIFO; this sample is triggered by the Trigger input to the FPGA. The Trigger pulse also starts the interrupt routine in the DSP which takes 4 s to copy each hit from the FIFO to the VME memory (Dual Port Ram);. The gate window in the data depends directly on the timing between the Trigger and the Gate signal generated by the BTB, with a minimum width of 6.4 s. Since the beginning of Borexino, the gate length have always been about 7 µs, a value that is appropriate for normal scintillation events. During the reported tests, by modifying the internal logic of the Borexino Trigger Board (BTB) we managed to have a larger gate window for all event (16 s). Moreover a devoted timing setup was installed for neutron events: as shown in the picture below after an internal event (trigger type 1) where the Muon Trigger Board signaled, a second event is generated immediately after the end of the muon event. The second event (trigger type 128) has a 1.6 ms gate window in which all the hits are collected.


Preliminary results The run 6165 was taken in the night between 20 and 21 October; the run last 15 hours with 165k neutrino events and 1653 trigger type 128 events. Among these only 33 events have a signal compatible with one or more neutrons, and number that is consistent with the expectations. The following considerations are the results of a very preliminary analysis on this run; deeper analysis will be performed as soon as possible. The plot on the right represent the time evolution of the event 15457 and 15458: the histogram is filled with the hits of both event referred to the trigger time of the second event (type = neutron) with 50 ns bin. In the plot is clear the 1.6 ms gate; the first peak at -1.6ms is due to the muon hits, while the second peak at -1.5ms is accounted as a neutron. The plot in the center is a logaritmic view of the previous; in this plot it's possible to see the hits accounted to the after-pulse. In the plot on the left there is a zoom of the same event; here the after-pulse signature is more clear: the bumps few s after the muon peak are present in any high energy event. In this plot however it's possible to fit the tails obtaining 2 slopes: 10 s and 1 s. A zoom of the neutron candidate is shown in the picture below: the rough energy in nhits is about 600 which is consistent with the energy observed for the observed neutrons.


The large gate is sensitive to more than one neutron or to any event in the 1.6ms; in the right plot a second event is displayed; while the second peak (at -1.58ms) is a good neutron candidate (530 hits) the third could be something else (250 hits). Unfortunatelly in this early stage the clustering and the position reconstruction is not yet present for these events, then it's not possible to correlate the energy with the radius. The biggest contribution is from 210Po which can create 0.6 peak/day in the gate (being 14C simple to reject). This system is also sensitive to the neutron in the buffer, even with a quenched energy.

A very interesting event is show in the larger picture below: this is a good candidate for a high multiplicity event. Each line should correspond to e neutron; the smaller plot show an histogram filled with the times of the peaks. An exponential fit gives = 260+-40 s with 2 44/42 which is the first direct measurement of the neutron capture time in PC. In any case the energy of the peaks may bee too low for neutrons in the IV: one explanation is that in event with a lot of hits (this events has 52k hits) few Laben board are saturated and produces no hits. This is caused by the small memory for the hits (700hit on 8 channels). We noticed that this happened on 3 events over the 33 acquired; in any case it's not yet clear if this can harm the position reconstruction.


Conclusions None ,actually. The work is still in progress. However: a) the test seems to give encouraging results. Definetely, it seems that multi-neutron events can be acquired with this modification. b) Further analysis is needed to see how well we can measure the number of hits in a single neutron event and with which accuracy the position of each neutron can be reconstructed c) In parallel with this activity, we are buying 4 FADC boards (CAEN V1731) which we plan to install as a new hardware system that can improve our capability to tag neutrons. It might well be that a combination of this new system with the work described in this short note will yield to satisfatory efficiency