Download MinutesWA104_21oct15

MINUTES OF THE WEEKLY WA104 TECHNICAL MEETING – 21/10/2015
Convener at CERN: Claudio Montanari
Present at CERN: Linda Bagby, Umut Kose, Marzio Nessi, Francesco Pietropaolo, Gian Luca Raselli, Maura
Spanu, Francesco Tortorici, Jae Yu, Andrea Zani
Present on Vidyo Connection: Sandro Centro, Angela Fava, Fernanda Garcia, Alberto Scaramelli, Jim Strait,
Dave Warner, Peter Wilson, Robert Wilson, Pavia group
More details about the items reported in these minutes can be found on the Indico page of the event:
https://indico.cern.ch/event/456174/
The meeting opens with Montanari recalling the previous week “Neutrino SBN Internal Review”, which took
place from October 12th to October 16th, and covered first the cryogenics and cryostats of the SBN program, and
then the ICARUS/WA104 refurbishing activities.
The meeting schedule and presentations can be found at the Indico page:
https://indico.cern.ch/event/446818/other-view?view=standard.
A brief summary of the meeting was also scheduled during the CENF General Meeting of Thursday October
23rd, by Peter Wilson. Although Wilson was not able to give the presentation, his slides can be found at the
following link:
https://indico.cern.ch/event/455067/contribution/4/attachments/1175458/1699054/CERN_Assessments_Summar
y_22Oct2015.pdf
After this, Spanu describes the latest activities at the evaporation area. A vacuum level fit for the evaporation
(2x10-6 mbar) is reached and few tests were performed, evaporating samples attached to a PMT mock-up put
inside the chamber. 0.5 g of TPB was used, per test, and the deposition density obtained on the samples (0.1
mg/cm2) is about half the desired value. Montanari notices that more TPB is deposited on the center of the PMT
mock-up, but this is not a problem, as the thickness is still far from the values causing a decrease in
transparency. On the other hand, he suggests to try with 1 g of TPB per cycle, and to test an evaporation on a real
PMT early next week.
Nessi asks about requested uniformity: Montanari replies that the basic request is 30%, but actually it is better to
require a minimum density of 0.15 mg/cm2, in order not to lose efficiency; then up to 0.3-0.4 mg/cm2 there are
no issues relative to decrease in transparency. Spanu points out that a new test with a quantity of 0.8 g of TPB is
already under way.
Raselli discusses PMT tests, showing which PMTs have already been characterised at warm and cold (in some
cases) temperature, but he notices that we are still missing a Multi-Channel Analyzer (MCA) module ordered
few weeks ago. He shows the installation of the bases on the PMTs, and then an example of Single Electron
Response, Peak-to-Valley ratio and Gain-Voltage curve for one PMT, both in warm and cold conditions. In LAr
after 5 days a 50% decrease in gain is measured. Peter Wilson asks about this decrease stability in time, Raselli
replies that they have to gather statistics before giving an answer. Montanari explains that for PMTs in LAr there
is usually continuous decrease in gain, over a period of few weeks to two months: such decrease can only be
monitored and compensated with time.
Uniformity measurements show a discrepancy with Hamamatsu data, but not worrisome. Nessi asks about
measurement mistakes but Raselli excludes them. It is true however that we do not know if measurements were
performed in the same way by Hamamatsu: Pietropaolo notices for example that we do not know which is the
incidence angle of the light used by the firm for their uniformity measurement. A discussion follows, mainly
between Nessi and Montanari, concerning how to solve this discrepancy: possibilities on the table are either to
go directly to Hamamatsu and see how they perform the measurements, or write them down precisely how we
are operating.
Finally, Raselli shows a distortion on the signal falling edge that he calls a reflection. Centro notes that if it does
not change by varying the cable length, then it is not a reflection; Montanari says that it comes from the base.
Raselli contacted Hamamatsu, and he says that they suggest to add so-called “dumping resistors” (10 to 200 
each) in series on the last two dynodes. Such resistors are already mounted on 14-dynode PMTs already in use
by the Collaboration for tests, whereas Dave Warner notices that his group had a similar problem with PMTs in
the BaBar experiment, and they solved such issue in the same way. Raselli shows then an example of waveform
of a 10-dynode PMT, taken after these dumping resistors were added to the base: it can be seen that the
distortion is strongly reduced. He then notices that the base producer has already been informed that a change in
the layout will be requested.
Nessi asks about next actions to be performed, in order to finish the tests in time: he recalls that all PMTs have to
be fully tested. He also suggests to write Hamamatsu again about this issue and discuss with them. Centro
notices that a resistor in that position on the base voltage divider should have no effect on the signals, so
explanations are due. Montanari states that all PMTs must be tested at room temperature within next week, and
10 more must be put in liquid; he also requests daily meetings to attest the progress and solve the signal
distortion issue. Nessi asks about other discrepancies in PMT properties, as measured by Hamamatsu and the
Collaboration: Raselli replies that every other measurement is compatible with Hamamatsu data.