Description of French Čerenkov Detectors
| This document briefly describes the basic features of the French Čerenkov Detectors and helps to deal with possible troubles. It should be read in conjunction with Larry Lee's detailed manual on the North American Čerenkov detectors. |
M. Versteegen, LPSC
Grenoble
Updated July 19th 2006 JS real
This How-to cover the description of the french Cerenkov, a description of the photomultiplier and is basis, and describe the way to change a basis or PMT.
French Čerenkov Detectors Overview
As can be seen on picture 1, French Čerenkov Detectors have two main components :
- The Čerenkov box which contains the aerogel and diffusive white paper.
- The yellow tubes which close the Čerenkov aerogel box. They contain the four PMTs which detect the photons emitted by the aerogel after diffusion.
Detailed about cerenkov design can be
found here.
At 687 MeV, the elastic electrons detected have a momentum of 340 MeV.
They are selected by a coincidence between the CEDs and FPDs. The
cerenkov is located rigth after the CEDs (see figure 1). Passing throw
5 cm of aerogel (Masthita company, silica aerogel with a refraction
index of 1.03), the electrons create around 350 cerenkov photons.
These photons diffuse in the ligth box and we expect to detect around 6
photo-electrons on the 4 PMTs for a single elastic electrons. The
pions for this kinematics (mainly coming from the neutron of the
deuterium target , but also from target windows) have a momentum which
is under the threshold of cerenkov emission. they can be detected by
emitting some delta-ray passing throw CEDs or other material within the
path. The delta-ray will have a momentum ranging from 0 to 0.1 MeV with
a mean value of 0.015 MeV. Thus the number of cerenkov photons created
will be between 0 and 400. The number pions which give a cerenkov
emission is 2.2%. On this 2.2% only 25% will give more than 2
photo-electrons on the 4 PMTs.
Picture 1. Overview of French Čerenkov Detector number 6
The scintillators of the CEDs can also be seen on picture 1, they are covered with black tape and installed beyond the Čerenkov box.
The PMTs are the XP4572/B02 produced by Photonis. The typical gain is 2.107 and their operating maximum HVmax is of 2600V.
20 PMTs are available. 16 are mount on the four cerenkov. The four spare PMTs are located in the French G0 cubicle, upstair of the counting house. Gain and noise measurements have been performed in Grenoble on each PMTs, the results of these tests can be found here. Magnetic shielding tests were also conducted in LCMI (Laboratoire des Champs Magnetiques Intenses) laboratory in Grenoble (see detailed report here). The most efficient magnetic shielding (yellow magnetic housing) which came up is detailed on picture 2. It is composed by 3 layers of iron 2 mm thick spacing by 2 mm of air and one layer of mu-metal 0.8 mm thick.
Picture 2. Cerenkov PMTs magnetic shielding
For each
PMTs, the gain of the tube was measured as a function of the HV for the
photoelectron signal :
|
octant # |
PM # |
SN # |
back off(cm) |
mesure gain |
HV (Volt) 2.5x107 |
HV (Volt) 2.x107 |
HV (Volt) 1.5x107 |
HV (Volt) 1.x107 |
|
2 |
1 |
60175 |
10+2+3 |
ok |
2252 |
2117 |
2009 |
1600 |
|
2 |
2 |
60174 |
10 |
ok |
2004 |
1918 |
1849 |
1776 |
|
2 |
3 |
60165 |
10 |
ok |
2013 |
1956 |
1870 |
1736 |
|
2 |
4 |
60176 |
10+2+3 |
|
2210 |
2193 |
2107 |
1991 |
|
4 |
1 |
60143 |
10+2+3 |
ok |
2175 |
2187 |
2089 |
1945 |
|
4 |
2 |
60145 |
10 |
ok |
1986 |
1857 |
1784 |
1647 |
|
4 |
3 |
60250 |
10 |
ok |
1838 |
1787 |
1715 |
1619 |
|
4 |
4 |
60164 |
10+2+3 |
ok |
2020 |
2015 |
1899 |
1600 |
|
6 |
1 |
60156 |
10+2+3 |
ok |
2197 |
2145 |
2061 |
1969 |
|
6 |
2 |
60161 |
10 |
ok |
2035 |
2094 |
2014 |
1908 |
|
6 |
3 |
60146 |
10 |
ok |
1817 |
1776 |
1710 |
1620 |
|
6 |
4 |
60040 |
10+2+3 |
ok |
2346 |
2245 |
2128 |
1600 |
|
8 |
1 |
60172 |
10+2+3 |
ok |
2291 |
2266 |
2181 |
2088 |
|
8 |
2 |
60162 |
10 |
ok |
2395 |
2294 |
2216 |
2139 |
|
8 |
3 |
60131 |
10 |
ok |
2270 |
2154 |
2074 |
1989 |
|
8 |
4 |
60173 |
10+2+3 |
ok |
2355 |
2300 |
2210 |
2095 |
Photomultiplier's
Basis:
The
voltage-divider/bases used on French PMTs are the same as the North
American ones. They are Photonis, model VD105K, negative high-voltage
bases, and complete technical data is
available on Photonis webpage. An amplifier with a gain of 10 was added
to the bases, and can be short circuited thanks to a switch,
directly located in the printed circuit. The amplifier is mechanicly
mounted as a second stage on the basis itself and is located IN the
shielding (contrary to NA which are outside). Picture 3 show the
electronic diagrams and the implementation on the printed circuit.
 |
 |
Picture 3. Basis diagram and implementation
Note : Make sure that
the switch does not indicate "anode" but "AMP OUT" when changing the
base. After July 2006, a variable resistor has been add to adjust the
baseline of the amplifier output. This baseline has to be put to zero
with the help of the voltmeter (or oscilloscope) using a small
screwdriver.
The potentiometer is the one monted on the amplifier stage (Not the one
on the lower voltage divider stage). To adjust the baseline to zero
with the voltmeter one has to adapt the impedance to 50 Ohms, using a
BNC T and a 50 Ohms terminator.
How to replace a PMT/base of a French Čerenkov Detectors
Before the detailed steps describing how to mount a PMT, here is a sketch to show what are the different parts concerned.
The PMT lies on a chock which measures 15 cm for the 1st and 4th tube, and 10 for the 2nd and 3rd central tubes. These chocks are aimed to optimize the shielding of the PMT from the magnetic field (see picture below). They allow to place the PMT at different back off from the diffusion box (at present either 10 or 15 cm, to be optimized with beam and field).
Picture 4. Cerenkov detector setup
A sheet of teflon wraps the PMT inside the mu-metal tube. A mechanical guide and an electronic base are then mounted on the PMT, assuring its stability and allowing signal and HV cables to be connected to the cap. This base contains the electronics to power the PMT and amplify its anode signal to be delivered to the acquisition system. This is shown on picture 4.
Picture 4. Overview of the PMT assembly
After having removed the faulting PMT or its base, here is the procedure to be followed to mount a new duo (preferably call a French expert on site).
The parts to be assembled are shown on the picture 5.
Picture 5. Separated parts from the French PMTs.
From left to right : cap, base, PMT, guide, chock, teflon
NB : Before any manipulation of the detector make sure that all HV powers are OFF as well as the LV supply.
Mounting steps :
- Insert the chock. Make sure that you have a 10 cm one for the 2 and 3 tubes, and a 15 cm one for the 1 and 4 tubes. (check if this is still valid after beam studies)
- Insert the teflon wrap. A side of the chock may be manufactured to facilitate the insertion : put the part where the teflon is doubled inside.
- Insert gently the PMT and make sure it goes down on the chock.
- Place now the mechanical guide as shown on the picture 4. The soft part must spouse the end of the PMT.
- Connect the base to the PMT.
- Connect
the base to the cap : high voltage power and signal cable. Make sure
that the clip on the base is on the "AMP OUT" position. Make sure that the baseline has been
adjust to 0.
- Put the cap in place on the yellow tube and screw it.
Note : The cap has two sets of screws, one opens the whole cap to remove the PMT, the other opens only the upper part to only remove the base.