# First Order Memory Dump on the E766/E690/E910/E895/E907 Threshold Cerenkov Counter

### Ed Hartouni

*January 7, 2002*

# Introduction

This documents describes much of what I know or can recall about the
threshold Cerenkov counter that we intend to use for Fermilab E907. I
assume familiarity with the literature posted on the E907 web page:

<http://ppd.fnal.gov/experiments/e907/Cerenkov/Cerenkov.html>

and people and documents referred to therein. The purpose of this
document is to provide as much information as possible for people
working with this detector system.

## The Box

The Cerenkov counter box includes essentially all of the Cerenkov
counter components. The box provides the mechanical platform for the
optics; it provides a light-tight, gas-tight volume viewed by the
photomultiplier tubes. It is designed to minimize the amount of material
along particle trajectories. In order to assemble components and align
the optics, it generally rides on an undercarriage capable of rolling in
and out of position. In the case of E907, this will be lateral motion
with respect to the beam line.

The box dimensions are WxHxD: 9'3"x10'11"x3'9" (2.8194m x 3.3274m x
1.143m). The front and back of the box have openings that are closed by
thin windows. Two primary mirror planes are mounted on a 3-point support
that is used to orient the planes to optimize the focus. The secondary
mirrors and photomultiplier mounts are located on the upper and lower
gas volume bulkheads. The photomultiplier tube bases are located in
electronics bays outside of the radiating gas volumes above and below
the upper and lower bulkheads. Figure 1 shows a side view of the
Cerenkov detector with the box nomenclature.

The front window consists of a Kevlar, Tyvek, and Aluminum composite
fabric bonded to an aluminum frame. This provides a strong, gas-tight
and light-tight window of very low mass. The window is shown in Figure
2. The back window was redesigned for BNL E895. This new window consists
of an aluminum frame, which attaches to the box and makes the gas seal
and a composite laminate which offers structural strength and the
necessary gas and light tightness. The window is shown in Figures 3 and
4.

+--------------------------------------+---+---------------------------+
| ![](media/imag                       |   | ![](media/ima             |
| e1.png){width="3.4763888888888888in" |   | ge2.png){width="2.3875in" |
| height="9.006944444444445in"}        |   | heig                      |
|                                      |   | ht="4.751388888888889in"} |
|                                      |   |                           |
|                                      |   | Figure 1. A schematic     |
|                                      |   | view of the cerenkov      |
|                                      |   | counter is shown on the   |
|                                      |   | left with the major       |
|                                      |   | components labeled. A     |
|                                      |   | picture of the cerenkov   |
|                                      |   | counter as it appeared in |
|                                      |   | BNL E910 is shown above.  |
|                                      |   | In the picture, the beam  |
|                                      |   | enters from the right.    |
|                                      |   | The cerenkov box is       |
|                                      |   | sitting on the            |
|                                      |   | undercarriage, which is   |
|                                      |   | sitting on the yellow     |
|                                      |   | rails. This allowed the   |
|                                      |   | box to be moved out of    |
|                                      |   | the beam to beam left.    |
|                                      |   | The three circular ports  |
|                                      |   | are the mirror adjust     |
|                                      |   | access ports. The two     |
|                                      |   | square boxes are for      |
|                                      |   | photomultiplier tube      |
|                                      |   | clearance.                |
+--------------------------------------+---+---------------------------+
| ![](media/image                      | ! |                           |
| 3.jpeg){width="2.9381944444444446in" | [ |                           |
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+--------------------------------------+---+---------------------------+
| Figure 2. Front of cerenkov counter  | F |                           |
| that is pushed out of the beam to    | i |                           |
| the left on the rails. The golden    | g |                           |
| colored fabric is the front window.  | u |                           |
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+--------------------------------------+---+---------------------------+
| ![](media/imag                       |   |                           |
| e5.jpeg){width="3.310416666666667in" |   |                           |
| height="1.8743055555555554in"}       |   |                           |
+--------------------------------------+---+---------------------------+
| Figure 4. The back window removed    |   |                           |
| from the counter. The inner surface  |   |                           |
| is shown here.                       |   |                           |
+--------------------------------------+---+---------------------------+

Once the front and back windows are removed the primary mirror planes,
and the secondary mirrors are visible. The upper and lower primary
mirror planes have 48 mirrors (each) which are glued to a composite
board. The board can be oriented to provide optical alignment of the
system. The mirrors are made from 1mm thick glass. The replacement
mirrors no longer exist; so great care must be given to handling the
primary mirrors. Currently, these mirrors are stored in wooden boxes in
the ME beam line at the Fermilab Meson Lab. The mirror planes are shown
in Figures 5, 6, 7 and 8.

  ---------------------------------------------------- -----------------------------------------------------
  ![](media/image6.jpeg){width="3.051388888888889in"   ![](media/image7.jpeg){width="3.4715277777777778in"
  height="3.0319444444444446in"}                       height="2.591666666666667in"}

  Figure 5. View of back of cerenkov counter with rear Figure 6. View of front of cerenkov counter with
  window removed. The primary mirror planes' carbon    front window removed. The primary mirrors are
  fiber skinned composite boards are visible. The      reflecting the image of the secondary
  strapping tape forms a simple handle system to       mirror/photomultiplier tube associated with each
  extract the planes. The planes are captured on the   optical element. The three mirror sizes are evident
  sides; the aluminum angle can be seen on the left    in this picture.
  boarder of the mirror planes.                        

  ![](media/image8.jpeg){width="3.0875in"              ![](media/image9.jpeg){width="3.3159722222222223in"
  height="2.3159722222222223in"}                       height="2.5652777777777778in"}

  Figure 7. The secondary mirrors are visible with the Figure 8. The lower primary mirror plane placed in
  lower primary mirror plane removed from the box.     its shipping crate.
  ---------------------------------------------------- -----------------------------------------------------

The mirror support fixtures are removed from the box once the primary
mirror planes are removed. These support fixtures are stored in David
Christian's office at Fermilab (on the 10^th^ floor of Wilson Hall).
Figures 9 and 10 show the box empty of the primary mirror planes.

  ----------------------------------------------------- ------------------------------------------------------
  ![](media/image10.jpeg){width="4.175694444444445in"   ![](media/image11.jpeg){width="1.9916666666666667in"
  height="3.1319444444444446in"}                        height="2.6555555555555554in"}

  Figure 9. Rear view of box with no primary mirror     Figure 10. Details of the primary mirror plane support
  planes. The primary mirror plane fixtures are seen on fixtures.
  the left side of the box.                             
  ----------------------------------------------------- ------------------------------------------------------

Once the photomultiplier tubes, bases and magnet shielding were removed,
the windows were put onto the box, a protective plywood window was
placed over the windows. The box was then removed from the undercarriage
and shipped to Fermilab. The box is currently stored at Fermilab's
Proton lab PC4 enclosure.

A schematic of the Cerenkov, with those items that are taken out of the
box, is shown in Figure 11.

Details of the optics design and fabrication can be found in David
Chrisitan's note on the Cerenkov counter.

## The Photomultiplier Tubes, et cetera

The photomultiplier tubes (pmt's) are Thorne EMI 9954B that are selected
on the bases of tube diameter (to ensure a good seal). The pmt's are
further modified with a coating of wave-shifter on the photocathode
window surface to increase the detection efficiency by shifting the blue
Cerenkov light in to the green.

The pmt's are run with low gain using 7 of the 12 available
multiplication stages. This was done to reduce gain variations due to
environmental conditions (e.g. temperature variations) and to eliminate
the need for individual high voltage

![](media/image12.pct){width="5.993055555555555in"
height="8.542361111111111in"}

Figure 11. A view of the Cerenkov counter that identifies the various
pieces that are removed from the box.

settings for each tube. The pmt base is shown in Figure 12.

![](media/image13.pct){width="5.995138888888889in"
height="4.550694444444445in"}

Figure 12. The base pin-out and schematic diagram of the photomultiplier
tube voltage divider and signal output "D" and "A" to amplifier.

A differential amplifier is provided in each base to increase the signal
to a 1 V. maximum level. This amplifier utilizes an ECL line driver in
multiple stages. Sensing the voltage difference between the final dynode
stages (the 7^th^ dynode), and the anode stage (the 8^th^ through 12^th^
stages and the anode), the ECL 10H116 line receiver produces a
differential output that is further amplified for analog and digital
outputs. The digital output is generated if the input signal exceeds an
externally set threshold. A schematic of the pmt amplifier is shown in
Figure 13.

![](media/image14.pct){width="5.997916666666667in"
height="4.258333333333334in"}

Figure 13. Schematic diagram of pmt base amplifier/discriminator. Refer
to Nevis DWG. 4143-2 2/28/83.

The base is shown in Figures 14a-f. The location of the HV, signal,
amplifier power and threshold are seen in Fig. 14a. The pmt socket can
be seen in Fig. 14b. The angle tabs on either side of the base provide
the means of connecting the base to the iron shield. The connection is
made by trapping a spring between the tab and a washer (captured by the
screw head). The force of the compressed spring seats the photocathode
against an o-ring, setting the position of the pmt, and sealing the gas
volume.

Figures 15a-e show the base being disassembled. The steps are to remove
the detent-captured part of the Aluminum box, unscrew the socket (Figs.
15a-b), unscrew the subminex connector bolts releasing the amplifier
(Figs. 15c-e). The amplifier board can be further removed by unsoldering
the signal wire connections to the "Dynode" and "Anode" points (denoted
by D and A in Figure 12). The pmt socket can then be removed by
unsoldering the HV lead.

  ------------------------------------------------------ ------------------------------------------------------
  ![](media/image15.jpeg){width="2.4381944444444446in"   ![](media/image16.jpeg){width="2.2333333333333334in"
  height="2.8055555555555554in"} 14a                     height="2.9458333333333333in"} 14b

  ![](media/image17.jpeg){width="2.0097222222222224in"   ![](media/image18.jpeg){width="2.1083333333333334in"
  height="2.826388888888889in"} 14c                      height="2.9430555555555555in"} 14d

  ![](media/image19.jpeg){width="2.4138888888888888in"   ![](media/image20.jpeg){width="2.234722222222222in"
  height="2.4625in"} 14e                                 height="2.029861111111111in"} 14f
  ------------------------------------------------------ ------------------------------------------------------

Figure 14a-f. External views of the Cerenkov pmt base assembly.

+-------------------------------------+--------------------------------+
| ![](media/image2                    | ![](media/image22.jpeg         |
| 1.jpeg){width="2.932638888888889in" | ){width="1.8798611111111112in" |
| height="1.9083333333333334in"}      | height="1.9381944444444446in"} |
|                                     |                                |
| 15a                                 | 15b                            |
+-------------------------------------+--------------------------------+
| ![](media/image2                    | ![](media/image24.jpeg         |
| 3.jpeg){width="2.938888888888889in" | ){width="2.6972222222222224in" |
| height="2.140972222222222in"} 15c   | height="2.036111111111111in"}  |
|                                     |                                |
|                                     | 15d                            |
+-------------------------------------+--------------------------------+
| ![](media/image25                   | Figures 15a-e. a-b) front box  |
| .jpeg){width="2.9340277777777777in" | cover removed with socket      |
| height="2.4277777777777776in"} 15e  | unscrewed from box; c-d) shows |
|                                     | the amplifier card released    |
|                                     | from back box cover by         |
|                                     | unscrewing retaining nuts; e)  |
|                                     | shows the insulating FR-4      |
|                                     | sheet removed from the         |
|                                     | amplifier card. Most frequent  |
|                                     | failures are the front-end     |
|                                     | diodes, and the first 3        |
|                                     | amplifiers of the input 10H116 |
|                                     | line receiver.                 |
+-------------------------------------+--------------------------------+

A generic test setup is shown in Figure 16. A photomultiplier is plugged
into a test base (one of which is selected from the set). The base
inputs and outputs are attached to the appropriate devices. The high
voltage can be as high as 2200 VDC. (Note that as of the writing of this
memo, the HV connectors may not have been reviewed by Fermilab safety
committee). The threshold should be setup to a low current voltage
supply capable of supplying --0.8 V to --1.8 V (the ECL voltage swings).
The "digital +" output goes to an ECL-to-NIM converter and the output of
the conversion to channel 2 of the scope. The "analog" output goes to
channel 1 of the scope. The signal, power and threshold cables must have
the "subminex" style cable ends, generally on a RG-174 50ohm cable,
standard LEMO connectors on the other side. These cables will have to be
found, scrounged or made. The high voltage cable is "standard".

The photomultiplier tube, base, scintillator and radiation source are
placed in a dark box with the cables accessible in a light area. As the
photomultiplier tube is brought up to voltage, the analog signal should
show up on the oscilloscope screen (assuming the threshold is set
correctly; -340 mV is typical maximum pulse height, widths can be as
small as 4ns). If the signal is above threshold, the digital output
should also be giving signals. It is possible that the tubes undergo
periodic, high rate pulsing (bursts), or spark (very large signals).
Tubes or bases may have low gain, etc. Connectors to cables can be bad,
tube-to-base connection may be bad, etc. (A set of 72 tubes which we may
be able to sort through to find replacements can be found in Lab G
attached to the old E690 rear scintillator hodoscope. These existed at
least up to summer 2001. They should be recovered as soon as possible
for possible use in E907).

A useful study would be to plot the threshold voltage setting versus the
signal minimum signal pulse height. This can be done by triggering the
scope on the digital output signal and measuring the minimum analog
signal pulse height for a given setting of the threshold voltage. The
total integrated charge for a given pulse height and duration can be
estimated by ∆Q/∆t = I = V/R, which can be solved for the charge: ∆Q =
∆t x V/R. This estimate is done on page 57 of the "E910 Cerenkov Logbook
#1". For R = 50Ω, V = 95 mV, and ∆t = 38.9 ns, the total charge
(assuming a square waveform) is ∆Q = 74 pC. E895 reported that the 17 mV
threshold corresponded to a 2 photoelectrons. This has to be checked
(measuring the pulse height spectrum from the base). Assuming this
statement would imply the single photoelectron integrated charge to be
less then 12 pC. One photoelectron sensitivity is a goal for E907
operation.

Properties of the Thorn/EMI 9954B can be found in the catalog obtained
from the spin-off company: electron tubes
(http://www.electrontubes.com). The tube has a maximum diameter of 51.5
mm and a length of 133 ± 3mm. The photocathode active area has a 46 mm
diameter. The tubes have the "hard base" configuration in which the pins
exit directly through the glass at the bottom of the tube. The tube
window is standard borosilicate. This window passes radiation above 300
nm wavelength. The transmission is 90% at 375 nm, 50% at 300 nm and 10%
at 275 nm. Borosilicate glass has radioactive elements at the limits:
\<60000 ppm K, \<1000 ppb Th and \<1000 ppb U. The number of decays per
minute is \< 400 /min. The tube has 12 dynodes with Be-Cu coating in a
linear focused configuration. This configuration has typical rise times
of 1.8 -- 2.7 ns, a transit time jitter of 0.5 -- 1.2 ns with a single
electron jitter of 2.2 ns. With the base design shown in Figure 12, the
V~d-d~ is 244 V, which would deliver a peak anode current of 122 mA in a
12 dynode base configuration. The bialkali (K-Cs-Sb) photocathode has a
peak quantum efficiency of 28% at 400 nm. The dark current emission is
typically 7.5 nA/cm^2^ corresponding to roughly 300 dark counts/s. In a
7 stage base configuration with gains ranging from 2x10^4^ to 2x10^5^
the output currents range from 1 to 10 A/lm, respectively, with the
higher of these being the recommended maximum limit.

Currently, electron-tubes can supply the 9954B at roughly \$588 per tube
in 3 to 4 weeks \[according to Mike Avery, Electron Tubes West Coast
Office

\(760\) 471 1053, <GBMICK@aol.com>\].

The tubes used in the Cerenkov counter are coated with a wave shifter to
increase their sensitivity to the blue Cerenkov light, by shifting the
light towards the green. The coating can be performed at existing
facilities at Fermilab, which must be scheduled. This involves dipping
the photocathode into the appropriate wave shifter solution and allowing
the solution to dry onto the photocathode window. Details can be
obtained from David Christian.

![](media/image26.pct){width="5.9944444444444445in"
height="6.808333333333334in"}

Figure 16. A generic test arrangement for the photomultiplier tubes and
bases. Note that phototube/base/scintillator assembly should be located
within a "dark box" for these tests.

## Gas system

The gas system for the Cerenkov counter is rudimentary. The counter
system is filled from the bottom of the volume, where the Freon 114
slowly displaces the air. Once this vessel is full, the system is
switched to "top off" the vessel. In E895, a large bladder volume
provided most of the top off reservoir, allowing the Cerenkov to
"breath" as a function of atmospheric pressure. The pressure changes are
conveyed to the gas volume through the front window, causing the gas
level to rise and fall.

Figure 17 shows the original E690 gas system.

The possibility of recovering the Freon from the counter should be
investigated, as single fills for the duration of the experimental run
are likely to be the operating mode for E907. In the event that a pmt
must be changed during a fill, some additional Freon will be required to
refill the counter. Procedures for reducing the amount of leakage during
a pmt change should be reviewed or developed for E907. The total volume
of the gas volume is of order 120 cu. ft.

  ---------------------------------------------------- --------------------------
  ![](media/image27.png){width="3.701388888888889in"   Figure 17. The E690 gas
  height="9.000694444444445in"}                        system (composite
                                                       picture). The Freon
                                                       pressure at the top and
                                                       bottom of the vessel are
                                                       measured by the two helix
                                                       pressure gauges noted on
                                                       either side of the
                                                       stainless steel tubing.
                                                       The original E766 plumbing
                                                       system was made of white
                                                       PVC tubing, which was
                                                       found to be translucent
                                                       (causing greatly increased
                                                       noise rates in pmt's near
                                                       the gas inlets). The Freon
                                                       is supplied via the small
                                                       black tube at the bottom
                                                       of the counter. A
                                                       bubbler/barometer allows
                                                       the gas level to be
                                                       detected within the vessel
                                                       providing a check on the
                                                       fill progress, and the
                                                       level of the gas when full
                                                       (but leaking). The two
                                                       valves seen at the bottom
                                                       of the counter allow the
                                                       fill to occur either at
                                                       the bottom of the vessel,
                                                       as is the case on the
                                                       original fill, or at the
                                                       top of the counter, which
                                                       is the "top off" mode.
                                                       E895 added a large bladder
                                                       at the top of the counter,
                                                       which was partially filled
                                                       and allowed the
                                                       atmospheric pressure to
                                                       change without emptying
                                                       and filling the counter.
                                                       The new back window and
                                                       the addition of the
                                                       bladder should make one
                                                       fill possible, with no
                                                       additional Freon required
                                                       for the run.

  ---------------------------------------------------- --------------------------

## Mirror Alignment

The mirror alignment is accomplished by observing the photocathode
images in the mirror mosaics for the upper and lower mirror planes.
Viewed from the position of the target, the photocathode images should
be centered in each mirror. The designed distance from target to the
mirror plane apex is 109.25". The only motion allowed is the positioning
of the upper and lower planes (each are independent). There are two
degrees of freedom for two of three support points.