TECHNICAL SPECIFICATION
OF VACUUM PHOTOTRIODES FOR
THE ENDCAP ELECTROMAGNETIC CALORIMETERS OF
THE COMPACT MUON SOLENOID (CMS) EXPERIMENT

May 11, 2001

Signed by

Dr H. Rykaczewski
 ETH-Zurich

Prof. R.M. Brown
Rutherford Appleton Laboratory

Mr I. Vasilyev, Director
 Research Institute Electron

TECHNICAL SPECIFICATION
OF VACUUM PHOTOTRIODES FOR
THE ENDCAP ELECTROMAGNETIC CALORIMETERS OF
THE COMPACT MUON SOLENOID (CMS) EXPERIMENT

May 11, 2001

INTRODUCTION

The CMS experiment is a large, general-purpose detector for a new CERN accelerator, the Large Hadron Collider, which is due for completion in 2005.  One of the principal CMS design objectives is to construct a very high performance, compact electromagnetic calorimeter, (ECAL), providing excellent angular coverage and energy resolution.  An ECAL based on scintillating lead tungstate, (PbWO4), crystals has been chosen as best fulfilling the experimental goals, and a design has been approved consisting of a cylindrical Barrel whose ends are closed by two disc-shaped Endcaps.

The Endcaps will cover the pseudorapidity range 1.48 < || < 2.60 at each end of the detector, (a polar angular range, measured from the collision point, relative to the axis defined by the colliding beams, of 8.5º to 25.5º at each end of the detector).  Each Endcap will contain about 8,000 PbWO4 crystals approximately 30 x 30 x 220  mm3, and each crystal will be read out through one 30 x 30 mm2 face by a 26.5 mm-diameter vacuum photo-triode, (VPT).  The VPTs must offer an adequate signal-to-noise ratio with the low light yield of PbW04 in the 4 T magnetic field of CMS, and must continue to operate satisfactorily over a ten year period in the presence of a radiation dose < 2kGy per annum and a neutron fluence  5 x 1013 n cm-2 per annum.

Extensive tests during the past three years have demonstrated that mesh-anode VPTs with the required characteristics can be produced.  This document specifies the parameters of VPTs for the CMS ECAL Endcaps.

VPT SPECIFICATION

Construction (see drawing PAC.433242.012 4):

Overall length  46.0 mm
The seal off stub shall be protected and covered completely by the base (socle).

External diameter = 26.5 +0-0.7 mm.
Measured over the conductive coating and the insulating film applied to the glass envelope.

External diameter of the VPT base (socle) = 26.5 +0-1.2 mm.

The envelope shall be coated with a conducting paint connected to the cathode pin and protected by a thin, insulating sleeve.

The angle between the normal to the VPT faceplate and the axis of the cylindrical envelope shall be  1°.

The face plate aplanarity shall be   50 m, concave or convex.

The active diameter of the anode mesh shall be  l9.0 mm and the photocathode diameter shall be  22 mm.

Radiation Hardness:

The faceplate shall be of US11401 (or equivalent) UV-transmitting glass, optimised for radiation resistance over the PbWO4 photoemission spectrum with a thickness of 1 00.1 mm.  The producer shall provide ten samples of faceplates to the customer from each glass production batch.

Sample faceplates shall, neglecting surface reflection losses, transmit at least 90% of PbWO4 scintillation light, following a radiation dose of 20 kGy and exposure to a fluence of 
5 x 1014 fast neutrons cm-2.  ‘Fast neutrons' in this context means nuclear evaporation neutrons.

The devices shall exhibit not more than a 10% loss in anode response following exposure to a radiation dose of 20 kGy and exposure to a fluence of 5 x 1014 fast neutrons cm-2.  For test purposes, each radiation dose will be delivered over a period of the order of a week.

The producer is required to guarantee the radiation hardness of the devices.

Electrical Connections:

Electrical connections will be 100  5 mm in length, made of copper flexible wires (PVMP-2 type) having a cross section of 0.12 mm2 with 1.6 mm-diameter polyethylene insulation, capable of withstanding a voltage up to 6 kV.  Each wire shall be terminated by a 1 mm-diameter crimped pin.  The customer will supply suitable pins and hand crimping tools at no cost to the producer.

The VPT's base (socle) will comprise a cylinder, 14 mm in length, made of insulator (type K-8 vicsint).  The connecting wires shall emerge from the base (socle) within a diameter of less than 14 mm.  The wires shall be color coded as follows: white (anode), yellow (cathode), blue (dynode).

Connections around the periphery of the base shall be according to drawing PAC.433242.012 4.  The VPT pins shall be treated to prevent oxidation.  The VPT pins must not be bent or misaligned.  All wire to VPT pin connections must be without broken copper strands.  A random sample of VPTs will be inspected by the customer, who will remove the base (socle) to ensure that this quality requirement is met.

Temperature Ranges:

Operational temperature range 		100C  T  350C
Non-operational temperature range	 -50C  T  500C

	Safety Regulations:

The VPTs must conform to CERN TIS Commission Safety Instruction, IS 41, March 1995, on fire safety and properties under irradiation.  The composition of all materials used for VPT construction shall be notified to the customer.

Modifications:

Any change of material and components with reference to the VPTs produced under the pre-production (ETH Zürich order B-3137-14300-10) will be subject to prior written agreement between the producer and the customer.

PARAMETERS TO BE MEASURED AND GUARANTEED BY THE PRODUCER

All measurements carried out by the producer shall be in zero magnetic field, ie under normal laboratory conditions, away from any known sources of significant AC or DC magnetic fields, but without taking specific actions to minimise the effect of stray magnetic fields.  In all tests the VPT will be operated at its 'normal anode and dynode operating voltage', defined by Vkk-a = 1000 V, Vkk-dl = 800 V.

Training and Burn-in

Each VPT will be 'trained' by illuminating the photocathode uniformly to produce an initial standing photocurrent of 100 nA and running the VPT at its normal operating voltage for two hours.

Following training, the illumination shall be readjusted as necessary to produce a photocurrent of 100 nA, and the initial anode current, Io shall be recorded.  The VPT shall be left to 'burn-in' at constant, uniform illumination for a further period of 4 hours, after which the final anode current, If; shall be recorded.  Any VPT for which If/Io < 0.90 shall be rejected.

VPTs passing the burn-in requirement shall be tested to ensure that they comply with the following requirements:

Anode Leakage Current

The anode (leakage + dark) current shall be  2 nA, at an anode voltage of 1100 V, after operation for ten minutes at 1200 V.

Photocathode Quantum Efficiency, P:

The quantum efficiency measured at a wavelength of 420 nm with a uniform illumination of the photocathode, P, shall be  0.15 [15%].

Current Gain in Zero Field, G:

The current gain in zero magnetic field, G, shall be measured with uniform illumination of the photocathode.  The measurement shall be made with a mask having a circular aperture of diameter 19 +1-0 mm centered on the front face of the VPT.  The measurement shall be made with a photocurrent of about 10 nA, and the gain G shall be G  7.0 at normal operating voltage.

Product PG

The product PG shall be  1.4.  In addition, tubes must not have an anomalously high value for the anode response.  Thus PG shall be < 3.8.  Nevertheless, the producer is encouraged to maximise the mean value of PG.  Should the mean value of PG increase during the course of production, causing the upper limit on PG to reject tubes in the main population, revised values of the upper and lower limiting values of PG may be agreed by the customer and producer.

The producer will identify each VPT with a unique, self-adhesive, bar-coded label supplied by the customer.  Each VPT will be accompanied by a 'passport' in machine-readable form, [e.g. as one or more Excel files on a floppy disk, in a format to be agreed with the customer].  The passport will identify the bar code of the VPT, and contain at least the results of the above measurements:

i)	anode current before and after burn-in,
ii)	anode leakage current after burn-in,
iii)	quantum efficiency after burn in, [and photocurrent under standard illumination from a blue LED if QE inferred from such a measurement],
iv)	anode current for current gain measurement, resulting gain, G, and
v)	productPG.

PARAMETERS NOT REQUIRED TO BE MEASURED BY THE PRODUCER

The following parameters will be measured by the customer on a sample basis.  The customer reserves the right to reject any VPT failing to meet the specified criteria.

Response in a Magnetic Field:

There shall be a complete absence of internal discharge at normal operating voltage, in zero field, and for magnetic fields between 1 and 4T, at angles in the range 0° to 26° to the VPT axis.

At normal operating voltage the ratio of the mean anode pulse height in a 4 T magnetic field, at 15° to the VPT axis, to the mean anode pulse height in zero field, R, measured using a pulsed, blue LED, shall be  0.75.

Over the angular range 8°  26°, the value of R shall not change by more than 10% of its value at l5°.

Excess Noise Factor, F:

The excess noise factor, F, shall be less than 4.5 at normal operating voltage in a 4 T magnetic field at 15° to the VPT axis