HF UPGRADE PROJECT

1. HF UPGRADE PROJECT Y. Onel For HF Upgrade Project Collaboration

2. Response to ECR-I  1-The discussion during the review concerning the estimated gain of a factor 1000 in interaction probability (on slides 26,27,28,29 of Yasars talk) was unclear. It would be useful to clarify and to explain the uncertainties in order to support future benchmarking. As explained in Taylan's talk slides 26-29 the calculations for the glass area is given on slide 58 as shown below- Interaction Probability Estimate Int. Prob. = (Events Above Pedestal/s)/(Neutron Rate x Area) Neutron rate = 4x104 n/cm2s (counted thermal neutrons inside moderator) [10]. Surface Area (one quadrant) of R7600U = 0.81 cm2 Surface Area of the glass of R7600U = 3.24 cm2 Surface Area of R7525 = 4.91 cm2 Glass Area of R7525 = 95.88 cm2 Flux = 4x104n/cm2s This gives a factor of 20 between 7600 and 7525 Moreover: The B10(n,alpha)Li reaction produces an alpha with a range in glass less than a few microns which must enter the PMT to count, and thus only the surface area ratios are important (the glass simply has negligible scintillation it is used for scintillation counting liquid scintillator vials in radiochemistry & radiobiology)> The gamma rays are produced uniformly throughout the glass volume. There the volume of glass is even larger for the R7525 than the R7600.. But these gamma count only by Compton electrons in the glass, giving a few photons above C threshold, and in the PMT only giving 1-2 pe crossing the anode or first 1-2 dynodes. This is the effect we see there is a reasonably large rate of 1-2 p.e. triggers in both tubes from the neutron absorption. This we can ignore for the purposes of HF it is still only 1-10% of all neutrons which is a small rate in any case. The alpha on the other hand can and do produce larger pulses, but a far smaller number get produced within microns of the glass inner surface. The R7600 has a cathode-dynode-anode stack only 8mm thick, and also nearly opaque from the sides, because of the insulators supporting the dynode sheets. On the other hand, As one can see the R7525, the cathode-dynode-anode stack is open. The activated (Cesium) metal from the dynodes and field shaping electrodes is ~ 40mm, and He nuclei can be accelerated nearly anywhere and hit the dynodes, cathode or anode generating secondaries, which are open. The area of the dynodes and active metal surfaces are at least a x10-20 larger than the R7600 (40mm/8mm x 2-4 more metal), and the ability of an alpha from any of the glass to penetrate into the dynodes and cathode gap is very obvious estimate 2-5 times more efficient for an alpha to enter dynodes or get accelerated to cathode. The factors are thus consistent with ~x 30 for glass area, x 10-20 for dynode/activated metal area x guesstimate 2-5 for getting into the dynode stack Gives a suppression of 600-3000 for the R7600 compared to the R7525. We therefore believe our measurements for 7600 and 7525 are consistent with a factor of 1000 (as estimated above between 400-2000).

3. Response to ECR-II 2-Do measurements exist at CMS or Hamamatsu which show the quantum efficiency as a function of delivered charge and as function of radiation dose in particular for the ultra bialkali photocathode? The only measurement exist by our group. CMS DN -2010/014 where the radiation measurements were performed at the CERN PS RADDAM facility for 7525 ( QE 24%) Bialkali and 7600 (QE 40%) Ultrabialkali. The results are very similar for for these PMTs which had a different QE. The final applied dose during several days of irradiation reached at 10^13 neutrons/cm^2 which is more then 10 years of the LHC operations plus 1 year of the SLHC conditions. These measurement showed similar 10% transmission loss effects due to the glass darking. We would also like to add that the ultrabialkali photocathode is a standard bialkali photocathode with the following properties: a) A diffusing (so-called milky) layer using ~20-50 nm sized inorganic oxide scatterers to spread the light sideways is first incorporated onto the substrate conducting film connection. b) Flood-oven co-evaporation (similar to MBE) of the alkali & Sb materials is used to create a far more uniform cathode (i.e.the alkali are not diffused into the surface as in a standard bialkali cathode). c) The stochiometry is very carefully controlled ? the photocathode is a weakly bound amorphous semiconductor, and defects caused by incorrect stochiometry result in phonon scattering and electron scattering which decrease the intrinsic QE. Indeed, the UBA photocathode is likely to be more robust since it approaches a pure compound much more closely. d) The cathode layer thickness is highly controlled and thinner than normal cathodes, since diffusion is not necessary. Not of these properties would affect the ability to draw current from the photocathode; the resupply of photocurrent is dominated by the resistance of the underlying conductive film and not the cathode itself. None of these properties would affect the rad-hardness; indeed, since the UBA photocathode is thinner than the normal cathodes, the absorption of radiation is less. Lastly, the PMT that was radiation-damaged with neutrons at CERN had a dose far exceeding normal operation, and had a slight decrease in effective gain consistent with glass-darkening.

4. Response to ECR-III 3-It was claimed that 20% of the light would be lost by suppression of the reflective sleeves that cause a sizeable fraction of the anomalous pulses (spikes). The proposed PMT has a quantum efficiency that is twice as high as the old one. Can the optical system be optimized with fewer or simpler components, yet deliver an overall performance that is good Yes. We will redesign the optical coupling between the light guide and PMT to optimize the light collection. Although there will be some light loss, the performance will be better than simply suppressing the sleeves. We have started the process of identifying candidate materials, and preparing a test setup for evaluating different designs and materials. 4-The pictures of the readout boxes show provision for 28 light guides, pmts, etc. The total number of needed pmts leads to 24 pmts for each HF-wedge; and only 24 R7600 pmts will fit in the space available. Will this complicate the light guide design since the light guides will be installed under some angle relative to the axis of the pmts?  Although the box appears to have provisions for 28 PMT's, it only holds 24 that correspond to the 24 Fiber bundles that come from the HF absorber. The row that does not contain PMT's had three unused holes in addition to the CLI ("Calibration light injector"). The CLI directs light from the LASER/LED system onto a bundle of fibers inside the HF wedge assembly that distribute the light to all of the PMT's. The three unused holes were provided for consistency in the mechanical structure and to allow the possibility to have gas for inertion to flow from the box into the HF wedge fiber compartment thus providing some degree of inertion for the iron HF absorber. These holes have been plugged at the wedge backplane to maximize the inertion of the readout box volume. 5-The CMS integration group should be consulted as soon as possible in order to deal with issues such as: building a few shorter boxes to be mounted in places where interference with CASTOR is present whether shielding needs to be developed to be put in place when collars are dismounted whether sufficient space is available for extra cables (even when using 4-channel readout/PMT) etc should be discussed with CMS integration group as soon as possible. Certainly integration group involvement is necessary on the all the issues mentioned as well as to finalize the infrastructure necessary to allow access to all of the readout boxes. Although there are a few places where space is tight between the readout boxes and the Castor table, presently there is sufficient space to access all readout boxes. This is a constraint that has been respected throughout the evolution of the collar and table elements. To accommodate the large increase in channel count thereadout box bulkheads and internal connections to the circuit boards will have to be redesigned. Once there is a better understanding of this we can determine to what extent the boxes can be shortened. We wish to continue to have the boxes be generic and independent of location specific details.

5. PMT Purchase

7. PMT Purchase