LHCb HCAL PMT problems initial report 1 Yu. Guz

LHCb HCAL & ECAL 2 Yu. Guz PMTs of type HAMAMATSU R are used in both HCAL and ECAL. The detector granularities are shown below: 6016 PMTs are used in ECAL and 1488 in HCAL. Total of 8000 PMTs R were purchased from HAMAMATSU in Since then, the PMTs installed in HCAL experience strange (and serious) problems, while in ECAL they behave as expected. HCALECAL

LHCb HCAL & ECAL: PMT working conditions 3 Yu. Guz Each PMT is equipped with an individual Cockcroft-Walton (CW) HV supply mounted on the PMT pins The HV value is controlled by an analog input on the CW PCB. CW: same schematics but different PCB and different way of mounting PMT mounting: see next slide HCAL: gains range from 10 4 (centre) to 2·10 5 (periphery) HV ranges from 900 to 1300 V Working anode current (at LHC stable running at luminosity of 2·10 32 cm -2 s -1 ): from 0.5 µA (periphery) to 15 µA (centre), 1.5 µA average ECAL: gains range from 10 3 (centre) to 2·10 4 (periphery) HV ranges from 600 to 900 V Working anode current (at LHC stable running at luminosity of 2·1032 cm-2s-1): from 0.2 µA (periphery) to 6 µA (centre), 1 µA average

LHCb HCAL & ECAL: PMT usage 4 Yu. Guz ECAL Mechanical stresses are possible in HCAL PMTs: the PMT is soldered to PCB through a hard plastic spacer, the pins can remain stressed after soldering the PMT entrance window is in tight contact with a polystyrene light mixer (pressed to its surface with the plastic nut shown on the photo) The ECAL mounting design does not have such features – less mechanical stress HCAL CW PCB PMT nut spacer magnetic shielding PMT/CW assembly HCAL: The PMT bulbs are covered by a black paint (of course except entrance window), for light tightness

Significant degradation of PMT parameters during LHC Run I (timescale of ~years) ! ~15% of HCAL PMTs affected ! not the case for ECAL! ~150 degraded PMTs dismounted from HCAL, can be used for studies. Three kinds of problems: Significant (positive) rate effect, up to % (initial selection: within ±2%). The rate effect is measured as a ratio of LED signals before and after the collisions start; the anode current changes from ~ nA to 1-20 μA Dark current in many PMTs (increasing with time: from 0..2 nA to few μA) Degradation of gain: factor of 2-10 per year HCAL: summary of problems with PMTs 5 Yu. Guz Common features:  None of the effects is correlated to occupancy or radiation dose (distance to the beam); the affected PMTs are randomly distributed over the HCAL surface  The presence of the three types of effect is correlated, but not 100%

Features of PMT dark current (DC):  “ohmic” behaviour, i.e. (roughly) proiportional to HV, therefore is not connected to the photocathode emission (PMT gain is ~HV 6.5 ).  Such PMTs show MOhm resistance between anode and some dynode(s) (!!!)  Once the non-zero DC appears, it gradually increases all the time the PMT is installed on HCAL (even during LHC shutdowns, without HV)  On statistics of several PMTs with DC, which were dismounted from HCAL in 2011 and 2012: the DC stops increasing after dismounting the PMT from HCAL HCAL: PMT dark current 6 Yu. Guz

HCAL dark PHYSICS HV, February Yu. Guz I>100 nA: 42 PMTs I>50 nA: 77 PMTs I>10 nA: 147 PMTs Distribution of dark currents in the 1488 HCAL PMTs (left); the dark current values w.r.t PMT positions on the detector (right)

HCAL dark PHYSICS HV, November Yu. Guz Generally, the dark current increased (during shutdown! No beam, no HV). New cells appeared with dark current > 10 nA; in particular, in one PMT out of those which were replaced last time (2011/2012). (in 2012 the dark current was <1 nA, rate effect -0.7%). I>100 nA: 58 PMTs I>50 nA: 100 PMTs I>10 nA: 187 PMTs

HCAL: PMT dark current: location 9 Yu. Guz One PMT (large dark current, conductivity of ~1 GOhm between anode and a dynode) was opened, in order to try to understand the location of conductive substance: -no conductivity between pins over the bulb surface  the black paint is not a culprit; -large (and unstable) conductivity between pins over the dynode support.

PMT rate effect (reminder: working anode current mkA):  LED monitoring system is running continuously during the data taking, the LEDs flash at “empty” bunch crossings  The rate effect of each individual PMT can be measured as variations of LED response before and after the start of collisions in LHC  The LED amplitude increases slowly after the collisions start (few hours).  The magnitude of such variation gradually increases in the affected PMTs: e.g., in November it is larger than in May HCAL: PMT rate effect 10 Yu. Guz

HCAL rate effect Nov Yu. Guz 139 PMTs have >5% 180 PMTs have >4% Significant positive rate effect in >10% of HCAL PMTs !!! A slow effect (hours), must be caused by PMTs, rather than electronics (e.g. CW) an example The rate effect is very large in some PMTs! t0 ref An example of time dependence of the amplitude of the calibration signal (top left). ref – right before the collisions start t0 – 6 hours later The distribution of rate effect values (bottom left) The r.e. values w.r.t. PMT positions on the detector

cf: ECAL rate effect, same period 12 Yu. Guz The rate effect in ECAL is negligible compared to that in HCAL. The average is slightly negative (-0.6%) ref t0

HCAL PMTs: sensitivity degradation over Yu. Guz Significant sensitivity degradation (up to factor of 16) occurred in 2012 for many PMTs with high rate effect (apart from high occupancy area).  LHC run I: high rate effect is accompanied by sizeable sensitivity degradation (info available from detector calibrations) No significant sensitivity degradation in ECAL (next slide).

14 Yu. Guz No significant sensitivity degradation in ECAL during the same period (Apr-2012 / Dec-2012). Of course we have to select the lower part of ECAL, to avoid zones with clear fiber radiation damage. There are 1024 PMTs in this selection, comparable to the total number in HCAL (1488). The values are much more relaxed ( ). cf: sensitivity degradation in ECAL (Apr-Aug 2012)

HCAL PMTs: possible reason of degradation ? 15 Yu. Guz A hypothesis: loss of vacuum via micro cracks in the HCAL PMTs. Supported by different design of the HCAL PMT/CW – unlike in ECAL, mechanical tension to PMT pins may occur (a hard spacer between PMT and PCB, see slide 4)  micro cracks in glass. Not the case for ECAL. An experiment: test with He (proposed by V. Rykalin) – compare life time of a good and a bad PMT in He. This test was performed: the result is ambiguous (next page)

HCAL PMTs: He test 16 Yu. Guz Two PMTs, good and bad, in a tight box flushed with He. Continuous monitoring with LED pulse; the LED itself was monitored by a PIN photodiode (HAMAMATSU S ) The bad PMT: dark current ~ 50 nA, rate effect ~ 20% The good one: dark current <5 nA, rate effect -1%

HCAL PMTs: He test 17 Yu. Guz The LED signal reduced at similar speed in both PMTs: by factor of ~2 after 35 days the dark current did not change However the mechanical situation was not completely similar to that at HCAL: the PMT entrance window was not touching a polystyrene light mixer good PMT bad PMT