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Studies of the effect of the LHC cycle on

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1 Studies of the effect of the LHC cycle on
crystals - update D Cockerill ECAL Week CERN, 10 July, 2002 D Cockerill, RAL ECAL Week 4-Dec-18

2 Radiation Hardness Specification for EE
The radiation hardness specification was initially set for the outer ring of EE crystals and was set to be the same as for barrel crystals. Specification :   Induced absorption for full saturation of the crystal transmission damage 0  m  1.5 m-1 at 420 nm lateral 60Co irradiation, dose 3 krad, dose rate 5-15 krad/h at 18°C m = 1.5 m-1 is equivalent to a 55% light yield loss for a path length of 53.6 cm. (At  = 2.6, dose rate = 0.6 krad/h (6 Gy/h) at L = 1034 cm-2 s-1) Is the present radiation hardness specification suitable or adequate for the EE ? D Cockerill, RAL ECAL Week 4-Dec-18

3 Radiation Hardness Specification for EE
Evaluate the consequences of the current specification: Model behavior of crystals at  = 0, 1.48, 1.6, 2.0, 2.5 and 2.9 Track crystal light yield response as a function of time during LHC running. Evaluate whether the monitoring system can adequately track the changes in light yield. D Cockerill, RAL ECAL Week 4-Dec-18

4 Crystal Dynamics Usually data not available for the dynamics of each trap type. Approximate by assuming a single trap type to describe an average behaviour for the creation and annihilation of colour centres. In this case the colour centre density with time is Equation (1) where a and b are average values for the crystal, and (a+bR)-1 is the characteristic response time for the system. The equilibrium is given by Equation (2) D Cockerill, RAL ECAL Week 4-Dec-18

5 Density of colour centres, Dall
Front irradiation, Gy/h. Mean loss 2.5%. But very far from LHC conditions in Endcap Can’t get Dall from these data. Can get characteristic time Constants. Dose rate at shower max, LHC 1034 cm-2 sec-1 . 14 Gy/h 0.15 Gy/h D Cockerill, RAL ECAL Week 4-Dec-18

6 Induced Absorption versus Light Yield Loss
Induced absorption and light yield loss measured after dose rate of 200 Gy/h, for 2h. (14x dose rate at η=3), E. Auffray, DPG, EE crystals where α is the measured induced absorption Fit data to find effective path length for light absorption, x Fit gives effective path length of x = 53.6 cm D Cockerill, RAL ECAL Week 4-Dec-18

7 Density of colour centres, Dall , from very high dose rates
Use 53.6 cm path length of light in crystals to re-plot induced absorption as % Light Loss. <Dall> = 31.6% Light Loss Simulate 31.6% ± 1, Dall ~ 20% and 40% Catania, June 2001 Induced absorption, 240 Gy/h, 2h x-axis from 0 to 3.0 m-1 mean ~ 0.75 m-1 % light loss D Cockerill, RAL ECAL Week 4-Dec-18

8 Crystal Dynamics - coefficients a, b
More recent data on time constants from GIF (P Rebecchi, TCG ) Barrel crystal, 4005 irradiation = 3.96 h = (a+ bR)-1 recovery = h = a-1 h a = h-1, b = 1.32 Gy-1 R=0.15 Gy/h Crystal 4005 2133 2162 recovery (h) 18.55 5.9 3.7 Characteristic time constant (a+bR)-1 at 0.15 Gy/h (h) 3.96 4.1 2.4 a (h-1) 0.054 0.17  0.04 0.27  0.04 b (Gy-1) 1.32 0.50  0.09 0.98  0.15 D Cockerill, RAL ECAL Week 4-Dec-18

9 Crystal light loss behaviour at LHC
Get net crystal light yield in real time at LHC by folding in : 1) Crystal dynamics. 2) LHC luminosity profile with time. 3) Dose rate profile with time. 4) Dose rate profile along crystal, 1 cm sections. 5) Light loss in each of 22 1cm long sections along the crystal. Since the dose rate is not constant at LHC, Equation (1) rewritten with R(t) : Equation (4) Equation (4) evaluated independently for each 1 cm section along the crystal. D(t) is the time varying colour centre concentration for the section. R(t) is the time varying LHC dose rate for the section. a, b, Dall are inputs to the model D, D0 and Dall in units of % light loss. D Cockerill, RAL ECAL Week 4-Dec-18

10 Crystal light loss behaviour at LHC
It is difficult to find an analytic solution for equation (4): Instead, evaluate in one hour steps. Use D(t) as input value for D0 in the next step at t = t+1. Evaluate for every hour, over 3 LHC cycles, 60 fills each cycle. 4800 hours, including 10 days off between each cycle. Can choose input time constants for the crystal dynamics. For this simulation, choose a recently measured crystal, crystal 4005. a (h-1) = 0.054, hours recovery b (Gy-1) = hours, characteristic time at 0.15 Gy/h Dall for values 10, 20, 31.6, 40, 50% Calculate loss at = 0, 1.48, 1.6, 2.0, 2.5 and 2.9 D Cockerill, RAL ECAL Week 4-Dec-18

11 Dose, and dose rate, profiles in crystals
Simulation of crystal response carried out at six values of  : 0, 1.48, 1.6, 2.0, 2.5 and 2.9 Dose profiles at each  taken from ECAL TDR, Figs A4 and A5, normalized to dose at shower max. At LHC know dose rates at shower max, from ECAL TDR, Fig 2.7. Use dose profile to get corresponding dose rates along crystal. D Cockerill, RAL ECAL Week 4-Dec-18

12 LHC Luminosity LHC luminosity (CERN AT/94-04)
D Cockerill, RAL ECAL Week 4-Dec-18

13 Crystal light yield behaviour at LHC, Dall=31.6%
Low luminosity, max High luminosity, 1034 max = 0 = 0 Annealing = 2.5 = 2.5 = 2.5 x 1034 cm-2 s-1 D Cockerill, RAL ECAL Week 4-Dec-18

14 Crystal light yield behaviour at LHC, Dall=31.6%
Low luminosity High luminosity D Cockerill, RAL ECAL Week 4-Dec-18

15 Light yield loss distributions, over time, at LHC for beam on periods only, Dall=31.6%
Low luminosity High luminosity D Cockerill, RAL ECAL Week 4-Dec-18

16 Average light yield at LHC, versus Eta
Starting luminosity, 1033 100% 80% r.m.s. variation in light yield D Cockerill, RAL ECAL Week 4-Dec-18

17 Average light yield at LHC, versus Eta
Starting luminosity, 1033 Low luminosity High luminosity 100% 80% 40% D Cockerill, RAL ECAL Week 4-Dec-18

18 Monitoring capability
The monitoring is used to follow the change in scintillation signal response. This has been shown to be directly proportional to the change in the monitoring signal. R = particle/ monitoring R is the spread in the value of R, believed to exist from one crystal to another The error on measuring the correct particle energy is Set a limit on the energy measurement error, say  : (%) The maximum light yield swing that can be followed by the monitoring is : From GIF, has a general value of 16%, and 6% for a few crystals. The target for  is that it should be smaller than other fractional energy measurement errors. For this simulation appraise consequences with,  set to 0.3% and R/ R of 16% and 6% D Cockerill, RAL ECAL Week 4-Dec-18

19 Monitoring capability, for E/E = 0. 3% r. m. s
Monitoring capability, for E/E = 0.3% r.m.s. light yield/<light yield> (%) versus Eta Starting Luminosity, 1033 0.5% D Cockerill, RAL ECAL Week 4-Dec-18

20 Monitoring capability, for E/E = 0. 3% r. m. s
Monitoring capability, for E/E = 0.3% r.m.s. light yield/<light yield> (%) versus Eta Starting luminosity Low luminosity High luminosity D Cockerill, RAL ECAL Week 4-Dec-18

21 Starting luminosity Light yield loss 18% at  = 2.5,
for average crystal with Dall=31.6% % Light Yield variation Only barrel within 0.5% at startup D Cockerill, RAL ECAL Week 4-Dec-18

22 Low Luminosity % Light Yield variation
Maximum variation at  = 2.5 2.9% for crystal with Dall=31.6% Light yield loss % at  = 2.5, for average crystal with Dall=31.6% D Cockerill, RAL ECAL Week 4-Dec-18

23 High Luminosity Light yield loss 29% at  = 2.5,
for average crystal with Dall=31.6% % Light Yield variation Maximum variation at  = 2.0 2.9% for crystal with Dall=31.6% D Cockerill, RAL ECAL Week 4-Dec-18

24 Conclusions (1 of 3) The radiation specifications for Endcap crystals are not ideal. Improved radiation tolerance would be of benefit. From barrel crystal data, the average number of potential colour centres in a crystal corresponds to a light yield loss of 31.6%. The distribution is wide (r.m.s. ~ 30%). The average light yield loss at starting luminosity is 18% at  = 2.5. The average light yield loss at low luminosity is 22% at  = 2.5. The average light yield loss at high luminosity is 29% at  = 2.5. D Cockerill, RAL ECAL Week 4-Dec-18

25 Conclusions (2 of 3) The low luminosity light loss of <22%> will increase the noise by 22%. Implies initial noise level target of ~ 120 MeV/ch, to ensure MeV/ch when running at low luminosity. Need to sort crystals for radiation tolerance. Need data for each crystal for induced absorption measured using the top/bottom boule cuts to enable sorting. May need to match VPTs to crystals (low radiation tolerance to higher gain VPT) since VPT yield distribution is also wide. D Cockerill, RAL ECAL Week 4-Dec-18

26 Conclusions (3 of 3) The fractional light yield change at startup is <0.5% in the barrel only. The fractional light yield change at starting luminosity is 2.7% at  = 2.5. The fractional light yield change at low luminosity is 2.9% at  = 2.5. The fractional light yield change at high luminosity is 1.8% at  = 2.5 but 2.9% at  = 2.0 . Variations in light yield for crystals with saturation values up to 40% can be monitored satisfactorily if R/R < 6%. Variations in light yield for crystals with saturation values up to only 20% can be monitored satisfactorily if R/R < 16%. D Cockerill, RAL ECAL Week 4-Dec-18

27 Further work What variations in the dynamics of colour formation, crystal to crystal ? GIF is an excellent facility to measure characteristic time constants a, b. Need ETH-Z/Cantonal hospital equilibrium data, for Dall. Need data on more crystals for damage and recovery time constants. The LHC, the ‘ultimate’ GIF, could allow Dall , a, b to be established for all crystals, using both monitoring/particle/LHC on/off data. In-situ crystal characterisation could complement the monitoring system and reduce the dependence on knowing R. D Cockerill, RAL ECAL Week 4-Dec-18


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