1. Studies of the effect of the LHC cycle on
crystals – update2
D Cockerill
D Cockerill, RAL
Printed/saved 17-Sep-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 uniform lateral irradiation
at  = 2.6 dose rates of 6 Gy/h (0.6 krad/h) at L = 1034 cm-2 s-1 and an effective path length for light in the crystal of 53.6 cm.
Is the present radiation hardness specification suitable or adequate for the EE ?
D Cockerill, RAL
Printed/saved 17-Sep-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
Printed/saved 17-Sep-18

4. Crystal Dynamics
Dynamics, colour centre creation and annihilation for PbWO4 ,CMS note 1999/069,
for a constant dose rate, R :
Di = density of colour centre type i
R = dose rate in Gy/h
biR = Colour centre creation rate ai. = Colour centre annihilation rate
Solution:
= total density of the potential colour centres related to colour centre i
= initial density of colour centres created, related to colour centre i
D Cockerill, RAL
Printed/saved 17-Sep-18

5. Crystal Dynamics
D Cockerill, RAL
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6. 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
Printed/saved 17-Sep-18

7. Crystal Dynamics
Equation (2) permits the ratio a/b and the total number of
colour centres, , to be extracted from data where only equilibrium losses versus
dose rate are available.
At very high dose rates Deq  Dall. Such data used in the simulation.
Comment :
Eqn (2) can be rewritten as Thus
a and b cannot be independently determined from the data when only equilibrium
measurements are made. However the ratio, a/b, can be extracted. This fixes a with
respect to b.
A plot of should be linear with 1/R, with slope a/b.
D Cockerill, RAL
Printed/saved 17-Sep-18

8. Equilibrium light loss data Uniform Lateral Irradiation
NB. Loss is  flat along crystal. Therefore colour centres have equal weight, irrespective of location at these densities. Use this factor in the simulation.
D Cockerill, RAL
Printed/saved 17-Sep-18

9. Extracting Dall from equilibrium data
Equilibrium light yield loss :
Dall
Fit :
to find Dall and a/b.
D Cockerill, RAL
Printed/saved 17-Sep-18

10. Equilibrium data check, versus 1/R
Data would be useful
in this region
D Cockerill, RAL
Printed/saved 17-Sep-18

11. 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
Printed/saved 17-Sep-18

12. Induced Absorption versus Light Yield Loss Uniform Lateral Irradiation
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
Printed/saved 17-Sep-18

13. 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
Printed/saved 17-Sep-18

14. Crystal Dynamics - coefficients a, b
Data from CMS Note 1999/069, Y/Nb Bogoroditsk crystals 2133, 2162.
Fit with Extract a, b
2133
2162
2133
2162
a (h-1)
0.17  0.04
0.27  0.04
b (Gy-1)
0.50  0.09
0.98  0.15
Characteristic time constant (a+bR)-1 at 0.15 Gy/h
1/( *0.15) = 4.1 h
1/( *0.15) = 2.4 h
D Cockerill, RAL
Printed/saved 17-Sep-18

15. 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
Printed/saved 17-Sep-18

16. 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
Printed/saved 17-Sep-18

17. Crystal light loss behaviour at LHC
It is difficult to find an analytic solution for the equation:
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 10, 20, 31.6, 40, 50%
Calculate loss at = 0, 1.48, 1.6, 2.0, 2.5 and 2.9
D Cockerill, RAL
Printed/saved 17-Sep-18

18. 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
Printed/saved 17-Sep-18

19. Light yield at one time instant,  = 2.5, t = 8 h
Dose rate along crystal
High luminosity, 1.48 Gy/h at max.
Colour centres along crystal, in units of % light loss / 22 cm.
Mean = 26.3 %
D Cockerill, RAL
Printed/saved 17-Sep-18

20. LHC Luminosity LHC luminosity (CERN AT/94-04) D Cockerill, RAL
Printed/saved 17-Sep-18

21. LHC Luminosity
3 LHC runs per year, 60 fills per run, 10 days between runs.
Each fill a random value between 30 and 100% of maximum 1034 cm-2 s-1 .
Initial fill values for 3x60 fills, one LHC year, x1034 cm-2 s-1.
A set of individual fills
D Cockerill, RAL
Printed/saved 17-Sep-18

22. 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
Printed/saved 17-Sep-18

23. Crystal light yield behaviour at LHC, <Dall> = 31.6%
Low luminosity
High luminosity
D Cockerill, RAL
Printed/saved 17-Sep-18

24. Light yield loss distributions, over time, at LHC for beam on periods only, <Dall> = 31.6%
Low luminosity
High luminosity
D Cockerill, RAL
Printed/saved 17-Sep-18

25. Average light yield at LHC, versus Eta
Starting luminosity, 1033
100%
80%
r.m.s. variation in light yield
D Cockerill, RAL
Printed/saved 17-Sep-18

26. Average light yield at LHC, versus Eta
Starting luminosity, 1033
Low luminosity
High luminosity
100%
80%
40%
D Cockerill, RAL
Printed/saved 17-Sep-18

27. 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
Printed/saved 17-Sep-18

28. 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
Printed/saved 17-Sep-18

29. 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
Printed/saved 17-Sep-18

30. Starting luminosity, 1033 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
Printed/saved 17-Sep-18

31. Low Luminosity, 2.1033 % 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
Printed/saved 17-Sep-18

32. High Luminosity, 1034 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
Printed/saved 17-Sep-18

33. Conclusions (1 of 3)
The radiation specifications for Endcap crystals are not ideal.
<Light yield loss> at  = 2.5
Starting luminosity 18%
Low luminosity 22%
High luminosity 29%
Fractional light yield change, at  = 2.5
Starting luminosity 2.7%
Low luminosity 2.9%
High luminosity 1.8% (2.9% at  = 2.0)
Improved radiation tolerance would be of benefit.
D Cockerill, RAL
Printed/saved 17-Sep-18

34. 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 150 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
Printed/saved 17-Sep-18

35. Conclusions (3 of 3)
The fractional light yield change at startup is <0.5% in the barrel only.
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
Printed/saved 17-Sep-18

36. 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
Printed/saved 17-Sep-18