- Email: [email protected]
Stability and calibration of a water Čerenkov detector prototype
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PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 75A (1999) 389-391
Stability and calibration of a water Cerenkov detector prototype J.(:. D'Olivo% A. Ferngndez b, M. Medina c, L. Netlen% S. Roman b, H. Salazar b, J. Vald4s-Galicia d, M. Va.rgas I:', g. Villasefior ~ and A. Zepeda e q n s t i t u t o de (:iencias Nucleares, UNAM, 04510 M4xico D.F., M~xico I, Facult.ad de (:iencias, BUAP, 72 000 Puebla, Me£ico ':lnstituto de Ffsica y Matemgticas, U. Michoacana, A. Postal 2-82, 58040 Morelia, Mich., M4xico ':tl;isica Espacia.l, IGf-UNAM, 04510 M~xico D.E., M4.xico ~'Dq)artamento de Ffsica, Cinvestav-IPN, 07000 Mdxico D.F., M4xico We present results of studies made with a reduced-scale water Cerenkov detector (WCD) prototype for the
Pierre Az~ger (;b.servatory. This detector is made of high-density polyethylene and was operated continuously for tour months. We studied time variations in the amplitude and shape of ~erenkov pulses due to cosmic ray muons and studied the correlation of these parameters with the bacterial population of the water. We also developed and tested a new technique to calibrate and monitor WCDs remotely, based on muons stopping and decaying inside the tank. \eVeconclude that high-density polyeit@ene tanks fulfill the requirements for use in the water (:erenkov detectors of the Pierre Auger Observatory.
1. I n t r o d u c t i o n The existence of ultra high energy eoslnic rays (IIHECR) poses a severe challenge in present day a.strophysies as their origin, composition and accelerat.ion mechanism are unknown [1]. The Pierre Auger International Collaboration was established t.o search for answers to these questions. The collaboration will install two observatories, one in the northern hemisphere and another in the southern hemisphere. Each observatory will ('onsist of two different subsystems: a surface array made of about 1600 water (:erenkov tanks (WCD), and a. set of three fluorescence eyes to measure the longitudinal development of air shower cascades [2]. The operatioll of a large number of tanks tor 2,5 years in a semi-desertic area puts severe restrict.ions on the design and durability of the materials used in their construction. Furthermore for such a larg~" surt3ee array it is indispensable to do the i,dit.ial calibration and subsequent monitoring of f~ach WCrD remotely. In this pal)~,r we present experimental evidew'o
that plastic tanks satisfy the main criteria of preserving stability of the water quality and light tightness over a long period. These requirements are among the most critical factors for the success of the project. We also present the results of a novel technique to perform the required calibration and monitoring of WCDs. This technique uses muons from the background of secondary cosmic rays and electrons from the decay of muons that stop inside the detector as a standard source. Besides the experimental results, we present a detailed simulation of our W C D prototype.
2.
Experimental
Setup
Tile WCD we used is a cylindrical tank of reduced dimensions with respect to the Auger design. It is made of polyethylene with an inner diameter of 1.54 m filled with commercially purified water up to a height of 1.2 m. It has a single 8" PMT located at the centre of the tank to collect the Cerenkov light. Ai! the inner surface of the tank is covered with tvvek whle|: ha~ a reflec-
0920-5632/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0920-5632(99)00301-1
390
J.C D'Olivo et al./Nuclear Physics B (Proc. Suppl.) 75.4 (1999) 389-391
tivity of about 90% in the relevant wavelength range of 300-500 nm. Two slightly different experimental setups were used for this work: one for detecting muons decaying inside the detector and the other for detecting nmons crossing the detector vertically. The main difference between them is the way in which the trigger is done. In the first, case the trigger is given by the coincidence of two consecutive pulses from the tank P M T within a time window of 25.6/1s. In the second case the trigger is simply given by the coincidence from two scintillation counters, one placed above and the other below the tank, in a time window of 100 ns. The pulse shapes were recorded using a digital oscilloscope with a resolution of 1 ns. A c u s t o m - m a d e CAMAC T D C module [4] was used to measure the time interval between consecutive pulses. This module also provided the trigger signal for double-pulse events. A total of 29, 84{5 double pulse events and 7,133 vertical nmons were registered.
3. S i m u l a t i o n The experimental results are compared to simulations done using an adapted version of AGASIM [5]. Besides adjusting the tank geometry, we wrote a. new module for reproducing the trigger and analysing the FADC traces. Three different runs were performed to simulate vertical muons, stopping and decaying muons, and arbitrarily crossing muons. For each run, we simulated 25,000 muons with realistic energy spectrum and angular distribution. For vertical muons, the sample was restricted to muons passing through the two scintillator paddles. To study stopping muons, the energy was limited to less than 300 MeV. For arbitrarily crossing muons we imposed no additional restrictions. After all steps of the simulation, the number of events producing a trigger was about 5 to 10% less than the number injected originally. We used the measured position of the peak of the charge distribution of vertical muons to normalize the simulation.
i z~o~o
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Figure 1. Size of the bacterial population in the water during a four month period. 4. R e s u l t s
The test tank was operated continuously for a period of four months. We verified that the tank was light-tight by checking that the average pulse shape and pulse rate were the same for 12 hour day and nig.ht data samples. The analysis of the average pulse shape and the charge distribution for vertical muons in various 24 hour periods revealed that the decay time and the number of photo-electrons, as estimated from the width of the charge distribution, increased slightly over the four months of the study. This can be interpreted as an improvement of the water quality which is correlated with a decrease of the bacterial population as shown in fig. 1. Another result obtained from the test tank is a newly developed calibration method using elec"~• 2oo
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Figure 2. T i m e between pulses after applying the cuts C1 < C2 and At < 8its. The solid curve is a fit to the d a t a using the function
P1 + P2 exp(-t/P3).
391
J.C D'Olivo et aL /Nuclear Physics B (Proc. Suppl.) 75,4 (1999) 389-391
trons from muon decay as a standard source with a known energy spectrum. The electron pulse follows the pulse generated by the parent muon with a.n exponential time distribution with a mean lifetime of about 2.2#s. Using adequate cuts, namely that. the charge of the second pulse should be bigger than the charge of the first pulse and restricting to a time window of 8/Is one can eliminate most of tile background consisting of PMT after-pulses and random coincidence of muon inside the tank (fig. 2). The charge distribution of the electron pulses (fig. 3) shows a pronounced beak a.t 44.5 I,C, a.bout a factor of 0.18 below the peak of vertical muons which was found to be at 248.5 pC (VEM). The charge distribution of the parent muons has its peak at 24.6 pC, corresponding to ~ 0.10 VEM (fig. 4). The experimental results are well reproduced by our simulations (shaded graphs).
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The results we obtained from the test tank indicate that high density polyethylene is a suitable material for the WCDs of the Pierre Auger Observatory. Furthermore, we demonstrated that decay electrons can be used for calibrating and monitoring WCDs in a remote way. We would like to thank C. Hojvat, C. Pryke from the Pierre Auger Collaboration. This work was partially supported by BUAP, CINVESTAV, CONACyT, UMSNH and UNAM. REFERENCES
1. P. Sokolsky, P. Sommers and B.R. Dawson, Phys. Rep., 217, No. 5, 225 (1992); M. Hillas, proceedings of the Xth IS VHECRI, Gran Sasso, Italy, 1998. 2. Auger Collaboration, " T h e Pierre Auger Project - Design Report", March 1997,
http ://www-td-auger. fnal. gov :82/. 3.
F. Alcaraz et al., Auger Project Technical Note, GAP-97-050, (1997). 4. A.GonzMez and L. Villasefior, Proc. of SOMI XII, San Luis Potosi, Mexico, 1997. 5. C.L. Pryke, Ph. D. Thesis, Leeds University, 1996.
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 75A (1999) 389-391
Stability and calibration of a water Cerenkov detector prototype J.(:. D'Olivo% A. Ferngndez b, M. Medina c, L. Netlen% S. Roman b, H. Salazar b, J. Vald4s-Galicia d, M. Va.rgas I:', g. Villasefior ~ and A. Zepeda e q n s t i t u t o de (:iencias Nucleares, UNAM, 04510 M4xico D.F., M~xico I, Facult.ad de (:iencias, BUAP, 72 000 Puebla, Me£ico ':lnstituto de Ffsica y Matemgticas, U. Michoacana, A. Postal 2-82, 58040 Morelia, Mich., M4xico ':tl;isica Espacia.l, IGf-UNAM, 04510 M~xico D.E., M4.xico ~'Dq)artamento de Ffsica, Cinvestav-IPN, 07000 Mdxico D.F., M4xico We present results of studies made with a reduced-scale water Cerenkov detector (WCD) prototype for the
Pierre Az~ger (;b.servatory. This detector is made of high-density polyethylene and was operated continuously for tour months. We studied time variations in the amplitude and shape of ~erenkov pulses due to cosmic ray muons and studied the correlation of these parameters with the bacterial population of the water. We also developed and tested a new technique to calibrate and monitor WCDs remotely, based on muons stopping and decaying inside the tank. \eVeconclude that high-density polyeit@ene tanks fulfill the requirements for use in the water (:erenkov detectors of the Pierre Auger Observatory.
1. I n t r o d u c t i o n The existence of ultra high energy eoslnic rays (IIHECR) poses a severe challenge in present day a.strophysies as their origin, composition and accelerat.ion mechanism are unknown [1]. The Pierre Auger International Collaboration was established t.o search for answers to these questions. The collaboration will install two observatories, one in the northern hemisphere and another in the southern hemisphere. Each observatory will ('onsist of two different subsystems: a surface array made of about 1600 water (:erenkov tanks (WCD), and a. set of three fluorescence eyes to measure the longitudinal development of air shower cascades [2]. The operatioll of a large number of tanks tor 2,5 years in a semi-desertic area puts severe restrict.ions on the design and durability of the materials used in their construction. Furthermore for such a larg~" surt3ee array it is indispensable to do the i,dit.ial calibration and subsequent monitoring of f~ach WCrD remotely. In this pal)~,r we present experimental evidew'o
that plastic tanks satisfy the main criteria of preserving stability of the water quality and light tightness over a long period. These requirements are among the most critical factors for the success of the project. We also present the results of a novel technique to perform the required calibration and monitoring of WCDs. This technique uses muons from the background of secondary cosmic rays and electrons from the decay of muons that stop inside the detector as a standard source. Besides the experimental results, we present a detailed simulation of our W C D prototype.
2.
Experimental
Setup
Tile WCD we used is a cylindrical tank of reduced dimensions with respect to the Auger design. It is made of polyethylene with an inner diameter of 1.54 m filled with commercially purified water up to a height of 1.2 m. It has a single 8" PMT located at the centre of the tank to collect the Cerenkov light. Ai! the inner surface of the tank is covered with tvvek whle|: ha~ a reflec-
0920-5632/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0920-5632(99)00301-1
390
J.C D'Olivo et al./Nuclear Physics B (Proc. Suppl.) 75.4 (1999) 389-391
tivity of about 90% in the relevant wavelength range of 300-500 nm. Two slightly different experimental setups were used for this work: one for detecting muons decaying inside the detector and the other for detecting nmons crossing the detector vertically. The main difference between them is the way in which the trigger is done. In the first, case the trigger is given by the coincidence of two consecutive pulses from the tank P M T within a time window of 25.6/1s. In the second case the trigger is simply given by the coincidence from two scintillation counters, one placed above and the other below the tank, in a time window of 100 ns. The pulse shapes were recorded using a digital oscilloscope with a resolution of 1 ns. A c u s t o m - m a d e CAMAC T D C module [4] was used to measure the time interval between consecutive pulses. This module also provided the trigger signal for double-pulse events. A total of 29, 84{5 double pulse events and 7,133 vertical nmons were registered.
3. S i m u l a t i o n The experimental results are compared to simulations done using an adapted version of AGASIM [5]. Besides adjusting the tank geometry, we wrote a. new module for reproducing the trigger and analysing the FADC traces. Three different runs were performed to simulate vertical muons, stopping and decaying muons, and arbitrarily crossing muons. For each run, we simulated 25,000 muons with realistic energy spectrum and angular distribution. For vertical muons, the sample was restricted to muons passing through the two scintillator paddles. To study stopping muons, the energy was limited to less than 300 MeV. For arbitrarily crossing muons we imposed no additional restrictions. After all steps of the simulation, the number of events producing a trigger was about 5 to 10% less than the number injected originally. We used the measured position of the peak of the charge distribution of vertical muons to normalize the simulation.
i z~o~o
BACTERIALPOPULATION
I(XX)O0
~XY30
4(XK)O
~60(0)
0
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]~/~3~7
'20/03p7
3 14,~4p7
4 ~5~4/97
5 o~5t97
6
7
ao/~/o7
17/o6~7
DATE
Figure 1. Size of the bacterial population in the water during a four month period. 4. R e s u l t s
The test tank was operated continuously for a period of four months. We verified that the tank was light-tight by checking that the average pulse shape and pulse rate were the same for 12 hour day and nig.ht data samples. The analysis of the average pulse shape and the charge distribution for vertical muons in various 24 hour periods revealed that the decay time and the number of photo-electrons, as estimated from the width of the charge distribution, increased slightly over the four months of the study. This can be interpreted as an improvement of the water quality which is correlated with a decrease of the bacterial population as shown in fig. 1. Another result obtained from the test tank is a newly developed calibration method using elec"~• 2oo
•
175
Entries
3082
P1
17.76 5E
6736
150 125
- -
-. . . . . . . . . .
,,>, ~oo 75
26 o
~ • :
. 2
' ~ ' ~ + . ' + " ~ - + , ' .~. . . ". . ~ 4
6
8
10
++ # d H ~ , . * ,+~+ , . ++= ++, ÷+ L "++ • s- . ,+~ ~ • • ,++, 12
14
T i m e b e t w e e n pulses ( e x p e r i m e n t )
16
1B
20 gs
Figure 2. T i m e between pulses after applying the cuts C1 < C2 and At < 8its. The solid curve is a fit to the d a t a using the function
P1 + P2 exp(-t/P3).
391
J.C D'Olivo et aL /Nuclear Physics B (Proc. Suppl.) 75,4 (1999) 389-391
trons from muon decay as a standard source with a known energy spectrum. The electron pulse follows the pulse generated by the parent muon with a.n exponential time distribution with a mean lifetime of about 2.2#s. Using adequate cuts, namely that. the charge of the second pulse should be bigger than the charge of the first pulse and restricting to a time window of 8/Is one can eliminate most of tile background consisting of PMT after-pulses and random coincidence of muon inside the tank (fig. 2). The charge distribution of the electron pulses (fig. 3) shows a pronounced beak a.t 44.5 I,C, a.bout a factor of 0.18 below the peak of vertical muons which was found to be at 248.5 pC (VEM). The charge distribution of the parent muons has its peak at 24.6 pC, corresponding to ~ 0.10 VEM (fig. 4). The experimental results are well reproduced by our simulations (shaded graphs).
250UA
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100
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4
Mean
24.60 +
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. . . . . . . . . . i
0
50
100
150
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350
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Charge of first pulse (experiment) 0. looo u3
E>o6co
Entries
5110
Constant
948.2
I M~n Stg~
pC 2 ,
22 92 J 7718 z
O1979 I
I 01493 1
I
W
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Lj
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o 5O
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200
250
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35O
Charge of first pulse (simulation)
40O
45O
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Figure 4. Charge distribution of first pulses after the cuts C1 < C2 and At < 8#s. 5. C o n c l u s i o n s a n d O u t l o o k
L5
Consiaql ! Mean
53 94 ~ 44 47 f
4c,
Sigma
15,37 ~
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IJJ 30 20
/
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Charge of second pulse (experiment) i
~z)
~
.
18o
20o
pC
Conslan~
242.9 ±
L940
Mem~ S,gma
4325 ± 12 26 I
0 3409 0 2210
[J~ 150 1 aO
Charge of second pulse (simulation)
pC
Figure 3. (lharge distribution of second pulses a.it,er the cuts C:t < (/', and At < 8 #s which enrich the sample with decay electrons. Comparison with the sinmlation, which has no random coincidence of muons, shows that the peak is indeed produced by the decay electrons.
The results we obtained from the test tank indicate that high density polyethylene is a suitable material for the WCDs of the Pierre Auger Observatory. Furthermore, we demonstrated that decay electrons can be used for calibrating and monitoring WCDs in a remote way. We would like to thank C. Hojvat, C. Pryke from the Pierre Auger Collaboration. This work was partially supported by BUAP, CINVESTAV, CONACyT, UMSNH and UNAM. REFERENCES
1. P. Sokolsky, P. Sommers and B.R. Dawson, Phys. Rep., 217, No. 5, 225 (1992); M. Hillas, proceedings of the Xth IS VHECRI, Gran Sasso, Italy, 1998. 2. Auger Collaboration, " T h e Pierre Auger Project - Design Report", March 1997,
http ://www-td-auger. fnal. gov :82/. 3.
F. Alcaraz et al., Auger Project Technical Note, GAP-97-050, (1997). 4. A.GonzMez and L. Villasefior, Proc. of SOMI XII, San Luis Potosi, Mexico, 1997. 5. C.L. Pryke, Ph. D. Thesis, Leeds University, 1996.