Measurement of EMI and HAMAMA TSU for the CHOOZ …lss.fnal.gov/archive/other/lpc-94-15.pdf · COLLEGE DE FRANCE Laboratoire de Physique Corpusculaire . Measurement of EMI and HAMAMA

COLLEGE DE FRANCE Laboratoire de Physique Corpusculaire

Measurement of EMI and HAMAMA TSU photomultiplier characteristics

for the CHOOZ experiment.

P. Salin I and S. Sukhotin2

1 College de France. 2 Kourchatov Institute, visitor at College de France.

Telephone: Direct: (l) 44 27 15 28 L.A. N° 41 II, pl. Marcelin Be11helot Standard: (l) 44 27 12 11 IN 2 P 3 - CNRS 75231 Paris Cedex 05 Tek~copie : (1) 43 54 69 89

Measurement of EMf and HAMAMA TSU photomultiplier characteristics.

I. Introduction

This work was done in the framework of the future CHOOZ experiment to search for neutrino oscillation at a distance of I km from a nuclear reactor. The detector design is shown on figure I. The proposal of the CHOOZ experiment l rests on using a large liquid scintillator detector in the center part of which gadolinium-loaded scintillator will be used. Antineutrinos from nuclear reactors will be detected via the reaction:

- + vc+p~e +n

L+ Gd ~ YI ,Y2... + <E> =8 MeV

with a neutron lifetime in the gadolinium scintillator of about 28 j.!s.

The fiducial volume is viewed by 160 photomultipliers (PM) which give the information

about the energy of e+ and n events as well as the time delay between these two. The time and energy balance information will provide a localisation of an event.

Monte-Carlo calculations2 of efficiency and light collection paranleters of the detector

showed that we have for the e± 150 photo-electrons/event at E;:;:; 1 MeV. It means that individual PMs will operate in single photoelectron regime. So, the PM parameters in this regime are very important and the Chooz collaboration needs to find good PM with best peak-to-valley ratio, low dark current counting rate etc. The College de France group is preparing a Flash ADC electronics, to sample the PM pulses3. The results of the present test will be a part of a full PM+FADC test to be installed in the laboratory in a near future.

Many types of PM have been already checked in other laboratories (see for example 4,5). We selected THORN-EMI-935 I and HAMAMATSU (R4558) 8" phototubes.

The followings characteristics have been investigated for both PM:

• Shape of single-electron anode pulse. • Single-electron transit time distribution. • Afterpulses. • Dark-current rate.

With EMI-935 I the tinle characteristic for CHOOZ regular and gadolinium-loaded

scintillators were measured with a 2078i source (Ee with E =976 keY).

2. Description of the electronic set-up:

For El\III-935 I we used the divider-type 8 6 with two independent high voltage power supplies: one for the photocathode and another one for all dynodes. So, we could vary independently the potential between photocathode and the first dynode and the potential between first dynode and the ground, according to figure 2.

2

Photomultiplier Gd-Ioaded Acrylic Gd-free scintillator internal vessel scintillator

The CHOOZ experiment Neutrino Detector

Figure 1.

3

HV2

HVI

14()() V ~ HV I ~ 1800 V 950 V ~ HV2 ~ 1200 V

R 1 = R2 = 305 k!l

R3 = ... = R8 = 161 k Q

R9 = 235 kQ ; RIO = 330 k!l

Anode

R 1]=487 k!l

R12=630 kQ

Ra=lO k!l

Figure 2 : High voltage divider-type B for EMI.

For EMI we varied the voltage between the photocathode and first dynode-V(pc -d I) and the voltage between first dynode and ground-V (d J-grnd) in wide ranges, which are indicated on figure 2. For HAMAMATSU the standard resistor divider, recommended by the firm, was used.

The electronic scheme is shown in figure 3.

I I

XP2020

I Scintillator : LED I I I -------------------- ­ ________ J

BLACK-BOX

CFD

ATT.

CFD

~--t\ ;enerator 1----- ­ ___------'

DC Stop

Stop C A

Start M A

DC C IN 1

IN2

Gate

Acquisition systenl hascd on VME and

CFD : consLant fraction discriminator OS9. DEL. : delay J\TT. : attcnllator

Figurc 3 : Elcctronic schcmc or thc mcasurcmcnt sct-llp.

4

3. Single photo-electron charge spectrum.

The measurements of dark-current spectra for both PM were done after some days of keeping PM in darkness with high voltage applied as follow: V(pc-dl) =550 V and V(dl-grnd) = 1050 V for EMI and V = 1675 V for HAMAMATSU. The dark-current rate after 48 hours in darkness was stable. At a threshold of 0.1 single photoelectron peak the counting rates were: for

EMI :::= 800 s- ) and 1400 s-I for HAMAMA TSU. The typical dark-current amplitude spectra (i.e. a number of events plot versus ADC channel) are shown in figure 4, for EMI and HAMAMATSU, just for demonstration the more interesting parameter of these spectra - a peak-to-valley ratio.

One can see an evident peak corresponding single-electron noise with a noise from dynodes in the left part of the charge distribution spectrum. HAMAMATSU behaves much worse than EMI, with bad peak-to-valley ratio. Taking this into account, more detailed measurements were done only with EMI-9351.

For EMI data, a fit was done to estimate the probability of emitting 2 or more photo­electrons in the dark current spectrum. The function fitted is the sum of an exponential function with parameters PI, P2 and two gaussians. Parameters P3, P4, P5 are for the first gaussian (P3 is the norn1alization constant, P4 is the mean value and P5 is the width). P6 is the normalization for the

second gaussian with a mean value 2P4 and a widttrJ2 P5. P7 is a value for the pedestal. The dark current spectrum and the fit are presented on figure 5. We estimate the contribution of 2 or more photo-electrons to be less than 3 % for the EMI-9351 photomultiplier in the dark current spectrum.

For the light source we used three kinds of LED (light-emitting diode) with different colours: red, green and yellow to make sure that the single photoelectron response does not depend on the light wave length. To obtain the shortest and fastest light pulse possible we polarized the LED in the avalanche mode i.e. in the forbidden polarity (negative) with a generator pulse. For some LED the slope of the characteristic is not steep enough, fortunately some present a very high slope (few AlV), with acceptable voltage pulse (a few tens volt).

To check that the generator pulse amplitude for the LED corresponds to a single-electron response from the PM the measurement with a long width of generator pulse (up to 450 ns) was done to see the expected plateau in time distribution. The result is presented in figure 6, with an amplitude distribution (top plot) and a time distribution (bottom plot). On the bottom plot, parameters of a linear fit are presented together with the fit, which is practically a constant.

The measurements with LED collected at the same time information from the ADC (to be sure that the PM is in the single-electron regime) and information from the TDC with transit time distribution. We went (step by step) through wide scale of potentials between photocathode and first dynode from 350 V up to 700 V, with a fixed potential between the first dynode and the ground (V (d l-grnd) = 1050 V). We present for six different values of the mentioned potential the following data: ADC distribution (fig. 7), TDC distribution for the main peak (fig. 8) and TDC distribution in a logarithmic scale (fig. 9) to demonstrate the deposit of afterpulses. Each bin in the TDC distributions equals 0.5 ns. Notice here, that for our measurements with a classical (start-stop) TDC, afterpulses correspond to the situation when a photoelectron was registered by TDC not within a nlain peak time window, but with some delay. In other words, sometimes a photoelectron transit time is bigger than the mean transit time. The results of these measurements were fitted. The

following parameters: (mean peak position/width)2 and a peak-to-valley ratio for amplitude distribution, as well as the peak position and the width of the transit time distribution were determined and their dependence versus the voltage between photocathode and first dynode are plotted on figurelO.

5

Dark current spectrum r EMI and HAMAMATSU

OJ 30000> OJ

'-+­

o L 25000

E 1 20000

15000

10000

50

o

OJ 16000 > OJ

'-+­

0 14000 L

12000E :=J

z

8000

6000

4000

2

0

ADC channel EMI 550V- 1050V

ADC channel HAMAMATSU HV= 1675V

Figure 4 : Typical dark-current amplitude spectra.

6

EMI PM.V(pc d1 )=550V.V(d1-grnd)=11 OOV.Threshold 0.1 pe.

en ........... c Q)

~ 30000 4­

o L Q)

.D

E ~ 25000

20000

15000

10000

5000

o

648.7 / 2 12.42

0.7956E-01 0.1953E +05

.91 P5 31.62 P6 359.6 P7 18.93 I

I

\ I \

\ ," \ : I: r

:\

\

\

channel ADC Dark current spectrum

Figure 5 : Fit for EMI dark-current spectrum.

C

7

I V(pc-d 1)=5 V(d 1 -grnd) 1050V Neg.polarity yellow

(f) +--'

Q) 225> Q)

o ' ­Q)

.D 175 E ::J 150Z

125

100

75

50

25

o

10 Entries

Mean

RMS 5.320

AOC channel AOC single P spectrum

(f) +--' C Q)

> Q) 250

' ­Q)

E 200 ::J

Z

100

50

o

1

O.1105E-01

TOC channel TOC - 1 bin O.5ns

Figure 6: ADC and TDC spectrum with a long width LED pulse.

8

Ingle PE spectra V(d1 grnd)

Ilndf 71.70 I 531500 Constant 1167. Mean 57.49 Si ma 20.67

1000

500

0 50 100 150

V(pc d1)=350v

x'/ndf SOLO I 78 Constant 8391. Mean 66.478000 Sigma 22.73

6000

4000

2000

0 50 100 150

V(pc-d1)=

x:Indf 62.42 I 63 Constant 1102. Mean 72.79

i Si ma 23.87

1000

500

0 50 100 150

V(pc d1) 600v

1050v. ff.V(pc-d1 ).LED -Neg.polar.

I 53 1110. 64.21 21.811000

500

0 50 100 150

V(pc d1)=450v

Kindt 34.90 I 53 Constant 2177. Mean 70.90

22.672000

1000

0 50 100 150

V(pc d 1 ) 550v

Sigma

2000

1500

1.000

500

0 50 100 150

V(pc-d 1) 700v

I 63 2140. 77.22 25.38

Figure 7: Single-electron amplitude distributions for different photocathode -first dynode potential.

100

9

80

6000

4000

2000

o 410

1bin 0.5ns V(d 1 grnd)= 1 050v. Oiff.V(pc d 1 ).LEO

--.~.-- ---r-'i';-;-/n--:df--=-86::-::-99~.~/--;---:-1-::-16

Constant Mean Si ma

420 430

V(pc-d 1)=350v

3000015000

2000010000

100005000

0 0 410 420 430

V(pc

15000

5000

o

5592. 426.9 2.536 8000

6000

4000

2000

o 410 420

V(pc-d1)

Neg.polar.

i'/ndf 2200. I 10 Constant 7080. Meon 421.1 5i ma 1.685

430

450v

I/ndf 2867. I 12 Canstont 0.2785E+05 Mean 420.3 5i ma 1.619

d1)=525v V(pc d 1)=

i'/ndf 362.2 I 12 I I/ndf 265.2 I 1230000Constant 0.1361E+05 Constant 0.2914E+05

Mean 418.1 Si ma 1.887

20000

10000

o 420 430410

V(pc-d1) 600v V(pc-d1 )=700v

Figure 8 : Transit time distributions for different photocathode-first dynode potential.

10

Toe 1bin=0.5ns V(d 1-grnd)= 1050v. Oiff.V(pc d 1 ).LEO Neg.polar.

ID 210 4 10 4

Entries 500000 Mean 1828. RMS 445.6

10 3 10 3

10 2 10 2

10 10

400 600 800

V(pc d 1) 350v

400

ID 2 Entries 500000 Mean 1866. RMS 426.9

600 800

V(pc-d1) 450v

10

400 600

ID Entries Mean RMS

2 500000

1759. 554.4

800

V(pc-d 1)=525v

400

2 Entries 1000000 Meon 1804. RMS 509.5

600 800

V(pc d1)=550v

10

400 600

ID Entries Mean RMS

800

2 500000

1764. 530.5

10

400 600

ID Entries Meon RMS

800

2 1000000

1769. 549.0 .

V(pc-d1) 600v V(pc-d 1)=700v

Figure 9: Transit time distributions with afterpulses for different photocathode-first dynode potential.

6

• 5.5.... = '0 ~

I:: 5 '33 8 =4.5 '~

~ 4 o .....= :r: 3.5

fZ ~

300

428

426 ~

~

= CQ= 424 .c

..:!.

.5 ~ 422 .... :: fIl 420 CQ= ....'""

.:c 418CQ

~ 416

11

(a)

! ! I

i II I I

I

I

400 500 600 700 800

Voltage [volts]

(c)

I

(b)

2.4

2.2

0 ':C 2r: ;,.. .! 1.8

CQ -j;o>. .s 1.6 ~

CQ ~

~ 1.4

1.2 300 400 500 600 700

Voltage [volts]

(d)

1.8

~ fIl .s 1.6 .­b '-'

.c.... 1.4

'i"C

~

~ 1.2 I ....

';; II f=r: !~ I I 0.8

300 400 500 600 700 800 300 400 500 600 700 800

Voltage [volts] Voltage [volts]

Figure 10 : Main parameters of EMI versus the voltage between the photocathode and the first dynode:

a) (mean peak position/width)2 (pedestal subtracted) : this parameter for single photoelectron regime is dominated by the gain coefficient of the first dynode;

b) peak-to-valley ratio; c) main peak position in transit time distribution; d) width of the transit time distribution.

All parameters after V(pc-dl) = 550V change rather slowly, that indicates that the voltage between photocathode and first dynode less than 550 V is not enough to create a proper electric field to collect photoelectrons to the first dynode. In the range 550-700 V one can see a slow rise of the peak-to-valley ratio: by increasing V(pc-dl), we improve the collecting conditions for the photoelectrons. We also see a systematic decrease of the position of the main peak in transit time distribution, due to an increase of the electric field inside of the PM bulb.

4. Transit time distribution.

The main peak in the transit time distribution plot, shows the regular gaussian shape when the voltage between photocathode and first dynode is larger than 500 volts. Using data from plot (d) on figure I 0, we estimated the width of the main peak as 0' = (1.0 ± 0.1) ns.

800

12

5. Multiple photoelectrons spectrum.

Knowing the generator amplitude corresponding to one single photoelectron response (fron1 the measurement with a long width of a generator pulse), we increased the LED generator amplitude and investigated the multiple photon response of the EMI PM. The resulting plot is presented in figure 11 with the fit of an exponential function (to describe a left part of the distribution) and 6 gaussians with qi = Nql and O'i = O'f{N, where N= 1,2, ... 6 (like for the figure 5). Paran1eters PI-P8 on the plot are: P7, P8 correspond to fit of exponential function and PI-P6 are proportional to the probabilities of emitting 1 to 6 photoelectrons. They follow to the Poisson distribution:

xn Pen) = exp(-x) I with x=1.94 n.

EMI Green PD. Start from generator

5000

4000

3000

2000

1000

o

P1 0.3663E+06 P2 0.3605E+06 P3 0.2187E+06 P4 0.1209E+06 P5 0.4394E+05 P6 0.2381 E+05 P7 12.93 P8 -0.5S96E 01

100 200 300 400 500 600

Figure 11 : Multiple photoelectron amplitude distribution.

6. Spectra with a lot of photoelectrons (linearity).

We measured also a response from the EMI PM when the nurnber of photoelectrons is larger than 10, by increasing the amplitude of the LED generator pulse. For each measurement we had a

13

regular gaussian amplitude distribution. Fitting it we obtained a mean peak position and a width( 0').

After that a parameter - (mean peak position/width)2 was calculated, which corresponds to the number of photoelectrons emitted from the photocathode. The result of these measurements is shown on figure 12 as the number of photoelectrons vs the mean peak position (ADC channel). One can see a satisfactory linearity of the PM, when the number of photoelectrons changes from 10 up to 200. So we checked the possibility to use the EMI PM both in the single photoelectron regime and with a big amount of light.

250

200

~ ~ 150 ~

<= ~

,.Q­100e

::s:z 50

0

0 200 400 600 800 1000

ADC channel (0.5 pC/channel)

Figure 12 : Amplitude linearity. 7. Afterpulses.

Some additional measurements were done to understand what parameters are more important for afterpulses. For each measurenlent two values were calculated: N . - is the number of events

mam

corresponding the main peak in the time distribution (±5 0' from the main peak position) and Naftp ­

is the number of events within a time window 100 ns after the nlain time window. The ratio

N . /N ft was chosen as a parameter to describe the deposit of the afterpulses. These ratios were mam a p obtained for:

1) different voltages between the photocathode and first dynode (V(pc-d1)) at fixed voltage between first dynode and ground (V(dl-grnd)=1050V) in the single photoelectron regime (figure 13 - a); 2) different V(dl-grnd) at fixed V(pc-dl) =550V, the threshold being changed to keep it at 0.1 times the single photoelectron peak for different V(dl-grnd):

(figure 13 - b ) for the single photoelectron regime; (figure 13 - c) for measurements with a mean number of photoelectrons about 10;

3) different thresholds at fixed V (d 1-grnd)= 11 OOV and V (pc-d 1 )=5 50V in the single photoelectron reginle:

(figure 13 - d) as a function of the ratio of a threshold to peak position (pedestal was subtracted).

For fig.13-a and 13-b we added the information about measured charge (from corresponding amplitude distributions) to demonstrate how to renormalize the anode signal when V(dl-grnd) and V(pc-d1) are changing.

14

plot "bl!plot "a"

6

.:I. C# ill Cl.5 c'; 8 0 ... 4 r.fJ ill r.fJ

'S CI.

"" ~ 3 ~ ,..... ~ '0 .:: 2 c# ~

:4 cc Q,j Q"

c 0.8 .; S S 0.6 til Q,j

.!!l :I Q"

t 0.4 c:: < 1"""1

~ ""-10.2 .S .... cc=:

I I I I

. x Measured charge(ADC channel) x Ratio[% J:Afterpulses to main peak x

x x

x

x

X

I ! ! I

0 Xx Measured charge(ADC channel) . Ratio[%J:Afterpulses to Main Peak

x

x

x

x

X

80'D

.:I. tr)'-' ~ 4.5 70~ tr)'Z Q. c c c c c

4 6O.cCIt

uo.c C# ';

V) u ~ UU 0

Q 50 Q... 3.5 < r.fJv) ~

<&,..." ill illcu r.fJ CI)CI) 'S 3 40 ;"" CI.c# .co.c uill<& u ""

ct: 30 "0"0 < 2.5 cuill ,..... V) "" ::"" ('!") ~ ~ 20 :ell illill :s 2

... ~ o~ c# ('!") ~ 1.5 10

V) ('l 0

300 400 500 600 700 800 900 950 1000 1050 1100 1150 1200 1250 V(pc-dl) (volts) V(dl'grnd) (volts)

plot "e" plot lid"

I I I I I I 3.7

Mean value of Npe=9.3±OJ .:i. C\I cu CI.

-c'; 3.2 8 0...

- r.fJ ill r.fJ

"3 CI.

2.7

""ill

- I ct:< 2.2 ,..... ~ :§'

I ­... C\I 1.7 ~

I I I I I

I I

! I I I I1.20 I I I I I I

900 950 1000 1050 1100 1150 1200 1250 0.1 0.2 0.3 0.4 0.5 0.6

V(dl.grnd) (volts) Threshold in Ispe

Figure 13 : Afterpulses versus different parameters.

The results of these measurements permit us to make the following conclusions: 1. afterpulses decrease with increasing of number of photoelectrons emitted from the

photocathode, due to the decrease of the probability to miss a photoelectron (remind here again that we used classical TDC in our measurements);

2. afterpulses do not change dramatically when changing V(pc-d1) and V(dl-grnd) from 350V up to 700 V and from 950 V up to 1200 V respectively;

3. even in the single photoelectron regime, afterpulses practically do not depend on the output signal threshold, at least when a threshold changes from 0.1 up to 0.6 of the mean value of single photoelectron peak position;

4. afterpulses are mainly located within a time window of 25 ns, which is itself delayed after the main peak about 45 ns (see figure 9).

15

8. Decay time of the scintillators.

With the EMI and another PM, HP 2020, the time parameters of scintillators (Od-free and

Od-loaded) were measured. A snlall quartz box with dimensions 8 x 8 x 50 mnl' was filled with

scintillator. On the top of the box a 207Bi source was placed. The box had optical contact with the HP 2020 . The distance between the box and the EMI-PM was chosen to obtain the single electron regime for the EMI. The amplitude distributions were measured for both PM. Signals from HP 2020 gave the trigger for EMI amplitude and time analyses. Time distributions of both PM were measured with a LED to demonstrate that time parameters of PM are much faster than those of the scintillator. Figure 14 shows the corresponding ADC and TDC spectra for both PM.

Single PE time distribution for EMI and H 020 with yellow

Indf 57.82 I 68 5000 · x/ndf 212.6 I 63 Constant 660.6 Constant 3954.700

600 4000

500 3000

400

300 2000

200 1000

100

o o 20 40 60 80

ADC-HP2020 PM- 1 sing pe

50 100 150

ADC-EMI 1 single pe

2000 2000

o 230 240 250 260

TOC-EMI PM 1 bin O.5ns TOC- P2020 PM 1 bin 0.5ns

Figure 14: ADC and TDC distributions of both EMI-9351 and HP 2020.

I Mean 22.82 Si ma 25.50

-I/ndf 657.4 I 10 Constant O.1035E+05 Mean 242.5 Si ma 1.391

14000 10000

12000

8000 10000

6000 8000

60004000

4000

-Ilndf 1267. I 10 Constant O.1332E+05 Mean 232.4 Si ma 1.700

16

In figure 15 and 16 we present the results of time-constant measurements for both scintillators (gadolinium-free and gadolinium-loaded).

100 200 300 400 500 600

tdc 1bin= 1ns

Figure 15: Time-constant measurements for gadolinium-free scintillator.

17

(f) +-' X

2/ ndf 833.9 / 556 c

ill P1 8.452> ill

'+-

10 40 L

ill ..D

E :::J z

10 3

10

200 300 400 500 600

tdc 1bin=1ns

I

", I '1 I

I

P6 P7 P8 P9 P10

-0.4141E 01 16.80

-0.2469 1 .8 68.83 3.9 3.734

-0.1027E 01 2.782

T1=4.0 ns

T2=24.2 ns

T3=96.0 ns

100

Figure 16 : Time-constant measurements for gadolinium-loaded scintillator.

18

A fit with three exponentials (to describe time constants of a scintillator) and a gaussian (for

afterpulses) was done for both figures. The results of the fit are (they are also presented on figures):

• For the gadolinium-free scintillator, the time constants are:

1:1 = (7.3 ± 0.4) ns ; 1:2 = (31.7 ± 1.6) ns 1:3 = (93.6± 4.7) ns

The corresponding probabilities are: y 1 = (70 ± 4)% Y2 = (28 ± 1)%. y 3 = (2 ± 0.1)%

• For the gadolinium-loaded scintillator, the time constants are: 1:1 = (4.0± 0.2) ns ; 1:2 = (24.2± 1.2) ns . 1:3 = (96.0 ± 4.8) ns

The corresponding probabilities are: Yl =(72±4)% Y2 =(26± 1)%.

Conclusions

The tests performed on EMI showed us that this tube suits well the purposes of CHOOZ experin1ent, mainly due to :

a) low dark curent noise rate;

b) good single-electron response;

c) narrow single-photoelectron transit time distribution;

d) good linearity up to 2 V for anode output signals.

Note also that the glass of the tube has a low contamination of radioactive materials. This last point is very important for a low background experiment like CHOOZ.

Acknowledgement

We thank the Thorn EMI and HAMAMA TSU societies for their help and for fruitful discussions. We are waiting for a new PM from HAMAMATSU (R5912), which must have much better peak-to-valley ratio than R4558. We thank also M.Sene for his help and advice.

I Proposal to search for neutrino vacuum oscillations to Llm2 =10-3 eV2 using a 1 km baseline reactor neutrino experiment NIM .....

2 See reference 1.

3 See reference 1.

4 Performances of large cathode area phototube for underground physics applications. G. Rannucci, R. Cavalletti, P. Inzani and S. Schoner. INFN/AE-92/09.

5 Characterisation and magnetic shielding of the large cathode area PMT's used for the light detection system of the prototype of the solar neutrino experiment Borexino. G. Rannucci et al. NIM A 337 (1993) 211-220.

6 Data sheet from Thorn EMI.RIP 069 Voltage Divider Design.