- Email: [email protected]
The photomultiplier tube testing facility for the Sudbury Neutrino Observatory
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Nuclear Instruments
and Methods in Physics Research A 373 ( 1996) 421-429
NUCLEAR INSTRUMENTS 8 METHODS IN PHVSICS RESEARCH SectIon A
ELSEVIER
The photomultiplier tube testing facility for the Sudbury Neutrino Observatory C.J. Jillings*,
R.J. Ford, A.L. Hallin, P.J. Harvey,’ R.W. MacLeod,2 H.B. Mak, P. Skensved, R.L. Stevenson Department
ofPhysics, Queen’s Universiry at Kingston, Kingston, Canada K7L 3N6 Received
3 January
1996
Abstract A facility to test the photomultiplier tubes (PMTs) for the Sudbury Neutrino Observatory (SNO) was constructed at Queen’s University. The facility measured the noise rate, relative efficiency, and charge and time spectra of single photoelectron anode signals for 44 PMTs daily. The system is described, and detailed results of the testing for two particular PMTs are presented as well as statistics for 9829 Hamamatsu RI408 PMTs accepted for use in SNO.
1. Introduction 1.1. The Sudbury Neutrino Observatory The flux of electron neutrinos from the sun has been measured in four separate experiments and is lower than the value predicted by solar models by a factor of 1.5 to 2 [l-4]. Our understanding of the nuclear fusion processes in the sun and the standard model of electro-weak interactions is not reconcilable with the results of these experiments [5]. To better understand this discrepancy, the Sudbury Neutrino Observatory (SNO) [6], a 100 t heavywater Cherenkov detector, is being constructed underground at the 6800-foot level at Into’s Creighton mine near Sudbury, Canada. SNO will be able to measure neutrino flavor oscillations by measuring the flux of all types of neutrinos from the sun and the flux and energy spectrum of the electron neutrinos. The energy spectrum and total flux of electron neutrinos will be detected using the charged-current interaction v, +d+p+p+e-
- 1.44MeV.
Because of final state effects between the two recoil protons, the recoil electron energy distribution has a sharp maximum approximately 1.44 MeV below the incident neutrino energy. The electron emits Cherenkov light in proportion to its energy, and PMTs detect this light. The * Corresponding author. E-mail [email protected].
’ Currently at Muse Research Inc., Kingston, Canada, 127. ‘Currently at Lawrence Berkeley National Laboratory, keley, USA 94720.
K7K Ber-
0168-9002/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SO168-9002(96)00067-8
detector geometry is such that two Cherenkov photons will seldom strike the same PMT in any one event. Therefore, the number of PMTs hit by Cherenkov light is related approximately linearly to the incident neutrino energy. SNO will measure the total flux of all neutrino types using the neutral-current disintegration of the deuteron
The neutron will be detected in ‘He proportional counters or will be captured by a chlorine nucleus, from salt dissolved in the heavy water, resulting in a cascade of gamma rays with a total energy of 8.8 MeV. These gamma rays will Compton scatter off electrons causing the electrons to emit Cherenkov light which will be detected by the PMTs. I.2 PMT characteristics The number of PMTs detecting Cherenkov light in an event depends on several factors: the total amount of Cherenkov light, the attenuation in the heavy water and other detector components, the geometrical coverage of the PMTS, and the efficiency of the PMTs. In a typical event, 30 to 150 (of 9456) PMTs will detect Cherenkov light and all anode signals will result from a single photoelectron (spe). The three most important PMT parameters are the noise rate (the rate of anode signals when there is no illumination of the PMT photocathode). the efficiency (the probability that a photon incident on the PMT produces an anode signal above threshold), and the transit time spread (the variation in the time between the emission of the photoelectron at the photocathode and the arrival of the signal at the anode). As any of these parameters is
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It is defined as the ratio of the height of the spe charge spectrum at its maximum value to its lowest value at lower charge than the maximum value. A high peak-to-valley ratio (good charge resolution) allows clear separation of the pedestal from the spe peak resulting in lower noise with higher efficiency. Pre-pulsing and after-pulsing rates are the sizes of secondary peaks in the time spectrum relative to the size of the main peak. The rate of increase of noise rate with anode-to-photocathode voltage difference near the operating voltage is important because a small drift in operating voltage must not cause a large increase in noise rate. To ensure that the PMTs used are of sufficiently high quality each PMT was tested for all these parameters.
2. The PMT testing systems 2.1. The single-PMT
Fig. 1, A schematic of the Hamamatsu R1408 PMT design. The 9 dynodes are shown as solid horizontal lines, The focusing grid is shown as the dashed lines above the dynodes. The mask above the focusing grid is kept at the same voltage as the first dynode and allows only those photoelectrons directed toward the middle of the dynodes to pass. All dimensions are in centimeters.
the energy resolution and event reconstruction are improved. The PMTs in SNO will be operated at a gain (the integrated anode charge resulting from a single photoelectron) of 10’. The gain is a function of the anode-tophotocathode voltage difference; the voltage yielding a gain of 10’ is referred to in this paper as the operating voltage. Several other PMT parameters are of concern. The charge resolution is measured by the peak-to-valley ratio of the spe charge spectrum which is the depth of the valley between the pedestal and the peak of the charge spectrum.
improved,
CArHODE /
FOCUS
ANODE
HlOH VOLTAGE
Fig. 2. The base used for SNO PMTs. All unmarked resistors are 1 mR. The values of the other components are Rl = 510 a, R2=240n, R3=5lfL, R4=lOOkR, R5=75fL, Cl=lnF, and C2 = 4.7 nF. The signal and high voltage are carried on the same cable.
testing system
Many collaborations have developed systems to test the PMTs used in their experiments [7-91. For SNO, two testing systems were constructed at Queen’s University. The first system tested PMTs individually in a small light-tight copper box between two magnetic field compensation coils [IO]. The net magnetic field was uniform over the volume occupied by the PMT and its strength could be varied from -48.6 ~_LT to +48.6 p,T, the strength of the earth’s natural field in Sudbury, Canada. The PMTs were covered with a mask centered about the axis of the test PMT such that only a circle of diameter 19.0 cm was exposed to the light source. The Cherenkov light source, a “‘Sr beta source encapsulated in UV-transmitting plastic, was mounted on a 1 cm Hamamatsu RI635 monitor PMT. The signals for both the monitor PMT and the test PMT were amplified and passed to constant-fraction discriminators. The discriminator outputs were passed to a coincidence unit whose output was the gate for a chargeto-digital converter (QDC) and the start signal for a timeto-digital converter (TDC). Scalars counted the singles rates for the monitor and test PMTs and were used for dead time correction and noise rate measurements. For relative anode efficiency measurements, the test PMT was placed with the Cherenkov source centered on the axis of the PMT at a distance such that the half-angle subtended by the exposed part of the test PMT was 7.0”. The relative anode efficiency was defined as
where C is the number of coincidences corrected for dead time, M is the number of monitor singles, and 9 is a scaling factor for the monitor PMT. The factor, R depended on the geometry of the source and monitor PMT and on the efficiency of the monitor PMT; it is, therefore,
C.J. Jillings et al. / Nucl. Instr. and Meth. in Phys. Res. A 373 (1996) 4,‘1-429 independent of the test PMT. It was measured regularly by retesting several standard test PMTs and was found to increase by approximately 10% over three years. This increase was due to radiation damage in the plastic which caused the plastic to become less transparent resulting in fewer coincidences for a given test PMT.
2.2. The automated test system The second system is an automated facility capable of testing 44 PMTs a day. This system, shown schematically in Fig. 3, measured the noise rate and gain as a function of voltage and measured the spe time and charge spectra at the operating voltage. From the spectra the relative anode efficiency, the spe timing resolution, the peak-to-valley ratio of the charge spectrum, and the pre-pulsing and after-pulsing rates were derived. Constant-fraction discriminators (CFDs) were used to reduce the timing walk due to variable charge in the anode signal. The gain of the amplifier for the PMTs was 20. The PMTs were tested in one of two dark rooms of dimension 3 m X 3 m X 3 m. Each dark room has 2 magnetic field compensation coils which reduced the field at the location of the PMTs to less than ten percent of the earth’s field. A 1 m high aluminum frame table with an electrically insulating top was centered in each room. Each table holds 48 PMTs. Black curtains, hung from the ceiling, divide the table into 4 sections each having 12 holes on a 3 by 4 grid. Twelve PMTs (1 standard and 1 I test PMTs) are mounted in each section for testing. In each section there is a diffuse light source centered 2 m above the PMTs. Using four light sources reduced the variation
PMT
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INTERFACE
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Fig. 3. A block diagram for the large test system. The amplifiers for the PMTs had a gain of 20.
of light intensity effects.
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at each test PMT due to solid angle
3. The light source The light source for the second testing system was a wavelength-shifted pulsed nitrogen laser. The nitrogen laser produces several sharp lines all near 337.1 nm. This ultraviolet light was directed to a 5 cc quartz glass dye cuvette containing a solution of BBQ (manufactured by the Exciton Chemical Company Inc. of Dayton, Ohio, USA). This dye produces a broad (- 10 nm) band of light centered at 386 nm where the quantum efficiency of the PMT is nearly maximum. The relaxation time of the dye was not important because the all the populated states are deexcited on the first pass of the light through the dye. Thus the width of dye laser light was that of the original laser. The beam of 386 nm light was passed through a diverging lens, attenuated, and directed to a bundle of 8 graded index optical fibres (Belden 2272) each of which carried light to a dark room section (2 rooms, 4 sections each) where it was diffused above the PMTs. The light was attenuated by two neutral density filters. The first filter reflected over 99% of the beam of the beam to an MRDS 10 photodiode producing a timing trigger. The second attenuator had eight possible settings: six neutral density filters (with transmissions of 50%, 40%. 30%. 20%, IO%, and l%), a solid aluminum blank, or no filter. These were placed a wheel which was turned by a computer-controlled DC motor. The spe testing was performed with a 1% transmission filter. The blank was used for the noise rate tests and the other filters were available to study the multiple-photoelectron response of the PMT. Two different models of laser were used. The first sixty percent of the PMTs were tested with an LN120C nitrogen laser manufactured by Laser Photonics of Orlando, Florida. This laser used a Blumlein discharge circuit and was operated for 18 hours daily and pulsed at 15 Hz. The time width of the output pulse was approximately 300 ns. The amount of maintenance required depended on the alignment of the laser head. If the head was not exactly aligned, the head required cleaning daily; however, when the laser head was precisely aligned, the laser ran for a week without maintenance. The dye required changing bi-weekly because of degradation due to the incident light. The pulse to pulse stability was -10% during the testing cycle. For the remainder of the PMT testing an LN203C nitrogen laser, also manufactured by Laser Photonics, which features a thyratron triggered discharge, was used. The width of the output pulse was approximately 600 ns. The pulse intensity was more stable (~4%). and the unit ran for over 2000 hours at 20 Hz with bi-monthly maintenance. The dye was changed weekly with this laser because of its higher intensity. No steps were taken to prevent multiple modes from
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being created in the optical fibre. This is a potential concern because multiple modes leads to time dispersion. Therefore, the time dispersion of light in the laser and optical-fibre system was measured to ensure that it was much less than the transit time spread of the PMTs. Light from the laser was passed through a neutral density filter and 35 m of optical fibre to a I cm PMT. The PMT was placed far enough from the end of the fibre that the PMT charge spectrum was dominated by spe events. Using a dye that output light with a wavelength of 365 nm, the time width of the time spectrum was 0.46?0.1 ns. For 386nm light, the width was 0.2920.05 ns; for 500 nm light, 0.26’_0.05 ns. Time spectra for 500 nm and 386 nm light are shown in Fig. 4. The shift in peak position between the 500 and 386 nm light is 1.3 ns corresponding to frequency-dependent change in refractive index of the optical fibres [I 11. To measure PMT parameters, an ideal light source would have no time dispersion and produce light of equal intensity over all the test PMTs. Practically, the time dispersion in the diffuser must be much less than the transit time spread of the test PMTs and the angular distribution of diffused light should be approximately constant over the area of the test PMTs. To spread the light from the fibre optic cable evenly over the PMTs, a diffuser was placed on the ceiling of each section of each dark room. The diffusers were hollow quartz glass cylinders, 4 cm in diameter and 2.5 cm in height. The diffusers were placed with one of the flat sides (the “bottom”) parallel to the surface of the table. The sides and tops of the cylinders were aluminized on the outside and the bottoms were transparent. Light entered the
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barrel of each diffuser through an optical fibre which was inserted horizontally. The cylinders were tilled with a diffusive liquid (Ludox TM, manufactured by DuPont) which Rayleigh scattered the light. Ludox TM is a suspension in water of approximately silica particles with an average diameter of 22 nm. Because of the size and chemical composition, Ludox TM can be modeled as a collection of randomly placed small spherical dielectric scatterers. To help design the diffusers, computer simulations were performed. The simulation assumed Rayleigh scattering in an otherwise perfectly homogeneous medium with a refractive index of 1.33, incoherent light entering the diffuser, and reflection and refraction according to Snell’s law. The attenuation constant of Ludox was measured to be 0.037+0.01 mm-‘. and used as an input to the simulation. To first order, the diameter of a mirrored cylindrical barrel is unimportant because Rayleigh scattering is forward-backward symmetric. The height of the cylinder should be as small as possible to minimize the time spread from multiple scatters. The distribution of time passed by photons in the diffuser is shown in Fig. 5. To compare the simulation with a real diffuser, the spe time spectrum of dye-laser light directly from an optical fibre (measured with a 1 cm Hamamatsu R163.5 PMT) was convolved with the spectrum from the simulation. Then the time spectrum of dye-laser light through an optical fibre and a diffuser was measured with the same 1 cm PMT. The trigger for the timing electronics was generated by the photodiode used in the testing system. Both spectra were normalized to an integrated number of counts of unity. The spectra agree and are shown in Fig. 6. According to the simulation, the dominant effect on light intensity at the various testing sites was found to be the solid angle subtended by the test PMT at each site. With corrections for the effect of solid angle, the variation of intensity at each site was less than 3%. Simulated
5 Time
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Time
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Fig. 4. The spe timing resolution for 386 and 500 nm wavelengths of light through 35 m of optical fibre.
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Fig. 5. The simulated
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Fig. 6. The convolution of the diffuser simulation and the time spectrum of light from the fibre compared with the time spectrum of light from the diffuser. The diffuser error bars are Z I CT and the dotted lines represent 1~ above and below the convolved spectrum.
0.08
0 4. Calibration
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Calibration T AiCDEFGHI
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of the dark rooms
During testing, the relative efficiencies were determined from the formula
of the test PMTs
4 6 Testing
8 Site
10
12
Fig. 7. The ratio of photons detected at each testing site to the number of laser pulses and the fitted value for the ratio for the calibration of dark room 2, section I. The letters are codes showing which the locations of the 12 PMTs during testing.
(2) where N is the number of counts and 4; is the intensity of the light at the PMTs. The value of 4 at each site was found from a calibration. Each section was calibrated independently [ 121. To calibrate a section, four runs were conducted with light levels low enough for the counting to be dominated by spe events (less than one count per PMT per IO laser pulses). The same 12 PMTs (labeled A to L) were used in the four runs. Between calibration runs, II PMTs were moved within the section while one PMT stayed at the same site to allow a direct comparison between runs. The number of counts for run I at the ith site was N,., = S,sr&x
(3)
The factor, S,. is a scaling factor for the run and accounts for the unknown intensity of the light source and the number of pulses of the light source for the run. The factor, 9,. is the relative intensity at site i integrated over the exposed area of the PMT. and &Xis proportional to the efficiency of the PMT at site i for the Zth run. The number of counts for each run for each PMT was measured. There were 28 parameters: the 12 efficiences, the 12 relative intensities, and 4 scaling factors for each run. However, only 26 parameters were independent. By definition, the site for the standard tube was given a relative intensity of unity. Also, the scaling factors for the runs and the relative efficiencies were not independent-the ef-
ficiencies could be doubled if the scaling factors were halved. Therefore. a judicious choice of the scaling factor for run I would have resulted in the quantity .zX to be exactly the relative efficiency, lx_ To determine the values of the free parameters, the Marquardt method for non-linear least-squares fitting was performed. The values of the intensities derived from the fit for dark room 2, section 1 varied by 12%. The number of photons detected by each PMT. divided by the number of laser pulses. for each run of the calibration for dark room 2 section 1 are shown in Fig. 7 along with the corresponding value from the fit. The reduced chi square for the fit to dark room 2 section 1 was 1.5. The relative efficiencies were compared to the efficiencies derived from the small test system. See Table I.
5. Analysis of spectra The spectra were analysed according to the following definitions. The gain was measured considering pulses whose integrated anode charge was between 0.25 and 2.5 times the mean anode charge for a gain of IO’. The anode efficiency, timing resolution, and noise rate calculation included only those anode pulses whose integrated charge was greater than 0.25 times the mean charge. To calculate the anode efficiency, the time spectra were integrated between + 1 FWHM to yield a total number of detected
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Table I The relative efficiencies of the PMTs used in the calibration of dark room 2 section I asmeasured by the small testing system and as derived from the fit PMT label
Small system test result +0.02
Fit to calibration data -to.01
A B C
I .26 I .22
1.24 1.21 1.26 1.16 1.23 1.20 1.18 1.19 I .26 1.24 I .09 1.11
D E
F G H I
J K L
I .20 1.16 I.17 1.22 1.21 1.20 1.27 1.28 1.04 1.14
These cuts ensured that the anode pulses satisfied the low charge cut and that anode pulses far out of time were not considered in the calculations. The spe timing resolution is defined as the one standard deviation (sigma) width of the time spectrum of spe signals. To calculate the timing resolution, only signals arriving within + 1.5 times the FWHM of the mean of the timing spectra were considered. To ensure that the noise rate is not overly sensitive to voltage, the noise rate (NR) was measured at gains of 0.3 X IO’ and 3.0 X 10’ as well as 1.0 X IO’. If the ratio NR(3 X lO’)/NR(I
in Phvs. Rrs. A 37.1 (1996)
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NR( 1 X lO’)/NR(0.3
Mrth.
-20 Magnetic
Field
on Efficiency
0 20 40 Field (MT)
Fig. 8. The effect of magnetic fields on PMT relative efficiency. Parallel refers to the direction of the vanes of the first dynode and perpendicular refers to the direction at right angles to the vanes of the first dynode. Both of these directions are perpendicular to the z direction of Fig. 1. The earth’s natural field is 48.6 )LT at Queen’s University. The efficiency has been normalized to unity at zero field.
the PMT axis [lo]. The directions are defined by the direction of the vanes (slats) of the first dynode. In the SNO detector the earth’s magnetic field points approximately 15” off the vertical axis. Because the PMT efficiency is reduced by less than 3% at magnetic field intensities of 40% of the earth’s natural field, it was decided to cancel only the vertical component of the magnetic field in the SNO detector with field-compensation coils.
X 10’) (4)
X IO’)
7. Single photoelectron was less than 1.3, the PMT was accepted. If the ratio was greater than 1.3, the noise rate verses voltage data were inspected by eye to determine if a small increase in voltage would result in a large increase in noise rate. The example PMTs below illustrate this point.
6. Sensitivity of the Hamamatsu magnetic fields
R1408 PMT to
The effect of the earth’s magnetic field on the relative efficiency of a PMT is important. This effect is primarily a function of the electron’s trajectory from the photocathode to the first dynode and of the electron trajectories between dynodes. Therefore, it will vary little between PMTs of a given design. Fields perpendicular to the axis of the PMT (the z-axis of Fig. I ) were found to have a much greater affect than those parallel to the axis. The response to the magnetic field was not axially symmetric. Fig. 8 shows the effect of a magnetic field in two directions perpendicular to
results for two PMTs
The testing results from two PMTs are presented as examples. The charge distribution for a PMT (labeled R) accepted for use in the SNO detector is shown in Fig. 9. The peak at 0 pC is the pedestal. There is a valley at 1OpC which separates the spe peak from the pedestal. The measured gain was 1.01 X IO’. The time spectrum (Fig. 9) shows low pre-pulsing and after-pulsing rates. The timing resolution was 1.57 ns. The relationship between gain and anode to photocathode voltage difference is represented approximately by an exponential function in the region of interest. Also, as the voltage is increased the noise rate increases smoothly and slowly. PMT S (Fig. 10) is a second example of an accepted PMT. The peak-to-valley ratio is excellent and the relative efficiency and spe timing resolution (1.62 ns) are acceptable. However, the noise-rate ratio (Eq. (4)) was higher than the criterion allowed. This is a shortcoming of the criterion. not the PMT. Inspection of the noise-rate-versusvoltage curve shows that for up to 150 V above operating voltage the noise increases smoothly and slowly with
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voltage. PMT S is an example of why the noise-rate ratio was used only as a warning flag to signal potentially unacceptable PMTs. Many PMTs, including this one, whose ratio was high, were acceptable. In particular. PMTs with low noise rates at a gain of 10’ and flat plateau curves between 0.3 X IO’ gain and 10’ gain could fail the noiserate ratio test even if they had a modest increase in noise rate between a gain of IO’ and 3 X IO’.
3ooirf
8. Single photoelectron accepted for SNO
1500 Voltage
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Fig. 9. Data from the test of PMT R. The charge plotted includes a factor of 20 from the amplifiers shown in Fig. 3. The vertical line in the gain and noise versus voltage plot indicates the operating voltage of 1891 V.
testing results for PMTs
Histograms for the timing resolution, relative efficiency, noise rate, and operating voltage, are presented for 9829 PMTs accepted for use in SNO (see Fig. 1I). The mean RMS timing resolution is found to be 1.5 ns with an RMS deviation of 0.08 ns. Given constraints imposed on Hamamatsu (such as dynode structure), 1.7ns was the best upper limit in transit time spread that could be provided at the time of production. The mean relative efficiency is I. IO with an RMS deviation of 0.09. The mean noise rate is 2300 counts a second. Approximately half way through the production Hamamatsu changed manufacture technique resulting in a slight reduction in efficiency (but still above our specification) but reduced noise rate. The cooler temperature in the detector (approximately 10 C instead of 20 C during the testing) should reduce mean noise rate to approximately 1000 counts a second. The mean operating voltage is 1875 V. Density plots of the timing resolution and operating high voltage and of relative efficiency and noise are presented in Fig. 1I. The timing resolution is expected to decrease with increasing high voltage because the photoelectron flight time is reduced when the voltage difference between the photocathode and the first dynode is increased. PMTs with a noise rate below 1500 counts a second tend to have lower relative efficiency.
9. Conclusions
1500 Voltage
2000
Fig. IO. Data from the test of PMT S. The charge plotted includes a factor of 20 from the amplifiers shown in Fig. 3. The vertical line in the gain and noise versus operating voltage of 1732 V.
voltage
plot
indicates
the
As far as PMTs are concerned, the three most important detector parameters are the energy resolution, the spatial resolution, and background rates. The background rates were addressed by choosing glass for the PMT bulbs which contain as little thorium and uranium as possible (less than 4 X IO-‘g/g of each). In the detector, reflectors will be added to the PMTs; with these, 65% of the solid angle is covered by PMTs. Given the efficiency of the PMTs we expect the energy resolution to be approximately 14% at 10 MeV Accidental noise pulses act as a “zero offset” in the energy calibration. If the mean noise rate is 1000 counts per second, the offset is I PMT. For spatial reconstruction, the transit time spread. noise rate. and efficiency all play a role. A transit time spread of
42x
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Fig. 1I. The testing results for 9829 PMTs accepted for use in SNO.
I .5 ns corresponds to a “distance spread” in heavy water of 23 cm (= I S/(cln)). The effect of noise rate on reconstruction accuracy can be estimated by considering the number of accidental pulses in a time window equal corresponding to the maximum distance between two PMTs in the detector ( = 17 m corresponding to 75 s). The number of accidental counts is then 0.75. Thus. a large fraction of triggers will contain accidental signals. If the efficiencies are increased, more PMTs are included in the event reconstruction and it is statistically improved. However, this improvement is small whereas including accidental signals can lead to inaccurate results. Therefore, the manufacturing change resulting in reduction in noise rate at the expense of a small drop in efficiency is beneficial. The ensemble of Hamamatsu RI408 PMTs are of high quality and meet the requirements of the SNO collabora-
tion. They represent a significant improvement ous large-area PMTs.
over previ-
Acknowledgement The authors would like to thank Alex Tekenos-Levy, Richard Payne, John Hazell. Peter Stokes, Alvin Bell, and Phillip Hart for help during the construction of the testing systems and during the PMT testing. We would like to thank our collaboration member, R. Van Berg of the University of Pennsylvania Department of Physics, for work on the PMT base for use in SNO. The photomultiplier tubes have been provided for SNO by Los Alamos National Laboratory under its contract with the United States Department of Energy.
C.J. Jillings et al. I Nurl. Instr. url
This work was supported by the Natural Sciences and Engineering Research Council of Canada.
References [I] R. Davis et al., Phys. Rev. Len. 20 (1968) 1205. [2] Kamiokande Collaboration. KS. Hirata et al.. Phys. Rev. D 44 (1991) 241. [3J J.N. Abdurasitov et al., Phys. Lett. B 328 (1994) 234. [4] GALLEX Collaboration, P Anselmann et al.. Phys. Lett. B 327 f 1994) 377. [S] N. Hata. S. Bludman and P. Langacker, Phys. Rev. D 49(7) (1994) 3622.
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[6] G.T. Ewan et al.. Sudbury Neutrino Observatory Proposal, SNO-1987-12 (1987). [7] I? Besson et al., Nucl. Ins&. and Me& A 344 (1994) 435. [8] C.R. Wuest et al.. Nucf. Instr. and Meth. A 239 ( 1985) 467. [9] T. Devlin et al., Nuci. Instr. and Meth. A 268 (1988) 24. [IO] R.W. MacLeod, Evaluation of Large Area Photomultiplier Tubes for the Sudbury Neutrino Observatory, MSc. Thesis, Queen’s University at Kingston ( 19901. [l I] R.J. Ford, Nitrogen/Dye Laser System for the Optical Calibration of SNO. M.Sc. Thesis, Queen’s University at Kingston (I 993). [12] C.J. Jillings, A photomultiplier Tube Evaluation System for the Sudbury Neutrino Observatory, MSc. Thesis, Queen’s University at Kingston (1992).
a
Nuclear Instruments
and Methods in Physics Research A 373 ( 1996) 421-429
NUCLEAR INSTRUMENTS 8 METHODS IN PHVSICS RESEARCH SectIon A
ELSEVIER
The photomultiplier tube testing facility for the Sudbury Neutrino Observatory C.J. Jillings*,
R.J. Ford, A.L. Hallin, P.J. Harvey,’ R.W. MacLeod,2 H.B. Mak, P. Skensved, R.L. Stevenson Department
ofPhysics, Queen’s Universiry at Kingston, Kingston, Canada K7L 3N6 Received
3 January
1996
Abstract A facility to test the photomultiplier tubes (PMTs) for the Sudbury Neutrino Observatory (SNO) was constructed at Queen’s University. The facility measured the noise rate, relative efficiency, and charge and time spectra of single photoelectron anode signals for 44 PMTs daily. The system is described, and detailed results of the testing for two particular PMTs are presented as well as statistics for 9829 Hamamatsu RI408 PMTs accepted for use in SNO.
1. Introduction 1.1. The Sudbury Neutrino Observatory The flux of electron neutrinos from the sun has been measured in four separate experiments and is lower than the value predicted by solar models by a factor of 1.5 to 2 [l-4]. Our understanding of the nuclear fusion processes in the sun and the standard model of electro-weak interactions is not reconcilable with the results of these experiments [5]. To better understand this discrepancy, the Sudbury Neutrino Observatory (SNO) [6], a 100 t heavywater Cherenkov detector, is being constructed underground at the 6800-foot level at Into’s Creighton mine near Sudbury, Canada. SNO will be able to measure neutrino flavor oscillations by measuring the flux of all types of neutrinos from the sun and the flux and energy spectrum of the electron neutrinos. The energy spectrum and total flux of electron neutrinos will be detected using the charged-current interaction v, +d+p+p+e-
- 1.44MeV.
Because of final state effects between the two recoil protons, the recoil electron energy distribution has a sharp maximum approximately 1.44 MeV below the incident neutrino energy. The electron emits Cherenkov light in proportion to its energy, and PMTs detect this light. The * Corresponding author. E-mail [email protected].
’ Currently at Muse Research Inc., Kingston, Canada, 127. ‘Currently at Lawrence Berkeley National Laboratory, keley, USA 94720.
K7K Ber-
0168-9002/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SO168-9002(96)00067-8
detector geometry is such that two Cherenkov photons will seldom strike the same PMT in any one event. Therefore, the number of PMTs hit by Cherenkov light is related approximately linearly to the incident neutrino energy. SNO will measure the total flux of all neutrino types using the neutral-current disintegration of the deuteron
The neutron will be detected in ‘He proportional counters or will be captured by a chlorine nucleus, from salt dissolved in the heavy water, resulting in a cascade of gamma rays with a total energy of 8.8 MeV. These gamma rays will Compton scatter off electrons causing the electrons to emit Cherenkov light which will be detected by the PMTs. I.2 PMT characteristics The number of PMTs detecting Cherenkov light in an event depends on several factors: the total amount of Cherenkov light, the attenuation in the heavy water and other detector components, the geometrical coverage of the PMTS, and the efficiency of the PMTs. In a typical event, 30 to 150 (of 9456) PMTs will detect Cherenkov light and all anode signals will result from a single photoelectron (spe). The three most important PMT parameters are the noise rate (the rate of anode signals when there is no illumination of the PMT photocathode). the efficiency (the probability that a photon incident on the PMT produces an anode signal above threshold), and the transit time spread (the variation in the time between the emission of the photoelectron at the photocathode and the arrival of the signal at the anode). As any of these parameters is
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It is defined as the ratio of the height of the spe charge spectrum at its maximum value to its lowest value at lower charge than the maximum value. A high peak-to-valley ratio (good charge resolution) allows clear separation of the pedestal from the spe peak resulting in lower noise with higher efficiency. Pre-pulsing and after-pulsing rates are the sizes of secondary peaks in the time spectrum relative to the size of the main peak. The rate of increase of noise rate with anode-to-photocathode voltage difference near the operating voltage is important because a small drift in operating voltage must not cause a large increase in noise rate. To ensure that the PMTs used are of sufficiently high quality each PMT was tested for all these parameters.
2. The PMT testing systems 2.1. The single-PMT
Fig. 1, A schematic of the Hamamatsu R1408 PMT design. The 9 dynodes are shown as solid horizontal lines, The focusing grid is shown as the dashed lines above the dynodes. The mask above the focusing grid is kept at the same voltage as the first dynode and allows only those photoelectrons directed toward the middle of the dynodes to pass. All dimensions are in centimeters.
the energy resolution and event reconstruction are improved. The PMTs in SNO will be operated at a gain (the integrated anode charge resulting from a single photoelectron) of 10’. The gain is a function of the anode-tophotocathode voltage difference; the voltage yielding a gain of 10’ is referred to in this paper as the operating voltage. Several other PMT parameters are of concern. The charge resolution is measured by the peak-to-valley ratio of the spe charge spectrum which is the depth of the valley between the pedestal and the peak of the charge spectrum.
improved,
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Fig. 2. The base used for SNO PMTs. All unmarked resistors are 1 mR. The values of the other components are Rl = 510 a, R2=240n, R3=5lfL, R4=lOOkR, R5=75fL, Cl=lnF, and C2 = 4.7 nF. The signal and high voltage are carried on the same cable.
testing system
Many collaborations have developed systems to test the PMTs used in their experiments [7-91. For SNO, two testing systems were constructed at Queen’s University. The first system tested PMTs individually in a small light-tight copper box between two magnetic field compensation coils [IO]. The net magnetic field was uniform over the volume occupied by the PMT and its strength could be varied from -48.6 ~_LT to +48.6 p,T, the strength of the earth’s natural field in Sudbury, Canada. The PMTs were covered with a mask centered about the axis of the test PMT such that only a circle of diameter 19.0 cm was exposed to the light source. The Cherenkov light source, a “‘Sr beta source encapsulated in UV-transmitting plastic, was mounted on a 1 cm Hamamatsu RI635 monitor PMT. The signals for both the monitor PMT and the test PMT were amplified and passed to constant-fraction discriminators. The discriminator outputs were passed to a coincidence unit whose output was the gate for a chargeto-digital converter (QDC) and the start signal for a timeto-digital converter (TDC). Scalars counted the singles rates for the monitor and test PMTs and were used for dead time correction and noise rate measurements. For relative anode efficiency measurements, the test PMT was placed with the Cherenkov source centered on the axis of the PMT at a distance such that the half-angle subtended by the exposed part of the test PMT was 7.0”. The relative anode efficiency was defined as
where C is the number of coincidences corrected for dead time, M is the number of monitor singles, and 9 is a scaling factor for the monitor PMT. The factor, R depended on the geometry of the source and monitor PMT and on the efficiency of the monitor PMT; it is, therefore,
C.J. Jillings et al. / Nucl. Instr. and Meth. in Phys. Res. A 373 (1996) 4,‘1-429 independent of the test PMT. It was measured regularly by retesting several standard test PMTs and was found to increase by approximately 10% over three years. This increase was due to radiation damage in the plastic which caused the plastic to become less transparent resulting in fewer coincidences for a given test PMT.
2.2. The automated test system The second system is an automated facility capable of testing 44 PMTs a day. This system, shown schematically in Fig. 3, measured the noise rate and gain as a function of voltage and measured the spe time and charge spectra at the operating voltage. From the spectra the relative anode efficiency, the spe timing resolution, the peak-to-valley ratio of the charge spectrum, and the pre-pulsing and after-pulsing rates were derived. Constant-fraction discriminators (CFDs) were used to reduce the timing walk due to variable charge in the anode signal. The gain of the amplifier for the PMTs was 20. The PMTs were tested in one of two dark rooms of dimension 3 m X 3 m X 3 m. Each dark room has 2 magnetic field compensation coils which reduced the field at the location of the PMTs to less than ten percent of the earth’s field. A 1 m high aluminum frame table with an electrically insulating top was centered in each room. Each table holds 48 PMTs. Black curtains, hung from the ceiling, divide the table into 4 sections each having 12 holes on a 3 by 4 grid. Twelve PMTs (1 standard and 1 I test PMTs) are mounted in each section for testing. In each section there is a diffuse light source centered 2 m above the PMTs. Using four light sources reduced the variation
PMT
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Fig. 3. A block diagram for the large test system. The amplifiers for the PMTs had a gain of 20.
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at each test PMT due to solid angle
3. The light source The light source for the second testing system was a wavelength-shifted pulsed nitrogen laser. The nitrogen laser produces several sharp lines all near 337.1 nm. This ultraviolet light was directed to a 5 cc quartz glass dye cuvette containing a solution of BBQ (manufactured by the Exciton Chemical Company Inc. of Dayton, Ohio, USA). This dye produces a broad (- 10 nm) band of light centered at 386 nm where the quantum efficiency of the PMT is nearly maximum. The relaxation time of the dye was not important because the all the populated states are deexcited on the first pass of the light through the dye. Thus the width of dye laser light was that of the original laser. The beam of 386 nm light was passed through a diverging lens, attenuated, and directed to a bundle of 8 graded index optical fibres (Belden 2272) each of which carried light to a dark room section (2 rooms, 4 sections each) where it was diffused above the PMTs. The light was attenuated by two neutral density filters. The first filter reflected over 99% of the beam of the beam to an MRDS 10 photodiode producing a timing trigger. The second attenuator had eight possible settings: six neutral density filters (with transmissions of 50%, 40%. 30%. 20%, IO%, and l%), a solid aluminum blank, or no filter. These were placed a wheel which was turned by a computer-controlled DC motor. The spe testing was performed with a 1% transmission filter. The blank was used for the noise rate tests and the other filters were available to study the multiple-photoelectron response of the PMT. Two different models of laser were used. The first sixty percent of the PMTs were tested with an LN120C nitrogen laser manufactured by Laser Photonics of Orlando, Florida. This laser used a Blumlein discharge circuit and was operated for 18 hours daily and pulsed at 15 Hz. The time width of the output pulse was approximately 300 ns. The amount of maintenance required depended on the alignment of the laser head. If the head was not exactly aligned, the head required cleaning daily; however, when the laser head was precisely aligned, the laser ran for a week without maintenance. The dye required changing bi-weekly because of degradation due to the incident light. The pulse to pulse stability was -10% during the testing cycle. For the remainder of the PMT testing an LN203C nitrogen laser, also manufactured by Laser Photonics, which features a thyratron triggered discharge, was used. The width of the output pulse was approximately 600 ns. The pulse intensity was more stable (~4%). and the unit ran for over 2000 hours at 20 Hz with bi-monthly maintenance. The dye was changed weekly with this laser because of its higher intensity. No steps were taken to prevent multiple modes from
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being created in the optical fibre. This is a potential concern because multiple modes leads to time dispersion. Therefore, the time dispersion of light in the laser and optical-fibre system was measured to ensure that it was much less than the transit time spread of the PMTs. Light from the laser was passed through a neutral density filter and 35 m of optical fibre to a I cm PMT. The PMT was placed far enough from the end of the fibre that the PMT charge spectrum was dominated by spe events. Using a dye that output light with a wavelength of 365 nm, the time width of the time spectrum was 0.46?0.1 ns. For 386nm light, the width was 0.2920.05 ns; for 500 nm light, 0.26’_0.05 ns. Time spectra for 500 nm and 386 nm light are shown in Fig. 4. The shift in peak position between the 500 and 386 nm light is 1.3 ns corresponding to frequency-dependent change in refractive index of the optical fibres [I 11. To measure PMT parameters, an ideal light source would have no time dispersion and produce light of equal intensity over all the test PMTs. Practically, the time dispersion in the diffuser must be much less than the transit time spread of the test PMTs and the angular distribution of diffused light should be approximately constant over the area of the test PMTs. To spread the light from the fibre optic cable evenly over the PMTs, a diffuser was placed on the ceiling of each section of each dark room. The diffusers were hollow quartz glass cylinders, 4 cm in diameter and 2.5 cm in height. The diffusers were placed with one of the flat sides (the “bottom”) parallel to the surface of the table. The sides and tops of the cylinders were aluminized on the outside and the bottoms were transparent. Light entered the
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barrel of each diffuser through an optical fibre which was inserted horizontally. The cylinders were tilled with a diffusive liquid (Ludox TM, manufactured by DuPont) which Rayleigh scattered the light. Ludox TM is a suspension in water of approximately silica particles with an average diameter of 22 nm. Because of the size and chemical composition, Ludox TM can be modeled as a collection of randomly placed small spherical dielectric scatterers. To help design the diffusers, computer simulations were performed. The simulation assumed Rayleigh scattering in an otherwise perfectly homogeneous medium with a refractive index of 1.33, incoherent light entering the diffuser, and reflection and refraction according to Snell’s law. The attenuation constant of Ludox was measured to be 0.037+0.01 mm-‘. and used as an input to the simulation. To first order, the diameter of a mirrored cylindrical barrel is unimportant because Rayleigh scattering is forward-backward symmetric. The height of the cylinder should be as small as possible to minimize the time spread from multiple scatters. The distribution of time passed by photons in the diffuser is shown in Fig. 5. To compare the simulation with a real diffuser, the spe time spectrum of dye-laser light directly from an optical fibre (measured with a 1 cm Hamamatsu R163.5 PMT) was convolved with the spectrum from the simulation. Then the time spectrum of dye-laser light through an optical fibre and a diffuser was measured with the same 1 cm PMT. The trigger for the timing electronics was generated by the photodiode used in the testing system. Both spectra were normalized to an integrated number of counts of unity. The spectra agree and are shown in Fig. 6. According to the simulation, the dominant effect on light intensity at the various testing sites was found to be the solid angle subtended by the test PMT at each site. With corrections for the effect of solid angle, the variation of intensity at each site was less than 3%. Simulated
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Fig. 4. The spe timing resolution for 386 and 500 nm wavelengths of light through 35 m of optical fibre.
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Fig. 6. The convolution of the diffuser simulation and the time spectrum of light from the fibre compared with the time spectrum of light from the diffuser. The diffuser error bars are Z I CT and the dotted lines represent 1~ above and below the convolved spectrum.
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During testing, the relative efficiencies were determined from the formula
of the test PMTs
4 6 Testing
8 Site
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Fig. 7. The ratio of photons detected at each testing site to the number of laser pulses and the fitted value for the ratio for the calibration of dark room 2, section I. The letters are codes showing which the locations of the 12 PMTs during testing.
(2) where N is the number of counts and 4; is the intensity of the light at the PMTs. The value of 4 at each site was found from a calibration. Each section was calibrated independently [ 121. To calibrate a section, four runs were conducted with light levels low enough for the counting to be dominated by spe events (less than one count per PMT per IO laser pulses). The same 12 PMTs (labeled A to L) were used in the four runs. Between calibration runs, II PMTs were moved within the section while one PMT stayed at the same site to allow a direct comparison between runs. The number of counts for run I at the ith site was N,., = S,sr&x
(3)
The factor, S,. is a scaling factor for the run and accounts for the unknown intensity of the light source and the number of pulses of the light source for the run. The factor, 9,. is the relative intensity at site i integrated over the exposed area of the PMT. and &Xis proportional to the efficiency of the PMT at site i for the Zth run. The number of counts for each run for each PMT was measured. There were 28 parameters: the 12 efficiences, the 12 relative intensities, and 4 scaling factors for each run. However, only 26 parameters were independent. By definition, the site for the standard tube was given a relative intensity of unity. Also, the scaling factors for the runs and the relative efficiencies were not independent-the ef-
ficiencies could be doubled if the scaling factors were halved. Therefore. a judicious choice of the scaling factor for run I would have resulted in the quantity .zX to be exactly the relative efficiency, lx_ To determine the values of the free parameters, the Marquardt method for non-linear least-squares fitting was performed. The values of the intensities derived from the fit for dark room 2, section 1 varied by 12%. The number of photons detected by each PMT. divided by the number of laser pulses. for each run of the calibration for dark room 2 section 1 are shown in Fig. 7 along with the corresponding value from the fit. The reduced chi square for the fit to dark room 2 section 1 was 1.5. The relative efficiencies were compared to the efficiencies derived from the small test system. See Table I.
5. Analysis of spectra The spectra were analysed according to the following definitions. The gain was measured considering pulses whose integrated anode charge was between 0.25 and 2.5 times the mean anode charge for a gain of IO’. The anode efficiency, timing resolution, and noise rate calculation included only those anode pulses whose integrated charge was greater than 0.25 times the mean charge. To calculate the anode efficiency, the time spectra were integrated between + 1 FWHM to yield a total number of detected
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Table I The relative efficiencies of the PMTs used in the calibration of dark room 2 section I asmeasured by the small testing system and as derived from the fit PMT label
Small system test result +0.02
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A B C
I .26 I .22
1.24 1.21 1.26 1.16 1.23 1.20 1.18 1.19 I .26 1.24 I .09 1.11
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These cuts ensured that the anode pulses satisfied the low charge cut and that anode pulses far out of time were not considered in the calculations. The spe timing resolution is defined as the one standard deviation (sigma) width of the time spectrum of spe signals. To calculate the timing resolution, only signals arriving within + 1.5 times the FWHM of the mean of the timing spectra were considered. To ensure that the noise rate is not overly sensitive to voltage, the noise rate (NR) was measured at gains of 0.3 X IO’ and 3.0 X 10’ as well as 1.0 X IO’. If the ratio NR(3 X lO’)/NR(I
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Fig. 8. The effect of magnetic fields on PMT relative efficiency. Parallel refers to the direction of the vanes of the first dynode and perpendicular refers to the direction at right angles to the vanes of the first dynode. Both of these directions are perpendicular to the z direction of Fig. 1. The earth’s natural field is 48.6 )LT at Queen’s University. The efficiency has been normalized to unity at zero field.
the PMT axis [lo]. The directions are defined by the direction of the vanes (slats) of the first dynode. In the SNO detector the earth’s magnetic field points approximately 15” off the vertical axis. Because the PMT efficiency is reduced by less than 3% at magnetic field intensities of 40% of the earth’s natural field, it was decided to cancel only the vertical component of the magnetic field in the SNO detector with field-compensation coils.
X 10’) (4)
X IO’)
7. Single photoelectron was less than 1.3, the PMT was accepted. If the ratio was greater than 1.3, the noise rate verses voltage data were inspected by eye to determine if a small increase in voltage would result in a large increase in noise rate. The example PMTs below illustrate this point.
6. Sensitivity of the Hamamatsu magnetic fields
R1408 PMT to
The effect of the earth’s magnetic field on the relative efficiency of a PMT is important. This effect is primarily a function of the electron’s trajectory from the photocathode to the first dynode and of the electron trajectories between dynodes. Therefore, it will vary little between PMTs of a given design. Fields perpendicular to the axis of the PMT (the z-axis of Fig. I ) were found to have a much greater affect than those parallel to the axis. The response to the magnetic field was not axially symmetric. Fig. 8 shows the effect of a magnetic field in two directions perpendicular to
results for two PMTs
The testing results from two PMTs are presented as examples. The charge distribution for a PMT (labeled R) accepted for use in the SNO detector is shown in Fig. 9. The peak at 0 pC is the pedestal. There is a valley at 1OpC which separates the spe peak from the pedestal. The measured gain was 1.01 X IO’. The time spectrum (Fig. 9) shows low pre-pulsing and after-pulsing rates. The timing resolution was 1.57 ns. The relationship between gain and anode to photocathode voltage difference is represented approximately by an exponential function in the region of interest. Also, as the voltage is increased the noise rate increases smoothly and slowly. PMT S (Fig. 10) is a second example of an accepted PMT. The peak-to-valley ratio is excellent and the relative efficiency and spe timing resolution (1.62 ns) are acceptable. However, the noise-rate ratio (Eq. (4)) was higher than the criterion allowed. This is a shortcoming of the criterion. not the PMT. Inspection of the noise-rate-versusvoltage curve shows that for up to 150 V above operating voltage the noise increases smoothly and slowly with
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voltage. PMT S is an example of why the noise-rate ratio was used only as a warning flag to signal potentially unacceptable PMTs. Many PMTs, including this one, whose ratio was high, were acceptable. In particular. PMTs with low noise rates at a gain of 10’ and flat plateau curves between 0.3 X IO’ gain and 10’ gain could fail the noiserate ratio test even if they had a modest increase in noise rate between a gain of IO’ and 3 X IO’.
3ooirf
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Fig. 9. Data from the test of PMT R. The charge plotted includes a factor of 20 from the amplifiers shown in Fig. 3. The vertical line in the gain and noise versus voltage plot indicates the operating voltage of 1891 V.
testing results for PMTs
Histograms for the timing resolution, relative efficiency, noise rate, and operating voltage, are presented for 9829 PMTs accepted for use in SNO (see Fig. 1I). The mean RMS timing resolution is found to be 1.5 ns with an RMS deviation of 0.08 ns. Given constraints imposed on Hamamatsu (such as dynode structure), 1.7ns was the best upper limit in transit time spread that could be provided at the time of production. The mean relative efficiency is I. IO with an RMS deviation of 0.09. The mean noise rate is 2300 counts a second. Approximately half way through the production Hamamatsu changed manufacture technique resulting in a slight reduction in efficiency (but still above our specification) but reduced noise rate. The cooler temperature in the detector (approximately 10 C instead of 20 C during the testing) should reduce mean noise rate to approximately 1000 counts a second. The mean operating voltage is 1875 V. Density plots of the timing resolution and operating high voltage and of relative efficiency and noise are presented in Fig. 1I. The timing resolution is expected to decrease with increasing high voltage because the photoelectron flight time is reduced when the voltage difference between the photocathode and the first dynode is increased. PMTs with a noise rate below 1500 counts a second tend to have lower relative efficiency.
9. Conclusions
1500 Voltage
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Fig. IO. Data from the test of PMT S. The charge plotted includes a factor of 20 from the amplifiers shown in Fig. 3. The vertical line in the gain and noise versus operating voltage of 1732 V.
voltage
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As far as PMTs are concerned, the three most important detector parameters are the energy resolution, the spatial resolution, and background rates. The background rates were addressed by choosing glass for the PMT bulbs which contain as little thorium and uranium as possible (less than 4 X IO-‘g/g of each). In the detector, reflectors will be added to the PMTs; with these, 65% of the solid angle is covered by PMTs. Given the efficiency of the PMTs we expect the energy resolution to be approximately 14% at 10 MeV Accidental noise pulses act as a “zero offset” in the energy calibration. If the mean noise rate is 1000 counts per second, the offset is I PMT. For spatial reconstruction, the transit time spread. noise rate. and efficiency all play a role. A transit time spread of
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Fig. 1I. The testing results for 9829 PMTs accepted for use in SNO.
I .5 ns corresponds to a “distance spread” in heavy water of 23 cm (= I S/(cln)). The effect of noise rate on reconstruction accuracy can be estimated by considering the number of accidental pulses in a time window equal corresponding to the maximum distance between two PMTs in the detector ( = 17 m corresponding to 75 s). The number of accidental counts is then 0.75. Thus. a large fraction of triggers will contain accidental signals. If the efficiencies are increased, more PMTs are included in the event reconstruction and it is statistically improved. However, this improvement is small whereas including accidental signals can lead to inaccurate results. Therefore, the manufacturing change resulting in reduction in noise rate at the expense of a small drop in efficiency is beneficial. The ensemble of Hamamatsu RI408 PMTs are of high quality and meet the requirements of the SNO collabora-
tion. They represent a significant improvement ous large-area PMTs.
over previ-
Acknowledgement The authors would like to thank Alex Tekenos-Levy, Richard Payne, John Hazell. Peter Stokes, Alvin Bell, and Phillip Hart for help during the construction of the testing systems and during the PMT testing. We would like to thank our collaboration member, R. Van Berg of the University of Pennsylvania Department of Physics, for work on the PMT base for use in SNO. The photomultiplier tubes have been provided for SNO by Los Alamos National Laboratory under its contract with the United States Department of Energy.
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This work was supported by the Natural Sciences and Engineering Research Council of Canada.
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[6] G.T. Ewan et al.. Sudbury Neutrino Observatory Proposal, SNO-1987-12 (1987). [7] I? Besson et al., Nucl. Ins&. and Me& A 344 (1994) 435. [8] C.R. Wuest et al.. Nucf. Instr. and Meth. A 239 ( 1985) 467. [9] T. Devlin et al., Nuci. Instr. and Meth. A 268 (1988) 24. [IO] R.W. MacLeod, Evaluation of Large Area Photomultiplier Tubes for the Sudbury Neutrino Observatory, MSc. Thesis, Queen’s University at Kingston ( 19901. [l I] R.J. Ford, Nitrogen/Dye Laser System for the Optical Calibration of SNO. M.Sc. Thesis, Queen’s University at Kingston (I 993). [12] C.J. Jillings, A photomultiplier Tube Evaluation System for the Sudbury Neutrino Observatory, MSc. Thesis, Queen’s University at Kingston (1992).