Author: popov

Performance studies of Hamamatsu R9800 photomultiplier tube with a new active base designed for use in the Hall D Broad-band Tagger Hodoscope
Vladimir Popov, Alexander Somov, Franz Klein, Natalie Walford, Nathan Sparks

ABSTRACT
Abstract– This paper presents the results of a study of a high rate photomultiplier and active base assembly designed for use in scintillator counters of the Hall-D Jefferson Lab Broad-band fixed-array Tagging Hodoscope. In its standard configuration the fixed-array hodoscope consists of 218 plastic scintillator counters made with Hamamatsu R9800 one inch in diameter photomultiplier tube. All tubes are equipped with designed at JLab new active base. The base comprised of a high voltage divider integrated with amplifiers powered by current flowing through the divider (U.S. Patent No. 6,791,269). This original design allows limiting the PMT anode current to 15-20µA at a 4MHz count rate, and allows tube operation at lower high voltage bias voltage while retaining a desired overall gain and dynamic range of output signals. Over two hundred PMT and base assemblies were manufactured and tested and studied with a pulse laser source and scintillator. The results of testing the R9800 photomultiplier in a new active base are presented and discussed.

SYSTEM TESTING
All produced R9800+active base assemblies were tested with Hamamatsu PLP-10-040 laser fast light source (60 ps FWHM typ.) and light wavelength 400 nm[ ]. The light source has a stabilized light pulse intensity and adjustable repetition frequency. The photomultiplier Single electron spectrum was used for pulse intensity calibration during test measurements. All detector assemblies were tested at 200 to 400 pe equivalent light pulses, high voltage bias from 850 to 1100 V and repetition frequencies up to 10 MHz. The output pulses waveform is different for PMT with an without active base, see pictures below.
The active base with attached phototube works as a Low Pass Filter (LPF) with cut off frequency ≈250MHz. From analysis of all 200+ tested with PLP-10 light pulser, active base assembly has an output pulse leading edge tf=(1.3±0.1)ns and reduced electronic noise (see. Fig 1 and 2). With plastic scintillator the output pulse leading time extension is low and insignificant. Overall, the active base has a little contribution to system timing, but it improves S/N and eliminates a small oscillation on the bare tube anode output (see. Fig. 1).

INTRODUCTION
GlueX is one of the new experiments being developed at Jefferson Lab, where its main physics goal is to search for mesons with gluonic excitations using beams of linearly polarized photons. The photon beam will be produced in the coherent bremsstrahlung process by 12 GeV electrons incident on a thin radiator. The energy of each photon in the energy range of interest can be determined by measuring the momentum of the recoil ("tagged") electron. Tagged electrons are deflected in a 6 m long dipole magnet and are detected in the tagger broad-band hodoscope. The hodoscope consists of 218 plastic scintillator counters positioned behind the focal plane of the tagger dipole magnet. The position of each counter corresponds to the specific momentum of the tagged electron. In order to equalize the counter rate, counter widths vary between 3 mm and 2.1 cm. The light produced by an electron in the scintillator will be detected by the Hamamatsu R9800 photo multiplier tube (PMT). The amount of light seen by the PMT corresponds to be about 300 photo electrons. The counters will be operated at a relatively high rate of about 2 - 4 MHz. Continuous operation of tubes at high rate may result in the damage to the last dynode by electron bombardment and lead to a subsequent degradation of the tube performance. The maximum average anode current of the R9800 PMT is about 100 uA. For the long term operation of PMTs, a smaller current of about 20 – 30 μA is desired. This requires to operate the PMT at a relatively small voltage of about 950 V, which corresponds to a tube gain of about 2·105. In turn, the small gain results in small signal pulse amplitudes, therefore an amplifier is required. The hodoscope counters should also be able to identify the accelerator beam bunch where the tagged electron is produced. This requires the timing resolution of the counter to be better than 250 ps. Though, the PMT has to be connected to the fast amplifier to meet all of the above mentioned requirements.

Fig.1 R9800 without active base waveform. HV=1100V, PLP-10 light pulse

Fig.2 R9800 in active base waveform. HV=950V, PLP-10 light pulse

Fig.3 R9800 without active base waveform. HV=1100V, plastic scintillator

Fig.4 R9800 in active base waveform. HV=950V, plastic scintillator

The PMT specs and the active base concept
The Hamamatsu R9800 is a 1 inch in dia. fast photomultiplier tube with maximum linear output about 30-50 mA and average anode current maximum 0.1 mA. In the Hall D Tagger Hodoscope the PMT will work with 6 mm thick plastic scintillator detecting scattered electrons in the energy range of 0.3 to 9 GeV (for 12 GeV electron beam). At this condition we are expecting about 10000 initial photons per tagged electron and 200 … 400 photoelectrons (pe) in the phototube. At the 4 Mhz event rate the tube will need operate at gain ≤ 5·105 to maintaining output current below 0.1 mA maximum. So, we should to use external amplifier to get sufficient output signal amplitude and keep the tube average anode current within the specs limit.
The problem the photomultiplier operation at high rate has been solved by active base containing amplifier which is powered by the current flowing through the divider. The divider comprises a fast amplifier with gain factor about 8.5 and linear output maximum up to 3 V on 50 Ohm load. The basic concept of the active base with amplifier is shown on the picture below.
The PMT divider is a passive resistor chain with total resistance about 11.5 Meg. It provides ≈800 μA current at 950 V. At this current tube has a linear range of anode average output current about 40 μA, but after amplifier the equivalent output current maximum is over 300 μA which is over triple higher then the bare tube output maximum and sufficient to support 200 … 300 mV the output pulses amplitude at 4 Mhz rate maximum.
All PMT+ active base assembly were studied using the PLP-10, Hamamatsu laser light source which provides about 60 ps width light pulses at adjustable repetition rate. The source intensity was calibrated using a single photon peak to insure the tube testing with certain amount of produced photoelectrons to simulate a close to desired photomultiplier operation condition (200 … 300 pe).
A sample of single electron Pulse Height Distribution histogram obtained with R9800 tube connected to active base and PLP-10 light pulser.

Results
The detector assembly rate capability as a function of amount of light (photoelectrons) at constant HV bias voltage is presented on Fig.5

The active base soldered to the R9800 tube leads

R9800 in active base. Rate test at HV=1000 V and different light pulse power. Note: this is test with PLP-10, very short light pulse, for plastic scintillator the output amplitude is about 1/3, due to wider light pulse.

Vladimir Popov. e-mail: popov@jlab.org, and Alexander Somov, e-mail: somov@jlab.org are with Jefferson Lab.
Natalie Walford, is natalie@jlab.org, Nathan Sparks, nsparks@jlab.org, and Franz Klein, fklein@jlab.org are with The Catholic University of America.