New proximity focusing photomultiplier resistant to high magnetic field

Nuclear Instruments and Methods 216 (1983) 439-445 North-Holland Publishing Company

439

N E W P R O X I M I T Y F O C U S I N G P H O T O M U L T I P L I E R R E S I S T A N T T O HIGH M A G N E T I C FIELD S. O R I T O Department of Ph)'sws, Unit:erstty of Tok.vo, 7- 3-1 Itongo Bunk.vo- ku, Tokyo 113. Japan T. K O B A Y A S H I LI('EPP. Faculty of Sctence, University of Tokyo. 7-3-1 Hongo Bunk.vo-ku, Tokyo 113, Japan K. S U Z U K I . M. I T O a n d A. S A W A K I ttamamatsu TV Co., l.td, 1126-1 h'hino-cho Harnamatsu, 4.t5, Japan Received 18 October 1982 and in revised form I1 February 1983

We have developed a new type of photomuhiplier which can be operated in magnetic field of up to order of 100 G. The photomuhiplier utilizes mesh dynodes and has a short distance betv.een the photocathode and the first dynode. Test results on such photomuhipliers with 3" bialkali photocathode show promising features of this new type of photomuhiplier.

1. Introduction P h o t o m u h i p l i e r s are widely used in the field of nuclear and high energy physics for scintillation counters and C h e r e n k o v counters etc. for their capability of resolving energy, position [1] or timing of the incident light. In some cases these counters are placed near to a magnet, where the stray field can be rather high. Since the usual p h o t o m u h i p l i e r s are operative in a magnetic field of only up to a few gauss [2]. long light guides or extensive shielding become necessary in such cases, which either deteriorate the energy resolution or are expensive. A n example is the large cylindrical colliding b e a m experiments which are operating or being planned. In such experiments m a n y t h o u s a n d s of shower counters are usually placed outside a solenoidal coil where a stray magnetic field of up to 100 G may be present. In this paper we report on test results on a new type of photomultiplier which can be operated in magnetic field of up to 100 G.

Fig. 1. Photograph of the photomultiplier. (.76ram Photocathode

2. T h e design of the photomultiplier The photomultiplier was designed considering the need for it to be operative in a high magnetic field and at the same time for it to have good linearity [7]. A p h o t o g r a p h and drawing of the p h o t o m u h i p l i e r are s h o w n in figs. I and 2. respectively. The distance between the p h o t o c a t h o d e and the first d y n o d e is approximately 7 m m and each d y n o d e is close to one 0167-5087/g3/00(.¥,0-0000/$03.00 .," 1983 N o r t h - H o l l a n d

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another (1.3 mm) to reduce the influence of the magnetic field. The trajectories of the photoelectrons from the photocathode to the first dynode are straight without any focusing. A 3" borosilicate envelope glass and a bialkali photocathode were chosen to match the spectra of the light from the scintillators or lead glass shower counters. Nine stages of SbCs mesh dynodes made by etching were used for the test photomuhipliers, ttowever the structure of the photomuhiplier is such that the number of dynodes could be easily increased. The height of the photomuhipliers is small for its diameter. This is advantageous in some points, for example, the apparatus is not bulky even if many thousands of photomuhipliers are used.

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The gain of a photomuhiplier is defined by the ratio of the anode output current to the emitted photocathode current. Fig. 5 shows typical gain curve as a function of the applied voltage. The gain can be easily adjusted by orders of magnitude by changing the number of dynode stages. Fig. 5 also shows typical dark current as a function of the applied voltage.

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In spite of its large diameter of three inches, cxcellent time properties are expected for this photomultiplier since the trajectories of the photoelectrons and the secondary electrons are short and parallel to each other• The time properties were measured by using light pulses from a laser diode at 800 nm with a time spread of less than 50 ps fwhm. The output waveform is shown in fig. 6. The transit time of 15 ns and the rise time of 5.3 ns were obtained at an applied voltage of 1000 V. The transit time spread (TTS) [3] was measured by using light pulses from fast LED at 550 nm with a time spread less than 50 ps fwhm. To simulate the severest

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In conventional photomultipliers, the electrons emitted from the photocathode and dynodes are greatly influenced by a magnetic field, and a 50% reduction of overall gain occurs usually at a few gauss. This is due to the long extensive focusing from the photocathode to the first dynode of small aperture and is also due to the large distance between each electrode. In the new photomultiplier described here, the photoelectrons follow short straight trajectories to the first dynode and the distance between each dynode is short, thus greatly improving the resistance to the magnetic

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field. Fig. 10 shows the gain dependence of the photomultiplier on the magnetic field at applied voltage of 1000 V. The photomultiplier can be operated in an axial magnetic field of up to 200 G and a transverse field of up to 60 G. The applied voltage of 1000 V was optimized for energy resolution and increasing the applied voltage improved the resistance to the magnetic field. For some applications, it is rather easy to use magnetic shielding up to the face of photocathode without introducing light guides. This improves the resistance to the transverse magnetic field as shown in fig. I1 when

3.5. Energy resolution

The energy resolution was measured using Cs 13v and an NaI(T1) crystal [4] of 3" diameter and 3" height. The optical contact was performed with silicone grease [5]. As shown in fig. 12, the energy resolution of 7.7% fwhm was observed to be compared with 7.0% for a typical Hamamatsu R 594, an equivalent 3" photomultiplier. Fig. 13 shows the variation of the energy resolution in

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the magnetic field. Combining fig. 10 and fig. 13, we can conclude that the loss of the gain in the magnetic field is mainly due to losses of photoelectrons before reaching the first dynode. In order to improve the energy resolution, a "hybrid" photomultiplier which utilizes thin Venetian blind dynodes for the first and second stages was produced. These Venetian blind dynodes are expected to have better collection efficiency. The energy resolution of 7.2% was obtained with the same Nal (TI) crystal and Cs ~37 as shown in fig. 14. The magnetic field dependence of the gain of this hybrid photomultiplier is shown in fig. 15, which is inferior to that of the photomultiplier with all mesh dynodes but is still much better than usual photomuhipliers.

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3.6. Linearity High current pulses suppress the linearity in conventional photomuhipliers due to space charge effects in the latter dynode stages. The linearity can be improved to some degree by the use of a properly designed voltage divider [2], but a more direct way to solve this is to reduce the density of the charge between dynodes. The new photomuhiplier has an advantageous structure from this point of view. As shown in fig. 16, the linearity of the photomuhiplier extends to 10 4 pC, which is approximately 10 times better than that of conventional 3" photomultipliers. This linearity can be further improved by using a properly designed voltage divider.

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Table 1 Characteristics of the photomultiplier compared with a typical Hamamatsu R594

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3. 7. Stability

A " c o u n t - r a t e stability" [6] test was performed by' using 137Cs and an NaI(TI) crystal by the following procedure. The photomultiplier was first operated at 10 kcps, and then the photopeak c o u n t i n g rate was decreased to 1 kcps. The photopeak position was measured and c o m p a r e d with the m e a s u r e m e n t made at a c o u n t i n g rate of 10 kcps. The count-rate stability is expressed as the percentage gain shift for the count-rate change. The count-rate stability of the photomultiplier described here was found to be 0.3~.

Diameter (in) Phot~x:athode material Quantum efficiency (%) Gain Rise time (ns) Transit time (ns) Transit time spread (ns) Resistivity against axial magnetic field 50% deviation (G) Energy resolution using 1~7Cs and NaI(TI) (c~) Pulse linearity 5% deviation (pC) Stability (%)

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4. Conclusion We have developed a new type of photomultiplier with small photocathode-to-first d y n o d e distance and specially designed mesh dynodes, which is operative in magnetic field of up to an order of 100 G. Excellent time properties (15 ns transit time, 5.3 ns rise time and 0.737 transit time spread), a high linearity (up to 104

S. Orito et al. / New proxtmt(v focusing photomultiplier pC) and reasonable energy resolution are observed. In table 1, we summarize the characteristics of the photomultiplier c o m p a r e d with a typical H a m a m a t s u R594. We wish to thank colleagues at Hamamatsu TV and the University of Tokyo for their support and encouragement. Sincere thanks are also due to Professor Koshiba for support and valuable disscusions.

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References [11 K. Kuroda, Nucl. Instr. and Meth. 196 (1982) 187. [2] Hamamatsu Technical Manual, Res-0790. [31 B. Leskovar and C.C. Lo, Nucl. Instr. and Meth. 123 (1975) 145. [4] 12A12, "Fhe Harshaw Chemical Company, 6801 Cochran Road, ~'~lon, Ohio 44139, USA. [5] SH200, Toray Silicone Co., LTD. 2-8 Nihonbashi Chuoh-ku Tokyo, Japan. [6] An American National Standard, IEEE Standard Procedures for Photomuhipliers for Scintillation Counting and Glossary for Scintillation Counting Field, IEEE Std 3981972. [7] K. Kuroda, D. Sillou and F. Takeutchi, Rev. Sci. Instr. 52 (3) (1981) 337.