THE LATEST PHOTODETECTORS FOR RADIATION DETECTION
Yuji Yoshizava, Seiji Suzuki, Toshikazu Hakamata HAMAMATSU PHOTONICS K.K. 314-5, Shiaokanzo, Toyooka Village, Ivata-gun Shizaoka Pref., Japan, 438-01
In the recent application of radiation, detection.,, performances required for photodetectors vary in the vide range. Meanwhile, new photodetectors have been continuously developed by HAMAMATStf for these requirements. The performances and the test results of such nev photodetectors are discussed in this paper.
LR5600 SERIES
In applications of radiation measurement and medical instrument, development of more compact and portable equipment has continually progressed. This has led to a strong demand for miniaturization of high sensitive photodetectors such as photomultiplier tubes ( PMTs ), However, it has been difficult to miniaturize conventional PMTs, because they have glass envelopes and sophisticated electrodes structure. Accordingly, PMTs have been chiefly used in high-precision photometric systems, while semiconductor sensors have been used in general purpose, compact
Effective Photocattede 18.0 15.S±0.2 / 10±0.2 / , TO-8 Type A Mela| Channe Metal Case Dynode A \ f^W
Top View Fig.l Cross section and top view of R5600 and portable equipment. To meet the increasing needs for small pbotodetectors with high sensitivity, HAMAMATSU has developed subminiatnre PMTs, R5600 series, using a metal package in place of the traditional glass envelope. These tubes have a size as small as semiconductor sensors without sacrificing high sensi tivity and high speed response offered by conventional PMTs. R5600 series will prove essential in development of portable photometric devices and in downsizing of equipment- Tie remarkable features of R560Q series are world's smallest size, fast time response, ability of low-light-level detection and good immunity to magnetic fields. Each feature is described in the followings.
R5600 is a subminiature PUT that incorporates an 8-stage electron multi plier constructed with stacked thin electrodes ( metal channel dynodes ), into a TO-8 type metal can package of 15mm in diameter and 10mm in height. Development of this metal package and its unique thin electrodes have made the fabrication of this subminiature PMT possible. The electrode structure of the electron multiplier was designed by means of advancedcomputer simulation and electron trajectory analysis. Furthermore, our long experience with micromachining tech nology has achieved close proximity assembly of these thin electrodes. Fig.1 shows a cross section of the R5600. Fig.2 shows a cross section of metal channel dynode and electron trajectories.
PHOTOCATHODE INPUT WINDOW
ELECTRONS METAL CHANNEL J^v, iL iL «E .E,
nrjrjrjrjr, • DDDD
ANODE / Fig.2 Cross section of metal channel dynode and electron trajectories
-2- Fig.3 shows an anode output waveform of R5600. A typical rise time of 650ps and a typical fall time of 1050ps are obtained when the tube is operated at -800V. As can be seen, virtually no ringing occurs in the output waveform. The reason is that the signal output wiring of R5600 is extremely short compared to conventional PtfTs and tiras there is only a slight resonance component. fig.A shows a typical TTS ( Transit Tine Spread ) of R5600. The TTS is the fluctuation in electron transit times repeatedly measured in a single photoelectron state. It is defined as the FWHM ( Full Width at Half Maximum ) of the frequency dis tribution, lith an ultra-short pulsed light, of which wavelength is 410nm and pulse width is 30ps, entering the photocathode and a supply voltage of -800V, R56Q0 exhibits a superior TTS of ZSOps ( typical ). This nigh-speed property obtained by application of -800V, which is relatively low for a PMT, lies to a great extent in an electrode design having extremely short distance between photocathode and anode. This is as short as 7mm. Moreover, the distance between each dynode is designed to be uniform, thus minimizing fluctuations in indivi dual electron transit time.
Fig. 5 shows a typical current amplification v. s. supply voltage characte ristics. An adequate current amplification of 3 x I0e5 ( typical ) is achieved at a rated voltage of -800V. The maximum rating of supply voltage is -1000V. Fig.6 shows a typical pulse height distribution ( PHD ) taken with R5600 opera ted in a single photoelectron state. Because the metal channel dynodes nave an adequate secondary electron emission yield, a distinct peak and valley appear in the PHD. Furthermore, the noise count is considerably suppressed to a very low level.
One of outstanding features of R5600 is good immunity to magnetic fields. In PMT operation, low energy electrons travel through the vacuum from the photo cathode to the anode. A magnetic field, if present near the tube, will have a detrimental effect on the electron trajectories, causing a loss of anode output. In general, tubes having a long path from the photocathode to the first dynode or having a small opening at the first dynode with respect to the photocayhode area, are more influenced by magnetic fields. In contrast, the electron trajec tory in R5600 is less sensitive to magnetic fields, because the metal channel dynodes are proximity-arranged and therefore a high electric field is developed across each dynode. Fig.7 shows typical output variations of R5600 and R647 ( a conventional small PMT ) caused by the external magnetic fields, when these tubes are operated in a magnetic field in each of X, Y and Z directions.
-3- 1000
100
O o w >
r c -800 (V) s UPPL 'VOL1 AGE: \ USETi w f ALLT MS 1037 (pssc) F'AUL S EWID1• H 1523 (psec) 04 I . 1 , 1 (ns/div ) -1S0O—1000—500 0 500 1000 1500 2000 2500 Fig.3 Anode output waveform TIME (ps) Fig.4 Transit time spread
2 .w
a: i O I O erf O
100 200 400 600 8001000 200 400 600 800 1000 SUPPLY VOLTAGE C V ) PULSE HEIGHT ( CH ) Fig.5 Current amplification Fig. 6 Pulse height distribution ( PHD ) in a single photoelectron state 100;
£U
O > 1 2 AXIS Magnetic Field Directions
-28-24-20-16-12-8 -4 0 4 8 12 15 20 24 28 MAGNETIC FIELDS ( mT)
Fig.7 Effect by magnetic fields on anode output
Fig. 8 shows typical photocathode spectral responses of R5600 series. The spectral response range is determined on the long wavelength side by the photo cathode material and on the short wavelength side by the window material. R5600 has spectral response in the visible range from 300nm to 650nm, with a peak 100 —1-—- ———i wavelength at 420nm. R5600-01 employs R5600 _j p=^ ~Vt /r\ . _fcs^_R5600 -01 a multialkali photocathode that extends f I / sensitivity to near infrared region. Zi / 10 1 The spectral response covers from 300nm > SIRS 500 -0 •K i\ to 820nn. R5600-03 uses a UV-transmit CO \ ' ! 1 ting glass as the window material, thus W CO providing high UV sensitivity. The -RS6C 0 ^ spectral response covers from I85nm to Q 650nm. 0.1 w Q O l
< 0.01 \ 100 200 300 400 SOO 600 700 800 900 WAVELENGTH (nm) Fig. 8 Spectral response characteristics
-5- 2.FINE MESH PMTs
In recent years, a demand for photodetectors, which can be operated in strong magnetic fields, has been increased, especially in the field of high energy physics. In the case of conventional PMTs, photoelectrons and secondary electrons are focused and multiplied by electric field between each electrode and dynode. The electrons trajectories are likely to be affected by magnetic fields of few gauss. As a result, the gain of these PMTs are decreased in such a condition. In that case , it is necessary to use a light guide to lead the signal light to a place where there is no magnetic fields, or to use a magnetic shield case. A light guide and its coupling to a PMT cause deterioration of the light amount and the time characteristics of the signal light. A magnetic shield case causes cost increase and energy loss for a next particle detector.
PMTs using fine mesh dynodes ( Fine Mesh PMTs ) features nearly parallel electric field and adjacent dynode stages. Fig. 9 shows a cross section of a Fine Mesh PMT and Fig. 10 shows a details of a fine mesh dynode. When an electron hits on the upper part of the fine mesh dynode, secondary electrons are emitted from the secondary emissive surface deposited on the fine mesh. This process is repeated through the last dynode stage and finally electrons are multiplied up to 10e6. In axial magnetic fields, there is little influence of the Lorentz force on the trajectories of the photoelectrons and secondary electrons, since the electric field is parallel to the tube axis. On the other hand, the influ ence of a transverse magnetic field is cinsiderably reduced due to the short electron trajectories. These Fine Mesh PMTs can be operated in magnetic fields of 1 tesla or more.
FINE MESH DYNODE ANODE
\ PHOTOCATHODE
Fig.9 Cross section of a Fine Mesh PMT Fig.10 Details of a fine mesh dynode
-6- Fig.11 shows a relative output with respect to magnetic fields as parameters of angles to a tube axis. ° «. i ( R2490-05 is the Fine Mesh PMT of 2 fc-4 •/** <; />— X VOLTAGE :2500V- u / W inch in diameter and has the 16-stage ' ^ dynode.) At a higher magnetic field, = k^ fli = ~--^i ^»B ^ .., K ^^ the deterioration of the current amp N V^ "X «ri«g N lification occurs. because some of |-. \ 30*0. — \_ electrons emitted from the fine mesh XN RRE 1 i ^ Odag dynode return to the sam'e fine mesh dynode due to Lorentz force. As can bJ 1—4 r be seen, there is an angle dependence B of the current amplification. At the larger angle. the deterioration of MAGNETIC FIELDS ( Tesla) the current amplification becomes Fig.ll Typical current amplification small, because the electron trajecto in magnetic fields of R2490-05 ries are improved and electrons emit ted from the fine mesh dynode become to reach to the next fine mesh dynode.
In addition, the other advantages offered by Fine Mesh PMTs are a fast time response and a high pulse linearity. These advantages are important factors for TOF ( Time Of Flight ) counters and calorimeters, respectively. Fig. 12 and Fig.13 show an anode output waveform and TTS of R2490-05. A typical rise time of 2.1ns and a typical TTS of 400ps are obtained at 2500V, respectively.
• 1 1 " 1" —I T 1 r —r • • _ FWHM : 400ps IB* I FWTM : lOOOps D 10' O - •£ O VOL.: 2500V f\ w IB' i >\ - mi > 1 XRIso Time IB S o 2.E-3 Soo ITS •Fall Tlmo w IB* 3.2E-3 Soc *Supply volt. 2500 V I I i r i 1
2 (ns/div.) TIME (ns) Fig.12 Anode output waveform Fig. 13 Transit time spread of R2490-05 ofR2490-05
-7- 3. Si HPD ( Silicone Hybrid Photodetector )
Recently, a combination of solid state technology and vacuum tube techno logy has tieea investigated. Si HPD is one of promising photodetectors that cone from these investigations. This new photodetector offers the following outstand ing features, wide dynamic range, stable operation, ability of multi-photoelect- ron discrimination, low power consumption and so on.
A schematic cross section of SI HPD is shown in Fig. 14. Its basic principle of operation is similar to standard PMTs. When light enters a photocathode, the photocathode emits photoelectrons into vacuum by an external photoelectric effect. These photoelectrons are electrostatically focused and directed by potentials of electrodes towards a small silicon photodiode ( Si PD ). The accelerated photoe^ectrons bombard the Si PD and oroduce a lot of electron-hole- pairs. With reversed bias Vultage to the Si PD, these electron-hole-pairs become signal output.
FOCUSING PHOTOCATHODE ELECTRODES Si PHOTODIODE
INCIDENT * OUTPUT LIGHT CURRENT
50V PHOTOELECTRON BIAS VOLTAGE FOR -15kV Si PHOTODIODE BIAS VOLTAGE FOR n ELECTRON BOMBARDMENT
Fig.14 Schematic cross section of Si HPD
Fig. 15 shows a typical current amplification characteristics v.s. supply voltage between photocathode and Si PD. The current amplification shows a linear response to supply voltages. This data shows a threshold voltage of around 4000V and a current amplification of 3000 at 17kV. The threshold voltage depends on
-8 the thickness of high conductive layer on Si PD. Theoretically, the current amplification is defined by followings;
G = ( V - Vo ) / 3.62
There G : current amplification V : supply voltage between photocathode and Si PD Vo : threshold voltage
1 1 [x103] !M! !! | 1 A.l,l>ia
Fig. 16 shows typical output waveforms which depend on bias voltage for Si PD. A rise time of around 2ns and a fall time of around 5ns are obtained at a bias voltage of 30V.
Fig.17 shows a multi-photoelectron spectrum of Si HPD. As can be seen, up to 8 photoelectrons can be discriminated. Fig.18 shows pulse linearity charac teristics with comparison of PMTs. In this data. R1828-01 is a 2 inch PMT with 12-stage linear focused dynode. R2490-05 is a 2 inch PMT with 16-stage fine mesh dynode. This characteristic relates to dynamic range. As mentioned in the chap ter 2, Fine Mesh PMT has a high pulse linearity compared to conventional PMTs, but Si HPD has higher characteristics than that of Fine Mesh PMTs. This data
-9- shows a linear response up to 2 Ampere within 2% deviation. Fig. 19 shows life characteristics of Si HPDs. This data shows around 10% degradetion of output with the operational current of 1mA after 1000 hours operation. This means that life characteristics of Si PD is at least ten times longer than that of conven tional PMTs,
800 As a promising photodetector. Si HPD has been developed now and will be offered as new product in ta 600 near future by HAMAMATSIL
K 400 s_Vi> f- 55 O O O 200
0 1000 PULSE HEIGHT ( CH ) Fig.17 Multi-photoelectron spectrum
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-10-