- Email: [email protected]
Why people like the Hybrid PhotoDiode
Nuclear Instruments
and Methods
in Physics
Research
A 387 (1997) 92-96
NUCLEAR INSTRUMENTS a METNODS IN PHVStCS RESEARCH Sect~onA
ELSEWER
Why people like the Hybrid PhotoDiode Riccardo DeSalvo* INFN sezione di Pisa, via Livornese 1291, I-56010 S. Piero a Grado (Piss), Italy
Abstract The Hybrid PhotoDiode (HPD) photocathode is coupled to a PIN existing photon detectors. Because and may be useful in a variety of
tube is a new kind of photodetector composed of a vacuum photodiode in which the diode. It presents peculiarities and performances radically different from those of most of these characteristics the HPD is generating vast interest in the scientific community demanding environments.
1. Introduction During the past few years great interest was expressed in the HPD photodetector. This interest is demonstrated, for example, by the large number of contributed papers to this conference on this subject. The reason for this interest whilst well established photodetectors such as photomultiplier tubes (PMT) and photodiodes (PD) are easily available, is that the HPD is a photodetector similar in construction to a PMT. but mechanically simpler, with noise-free gain and the stability of a PD. This paper will review those characteristics of this detector which make it so attractive.
2. The working principle The HPD is a vacuum photodiode tube in which the collection electrode is replaced by a reversed-biased planar silicon diode.’ Photoelectrons are accelerated by a negative photocathode voltage of a few kV and then penetrate the silicon diode surface. The photoelectrons dissipate their acceleration energy near the surface of the silicon diode by exciting and ionizing the valence electrons of the silicon atoms. During this energy dissipation process, electronhole (e/h) pairs are freed in the silicon at the rate of one for every 3.6 eV of energy deposited. If a photocathode voltage HV is applied, a charge-gain of HV13.6 is achieved in a nonmultiplicative way. There is a gain threshold of about 1 keV due to the energy lost by the photoelectron in penetrating the thin contact layer of the diode. This kind of dissipative gain, being free from the *Tel. +39 50 880349. fax +39 50 880350, e-mail [email protected] ’ The name HPD derives from the fact that it is built as an hybridisation of a silicon photodiode inside a vacuum photodiode. The HPDs are also commercially known as HPMTs. 0168-9002/97/$17.00 Copyright PII SO168-9002(96)00969-2
0 1997 Elsevier
multiplicative noise intrinsic in the first stages of both multiplicative and avalanche gain devices, is extremely clean and stable. The reverse-bias of the silicon diode provides an efficient and fast collection of the e/h pairs on the signal electrodes. The light image projected on the photocathode is imaged onto the silicon diode by either electrostatic or proximity focusing eiectron-optics. Segmentation of the diode results in an efficient positionsensitive device. The resulting detector is simple, stable, fast, linear, position-sensitive and, in the proximity focused version, insensitive to magnetic fields. The combination of these characteristics makes it an ideal detector for many applications despite its presently high cost. The HPD concept was already invented and tested in 1957 [I] but it was soon abandoned because of the immaturity of the silicon chip technology and because the preamplifier technology had not yet advanced sufficiently to make of the HPD a practical photodetector for the detection of single photoelectrons. The HPD gain is limited by practical considerations to a few thousand and at this signal level single-photon signals are not observable without modem low-noise charge amplifiers. As a result, the HPD was outclassed by the PMT with its higher but noisier gain. The recent demands for fast position-sensitive photodetectors which are insensitive to magnetic fields has revived the interest in HPDs. Based on these demands, and ignorant of the earlier developments, the author reinvented the device in 1987 [2]. The rush for development started in 1989 stimulated by the LAA project in collaboration with DEP’, Hamamatsu and RTC Philips (presently not producing HPDs) and continued with funding from INFN gruppo V and with support from an EU HCMP Network. The development is still advancing vigorously as HPDs are being engineered for specific uses.
’ DEP Delft Roden, NL-9300
Science B.V. All rights reserved
Electronische Production, Dwazziewegen AB Roden. The Netherlands.
2,
R. DeSalvo
I Nucl. Instr. and Meth. in Phps. Res. A 387 (1997) 92-96
93
3. The main characteristics The HPD silicon chip can be viewed as an infinite flat capacitor charged with the reverse-bias voltage. The signal is generated as the e/h charges generated close to the entrance surface of the diode, are separated by the bias field and collected on the two electrodes. The charge flowing to the closest electrode, for example holes’, reaches it immediately but it is held there in the form of image charge whilst the opposite charge drifts towards the more distant electrode. The signal current is generated as the image-charge is transferred from one electrode to the other. The signal lasts for the entire drift-time between the two electrodes and, since the bias field is roughly uniform, the resulting signal shape is roughly a square wave. Risetimes of less than I ns have been observed with small capacitance HPDs (for example in cross-focused tubes in the DEP-PP0270 series) whilst the pulse duration is inversely proportional to the bias voltage (3 ns FWHM at a bias of 300 V). 300 km thick chips were used to obtain these results. Faster HPDs could be manufactured using thinner silicon chips. In large capacitance HPDs, (for example in single-pixel proximity-focused units like the DEP-PPO350 series) the signal shape becomes triangular [3]. The rise-time is equal to the drift-time of the charge which is inversely proportional to the bias voltage whilst the fall-time corresponds to the RC time-constant of the diode capacitance and the input impedance of the readout electronics. The signal returns to the square-wave shape if a sufficiently low input impedance is used. As the gain of the HPD is dissipative rather than multiplicative, it presents a linear behavior with high voltage rather than an exponential one (see Fig. 1). This is a highly prized characteristic of the HPD since the gain remains perfectly stable with time and does not depend on the device temperature. The linear behavior of the HPD begins once a threshold voltage of the order of 1 kV is reached. The electrons then lose energy in the fullydepleted region of the diode. The energy lost in the contact layer does not contribute to the gain process. In theory, an arbitrarily high gain could be obtained from an HPD but due to practical limitations in applying a very high voltage to the device, the gain must be limited to a few thousands. Typically, HPDs work satisfactorily between say 4 kV where they have a gain of about 1000, up to about 25 kV. Excellent temporal gain stability of the order of 0.1% photoelectrons week have been observed [4]. Temperature response variations are dominated by the photocathode properties alone. Unlike multiplicative or avalanche devices, the HPD gain process is insensitive to the electric
’ HPDs can be built with both silicon chip orientations, in the e-type HPD electrons drift along the long path while in the t-type the holes do it.
Fig. 1. Gain curve of a HPD. The normalization was obtained measuring the PC current at 200 V positive bias. The top insert (top left scale) details the onset of the gain; the gain of 2 around zero PC voltage is produced by the silicon diode sensitivity to light traversing the photocathode. The bottom plot (bottom right scale) shows the real HPD gain versus PC high voltage. A linear fit over the high points of the distribution determines an effective gain threshold of 2.1 kV for this tube.
field in the silicon diode and thus presents no space-charge effects. As a consequence, the device output is linear with charge over many orders of magnitude even with sub-ns light pulses. Linearity of the pulse height is also maintained over several orders of magnitude and is only limited by the reverse-bias voltage [4]. From the point of view of temporal characteristics, the HPD presents no transit-time fluctuations, neither in the cross-focused (spherical field) nor in the proximity-focused (planar field) versions. The HPD temporal resolution is dominated by the amplifier voltage RMS noise over the pulse height and the pulse rise time. As a consequence, the timing resolution for a fast light pulse is found to be where npr is the number of photoproportional to l/n electrons per pulse”Pi3). Probably the most striking property of the HPD is its extraordinary capacity to resolve single photons. In Fig. 2, pulse height spectrum for light pulses having an average of 2.5 photoelectrons per pulse is shown in detail. The single photoelectron peak is completely separated from the pedestal and the double photoelectron peak, and so on. Only a small low-side tail is present between the photoelectron peaks. This excellent performance is again due to the fact that the gain of the HPD is dissipative and not multiplicative. This aspect merits a more detailed study. In multiplicative devices the first gain step is always small. It is a few for PMTs and two for avalanche devices. As each multiplicative step is subject to Poissonian fluctuations, the first gain step cannot present a fluctuation smaller than 4 Rather than the I /v/n,, behaviour characteristic of transit time dominated systems.
III. HYBRID PHOTODIODES
R. LIeSalvo / Nucl. Instr. and Meth. in Phys. Res. A 387 (1997) 92-96
0
5000
loo00 phl)tcdho*~
Fig. 2. HPD pulse-height spectrum for 2.5 photoelectrons light pulse excitation [courtesy of 51.
nG +
G
ronq.3
per
dn. This initial fluctuation is maintained and amplified in the following gain stages. High photoelectron resolution cannot therefore be expected from such devices. The single dissipative step in the HPD gain process gives the nth photoelectron peak a fractional width w,, of:
w* = -\/
15000 hiwl
Fig. 3. Charge and statistical
means that the HPD, even with the same photocathode quantum efficiency of other phototubes, acts as if it was effectively detecting more photoelectrons than a multiplicative device. Moreover, the following formulas apply exactly for HPDs’
CT:,
n pe = A2/(r’
’
G-= Gdn,<,
where G is the gain and a,, (rms, electrons) is the electronic noise charge (ENC) of the HPD/preamplifier combination. The tail on the low side of the peak is due to the fact the accelerated photoelectron exthat, occasionally, periences a hard scatter on one of the silicon atoms and some energy is lost from the gain process. This low-energy tail gives the HPD a small excess noise. This excess noise is almost negligible when compared to the excess noise of 2 or 3 which is common in PMT and avalanche devices. It has to be noted that a few multiplicative light detectors such as micro channel plates may present separated photoelectrons peaks that approach the resolutions of Fig. 2 despite their large excess noise. These photoelectron resolutions are obtained by saturated pulses whose pulseheight is limited by local space charge saturation of the gain-generating electric field. Under these conditions the dynamic range of these devices is limited by the available surface of the device active area divided by the streamer dimensions. The negligible excess noise in HPDs is a seldom considered but very important advantage of these devices over other photodetectors. The standard deviation of the pulse-height spectra produced by light pulses in an HPD is controlled only by the Poissonian statistics or by the statistics of the incoming photoelectrons5. In practice this
5The excess noise in other detectors disrupts the underlying photon statistics and non-negligible correction factors become necessary.
gain curves of a HPD.
.
G=cr=/A, where A is the signal pulse height and D is the light pulse statistical standard deviation
with um,,I,
the measured standard deviation and a,, the pedestal one. A further proof of the negligible HPD excess noise is given in Fig. 3 where the charge gain [(HV-Thr.)/3.6] and the statistical gain [uZ/A] plots are superimposed and are practically indistinguishable. Note that the threshold of this HPD is lower than that of the older device used in Fig. 1.
4. Magnetic field behavior The gain process in the silicon is insensitive to an external magnetic field. The gain of the HPD is function only of the impact angle of the photoelectrons on the silicon chip. In proximity-focused HPDs the electric field between the photocathode and the silicon diode is planar and uniform. The photoelectrons leave the photocathode at virtually zero kinetic energy and follow a spiral trajectory around the magnetic field lines. These trajectories can be
6 These formulas are commonly used with multiplicative tors disregarding the correction factors.
detec-
R. DeSalvo
I Nucl. Instr.
and Meth.
calculated exactly [6]. The photoelectrons cannot fail to hit the diode surface unless the magnetic field is perpendicular to the electric field and above the value for which the photoelectrons cycloid trajectory is smaller than the photocathode to chip separation. Some photoelectrons generated near the edge of the photocathode may be forced outside the active surface of the silicon diode by a tilted magnetic field thus producing a loss of sensitivity. It is easy to predict which area of the photocathode will remain active and which will be rendered insensitive by this effect. As the magnetic field cannot give energy to the photoelectrons, all the photoelectrons that impact on the diode will have exactly the photocathode acceleration energy. Photoelectrons that impact at a significant angle from the perpendicular take a longer path to cross the conductive contact layer of the diode and lose a larger fraction of the original energy and will generate a smaller signal. If the magnetic field is uniform over the dimensions of the detector, since the electric field is also uniform and all photoelectrons start at virtually zero energy, they are all subject to the same bending and hence the same energy loss. The observed performance of an HPD in a magnetic field matched the theoretical predictions very closely. Fixed signal losses of few percent have been measured for field intensities as high 3 or 4 T at field inclinations of up to 45” [7]. No effect at all is present if the magnetic and the electric fields are parallel. The HPD response changes observed in a magnetic field derive from the modulation of the gain threshold voltage of Fig. 1. Improvements in the diode design are currently reducing the value of this threshold, compare the threshold of Fig. 3 with that of Fig. 1. As a consequence, the already low HPD magnetic field sensitivity is expected to decrease further in new low-threshold HPDs. The almost negligible magnetic field sensitivity is one of the strongest advantages of the proximity focused HPD over all other vacuum tube photodetectors. It has been observed that the HPD thermal-electron shot-noise is magnetic field dependent. At certain values of magnetic field inclination, the accelerated photoelectrons emitted at the edges of the photocathode may hit the tube structure. On impact some secondary photons may be emitted to re-excite the photocathode. This effect has been observed at certain characteristic field inclinations and strengths and can be avoided by correctly orienting the device or by the manufacturer masking the photocathode. Cross-focused HPD tubes remain sensitive to magnetic fields although the higher accelerating field and the better photoelectron collection optics make them less sensitive than linear focused PMTs. In most cases cross-focused HPDs do not need mu metal shields to protect against the effects of the earth or other weak magnetic fields. The elimination of the magnetic shield often offsets the thicker electrical insulation required by the higher photocathode operating voltage in HPDs.
in Phys. Res. A 387 (1997)
5. Photoelectron
95
92-96
detection efficiency and uniformity
Despite the fact that photocathodes are normally quite uniform over their entire surface, most light detectors show response non-uniformity due to local photoelectron collection inefficiencies. The cross-focused HPD has an almost perfectly spherical photoelectron collection field whilst the silicon chip is essentially perpendicular to the photoelectron trajectory. The proximity-focused HPD has a parallel field. Both structures present essentially ideal photoelectron collection geometries and virtually no photoelectron can be lost. As a result, the HPD response is always flat to within a few percent over the entire photocathode surface both in single and multi-pixel versions. The only photoelectron detection inefficiency arises from the energy loss through hard scattering of photoelectrons in the silicon. The virtually 100% efficiency of the HPD for detecting photoelectrons compares very well with PMTs in which photoelectron detection efficiencies of 50% and photocathode response variations of tens of percent are common. Indeed HPDs have been observed to detect as many as twice as many photoelectrons than an equivalent photocathode PMT [S]. A few fringe benefits are also present in HPDs. The higher voltage gradient at the photocathode increases its quantum efficiency by typically 10%. compared with a PMT [5]. The high electric field at the photocathode surface helps to collect those photoelectrons emitted with so low an energy that they may be reattracted to the photocathode by their own image-charge. As most HPDs are built using a transfer technology, as opposed to the in-tube processing of most PMTs, the glass of the photocathode substrate can be chosen almost freely. For example, a high refractive index glass can be used to obtain a better match to scintillation crystals and so achieve a better photon-collection efficiency at the photocathode. Additionally, not having the in-tube processing limitations, better engineered photocathodes may be made.
6. Readout segmentation The photoelectron pattern on the photocathode can be imaged on the silicon chip anode with micron accuracy both in crossed and proximity-focused tubes, with and without magnetic fields. The silicon diode segmentation is not a limit since with appropriate bias voltages very little diffusion can occur within the chip. The HPD position resolution capabilities are in practice limited only by the concentration of the light on the photocathode. Clear windows are most commonly used but fiber input plates are available for high resolution requirements. Presently, only moderately pixelated detectors are commercially available (7-pixel DEP-PP0380 series) but developmental HPDs having 70 X 500 micron pixels with internal readout have already been built and tested [9]. Moderately
III. HYBRID PHOTODIODES
96
R. DeSalvo I Nucl. Instr. and Meth. in Phys. Res. A 387 (1997) 92-96
pixelated 19-pixel developmental tubes have been evaluated whilst 20 to 100 pixel tubes are in the industrial developmental phase. The multipixel HPD tubes tested so far have shown the expected position resolution and an excellent response uniformity even across pixel boundaries and cross-talk at the one percent level [lo]. This is a large advantage over segmented PMTs typically plagued by large cross-talk, large detection variations between pixels and large pixel-to-pixel gain variations.
is a problem for mass use. Its price is expected to come down with volume and then it will have a chance to replace other light detectors even in less demanding environments.
References [l] N. S&u, 1957.
7. Conclusions The HPD is a new type of photodetector which is free of the multiplicative gain restrictions of PMTs and with an enormous potential for pixelation. Its unique photoelectron counting capabilities make it the detector of choice for precision photon-counting applications. Its insensitivity to magnetic field make it useful in high and low magnetic field operations whilst the gain stability and response uniformity of this detector may provide a technical solution to monitoring tasks. Because of its superior performances it is expected to start replacing the PMTs in demanding environments such as in magnetic fields and when precision pixelation or precision photon-counting is required. The HPD may represent a valid replacement of PDs as well when the capacitive noise of the PD is a limit. The HPD is presently built in small numbers and its price
Electron Device Conf., Washington
DC, October,
[2] R. DeSalvo, Hybrid Photo Diode tube, CLNS 87-92, Cornell University, Ithaca, NY 14853. [3] G. Anzivino et al., Nucl. Instr. and Meth. A 367 (1995) 384. [4] R. DeSalvo et al., Nucl. Instr. and Meth. A 315 (1992) 375. [5] C. Datema et al., these Proceedings (1st Conf. on New Developments in Photodetection, Beaune, France, 1996) Nucl. Instr. and Meth. A 387 (1997) 100. [6] H. Amaudon et al., Nucl. Instr. and Meth. A 342 (1994) 558. [7] G. Anzivino et al., Nucl. Instr. and Meth. A 365 (1995) 76. [8] G. Anzivino et al., Recent developments of the HPD, Proc. IV Int. Conf. on Calorimetry in High Energy Physics, Isola d’Elba, Italy, 19-25 September 1993. [9] C. D’Ambrosio et al., Nucl. Instr. and Meth. A 332 (1993) 134; A 345 (1994) 279. [lo] L. Waldron et al., these Proceedings (1st Conf. on New Developments in Photodetection, Beaune, France, 1996) Nucl. Instr. and Meth. A 387 (1997) 113; P. Cushman et al, ibid., 107.
and Methods
in Physics
Research
A 387 (1997) 92-96
NUCLEAR INSTRUMENTS a METNODS IN PHVStCS RESEARCH Sect~onA
ELSEWER
Why people like the Hybrid PhotoDiode Riccardo DeSalvo* INFN sezione di Pisa, via Livornese 1291, I-56010 S. Piero a Grado (Piss), Italy
Abstract The Hybrid PhotoDiode (HPD) photocathode is coupled to a PIN existing photon detectors. Because and may be useful in a variety of
tube is a new kind of photodetector composed of a vacuum photodiode in which the diode. It presents peculiarities and performances radically different from those of most of these characteristics the HPD is generating vast interest in the scientific community demanding environments.
1. Introduction During the past few years great interest was expressed in the HPD photodetector. This interest is demonstrated, for example, by the large number of contributed papers to this conference on this subject. The reason for this interest whilst well established photodetectors such as photomultiplier tubes (PMT) and photodiodes (PD) are easily available, is that the HPD is a photodetector similar in construction to a PMT. but mechanically simpler, with noise-free gain and the stability of a PD. This paper will review those characteristics of this detector which make it so attractive.
2. The working principle The HPD is a vacuum photodiode tube in which the collection electrode is replaced by a reversed-biased planar silicon diode.’ Photoelectrons are accelerated by a negative photocathode voltage of a few kV and then penetrate the silicon diode surface. The photoelectrons dissipate their acceleration energy near the surface of the silicon diode by exciting and ionizing the valence electrons of the silicon atoms. During this energy dissipation process, electronhole (e/h) pairs are freed in the silicon at the rate of one for every 3.6 eV of energy deposited. If a photocathode voltage HV is applied, a charge-gain of HV13.6 is achieved in a nonmultiplicative way. There is a gain threshold of about 1 keV due to the energy lost by the photoelectron in penetrating the thin contact layer of the diode. This kind of dissipative gain, being free from the *Tel. +39 50 880349. fax +39 50 880350, e-mail [email protected] ’ The name HPD derives from the fact that it is built as an hybridisation of a silicon photodiode inside a vacuum photodiode. The HPDs are also commercially known as HPMTs. 0168-9002/97/$17.00 Copyright PII SO168-9002(96)00969-2
0 1997 Elsevier
multiplicative noise intrinsic in the first stages of both multiplicative and avalanche gain devices, is extremely clean and stable. The reverse-bias of the silicon diode provides an efficient and fast collection of the e/h pairs on the signal electrodes. The light image projected on the photocathode is imaged onto the silicon diode by either electrostatic or proximity focusing eiectron-optics. Segmentation of the diode results in an efficient positionsensitive device. The resulting detector is simple, stable, fast, linear, position-sensitive and, in the proximity focused version, insensitive to magnetic fields. The combination of these characteristics makes it an ideal detector for many applications despite its presently high cost. The HPD concept was already invented and tested in 1957 [I] but it was soon abandoned because of the immaturity of the silicon chip technology and because the preamplifier technology had not yet advanced sufficiently to make of the HPD a practical photodetector for the detection of single photoelectrons. The HPD gain is limited by practical considerations to a few thousand and at this signal level single-photon signals are not observable without modem low-noise charge amplifiers. As a result, the HPD was outclassed by the PMT with its higher but noisier gain. The recent demands for fast position-sensitive photodetectors which are insensitive to magnetic fields has revived the interest in HPDs. Based on these demands, and ignorant of the earlier developments, the author reinvented the device in 1987 [2]. The rush for development started in 1989 stimulated by the LAA project in collaboration with DEP’, Hamamatsu and RTC Philips (presently not producing HPDs) and continued with funding from INFN gruppo V and with support from an EU HCMP Network. The development is still advancing vigorously as HPDs are being engineered for specific uses.
’ DEP Delft Roden, NL-9300
Science B.V. All rights reserved
Electronische Production, Dwazziewegen AB Roden. The Netherlands.
2,
R. DeSalvo
I Nucl. Instr. and Meth. in Phps. Res. A 387 (1997) 92-96
93
3. The main characteristics The HPD silicon chip can be viewed as an infinite flat capacitor charged with the reverse-bias voltage. The signal is generated as the e/h charges generated close to the entrance surface of the diode, are separated by the bias field and collected on the two electrodes. The charge flowing to the closest electrode, for example holes’, reaches it immediately but it is held there in the form of image charge whilst the opposite charge drifts towards the more distant electrode. The signal current is generated as the image-charge is transferred from one electrode to the other. The signal lasts for the entire drift-time between the two electrodes and, since the bias field is roughly uniform, the resulting signal shape is roughly a square wave. Risetimes of less than I ns have been observed with small capacitance HPDs (for example in cross-focused tubes in the DEP-PP0270 series) whilst the pulse duration is inversely proportional to the bias voltage (3 ns FWHM at a bias of 300 V). 300 km thick chips were used to obtain these results. Faster HPDs could be manufactured using thinner silicon chips. In large capacitance HPDs, (for example in single-pixel proximity-focused units like the DEP-PPO350 series) the signal shape becomes triangular [3]. The rise-time is equal to the drift-time of the charge which is inversely proportional to the bias voltage whilst the fall-time corresponds to the RC time-constant of the diode capacitance and the input impedance of the readout electronics. The signal returns to the square-wave shape if a sufficiently low input impedance is used. As the gain of the HPD is dissipative rather than multiplicative, it presents a linear behavior with high voltage rather than an exponential one (see Fig. 1). This is a highly prized characteristic of the HPD since the gain remains perfectly stable with time and does not depend on the device temperature. The linear behavior of the HPD begins once a threshold voltage of the order of 1 kV is reached. The electrons then lose energy in the fullydepleted region of the diode. The energy lost in the contact layer does not contribute to the gain process. In theory, an arbitrarily high gain could be obtained from an HPD but due to practical limitations in applying a very high voltage to the device, the gain must be limited to a few thousands. Typically, HPDs work satisfactorily between say 4 kV where they have a gain of about 1000, up to about 25 kV. Excellent temporal gain stability of the order of 0.1% photoelectrons week have been observed [4]. Temperature response variations are dominated by the photocathode properties alone. Unlike multiplicative or avalanche devices, the HPD gain process is insensitive to the electric
’ HPDs can be built with both silicon chip orientations, in the e-type HPD electrons drift along the long path while in the t-type the holes do it.
Fig. 1. Gain curve of a HPD. The normalization was obtained measuring the PC current at 200 V positive bias. The top insert (top left scale) details the onset of the gain; the gain of 2 around zero PC voltage is produced by the silicon diode sensitivity to light traversing the photocathode. The bottom plot (bottom right scale) shows the real HPD gain versus PC high voltage. A linear fit over the high points of the distribution determines an effective gain threshold of 2.1 kV for this tube.
field in the silicon diode and thus presents no space-charge effects. As a consequence, the device output is linear with charge over many orders of magnitude even with sub-ns light pulses. Linearity of the pulse height is also maintained over several orders of magnitude and is only limited by the reverse-bias voltage [4]. From the point of view of temporal characteristics, the HPD presents no transit-time fluctuations, neither in the cross-focused (spherical field) nor in the proximity-focused (planar field) versions. The HPD temporal resolution is dominated by the amplifier voltage RMS noise over the pulse height and the pulse rise time. As a consequence, the timing resolution for a fast light pulse is found to be where npr is the number of photoproportional to l/n electrons per pulse”Pi3). Probably the most striking property of the HPD is its extraordinary capacity to resolve single photons. In Fig. 2, pulse height spectrum for light pulses having an average of 2.5 photoelectrons per pulse is shown in detail. The single photoelectron peak is completely separated from the pedestal and the double photoelectron peak, and so on. Only a small low-side tail is present between the photoelectron peaks. This excellent performance is again due to the fact that the gain of the HPD is dissipative and not multiplicative. This aspect merits a more detailed study. In multiplicative devices the first gain step is always small. It is a few for PMTs and two for avalanche devices. As each multiplicative step is subject to Poissonian fluctuations, the first gain step cannot present a fluctuation smaller than 4 Rather than the I /v/n,, behaviour characteristic of transit time dominated systems.
III. HYBRID PHOTODIODES
R. LIeSalvo / Nucl. Instr. and Meth. in Phys. Res. A 387 (1997) 92-96
0
5000
loo00 phl)tcdho*~
Fig. 2. HPD pulse-height spectrum for 2.5 photoelectrons light pulse excitation [courtesy of 51.
nG +
G
ronq.3
per
dn. This initial fluctuation is maintained and amplified in the following gain stages. High photoelectron resolution cannot therefore be expected from such devices. The single dissipative step in the HPD gain process gives the nth photoelectron peak a fractional width w,, of:
w* = -\/
15000 hiwl
Fig. 3. Charge and statistical
means that the HPD, even with the same photocathode quantum efficiency of other phototubes, acts as if it was effectively detecting more photoelectrons than a multiplicative device. Moreover, the following formulas apply exactly for HPDs’
CT:,
n pe = A2/(r’
’
G-= Gdn,<,
where G is the gain and a,, (rms, electrons) is the electronic noise charge (ENC) of the HPD/preamplifier combination. The tail on the low side of the peak is due to the fact the accelerated photoelectron exthat, occasionally, periences a hard scatter on one of the silicon atoms and some energy is lost from the gain process. This low-energy tail gives the HPD a small excess noise. This excess noise is almost negligible when compared to the excess noise of 2 or 3 which is common in PMT and avalanche devices. It has to be noted that a few multiplicative light detectors such as micro channel plates may present separated photoelectrons peaks that approach the resolutions of Fig. 2 despite their large excess noise. These photoelectron resolutions are obtained by saturated pulses whose pulseheight is limited by local space charge saturation of the gain-generating electric field. Under these conditions the dynamic range of these devices is limited by the available surface of the device active area divided by the streamer dimensions. The negligible excess noise in HPDs is a seldom considered but very important advantage of these devices over other photodetectors. The standard deviation of the pulse-height spectra produced by light pulses in an HPD is controlled only by the Poissonian statistics or by the statistics of the incoming photoelectrons5. In practice this
5The excess noise in other detectors disrupts the underlying photon statistics and non-negligible correction factors become necessary.
gain curves of a HPD.
.
G=cr=/A, where A is the signal pulse height and D is the light pulse statistical standard deviation
with um,,I,
the measured standard deviation and a,, the pedestal one. A further proof of the negligible HPD excess noise is given in Fig. 3 where the charge gain [(HV-Thr.)/3.6] and the statistical gain [uZ/A] plots are superimposed and are practically indistinguishable. Note that the threshold of this HPD is lower than that of the older device used in Fig. 1.
4. Magnetic field behavior The gain process in the silicon is insensitive to an external magnetic field. The gain of the HPD is function only of the impact angle of the photoelectrons on the silicon chip. In proximity-focused HPDs the electric field between the photocathode and the silicon diode is planar and uniform. The photoelectrons leave the photocathode at virtually zero kinetic energy and follow a spiral trajectory around the magnetic field lines. These trajectories can be
6 These formulas are commonly used with multiplicative tors disregarding the correction factors.
detec-
R. DeSalvo
I Nucl. Instr.
and Meth.
calculated exactly [6]. The photoelectrons cannot fail to hit the diode surface unless the magnetic field is perpendicular to the electric field and above the value for which the photoelectrons cycloid trajectory is smaller than the photocathode to chip separation. Some photoelectrons generated near the edge of the photocathode may be forced outside the active surface of the silicon diode by a tilted magnetic field thus producing a loss of sensitivity. It is easy to predict which area of the photocathode will remain active and which will be rendered insensitive by this effect. As the magnetic field cannot give energy to the photoelectrons, all the photoelectrons that impact on the diode will have exactly the photocathode acceleration energy. Photoelectrons that impact at a significant angle from the perpendicular take a longer path to cross the conductive contact layer of the diode and lose a larger fraction of the original energy and will generate a smaller signal. If the magnetic field is uniform over the dimensions of the detector, since the electric field is also uniform and all photoelectrons start at virtually zero energy, they are all subject to the same bending and hence the same energy loss. The observed performance of an HPD in a magnetic field matched the theoretical predictions very closely. Fixed signal losses of few percent have been measured for field intensities as high 3 or 4 T at field inclinations of up to 45” [7]. No effect at all is present if the magnetic and the electric fields are parallel. The HPD response changes observed in a magnetic field derive from the modulation of the gain threshold voltage of Fig. 1. Improvements in the diode design are currently reducing the value of this threshold, compare the threshold of Fig. 3 with that of Fig. 1. As a consequence, the already low HPD magnetic field sensitivity is expected to decrease further in new low-threshold HPDs. The almost negligible magnetic field sensitivity is one of the strongest advantages of the proximity focused HPD over all other vacuum tube photodetectors. It has been observed that the HPD thermal-electron shot-noise is magnetic field dependent. At certain values of magnetic field inclination, the accelerated photoelectrons emitted at the edges of the photocathode may hit the tube structure. On impact some secondary photons may be emitted to re-excite the photocathode. This effect has been observed at certain characteristic field inclinations and strengths and can be avoided by correctly orienting the device or by the manufacturer masking the photocathode. Cross-focused HPD tubes remain sensitive to magnetic fields although the higher accelerating field and the better photoelectron collection optics make them less sensitive than linear focused PMTs. In most cases cross-focused HPDs do not need mu metal shields to protect against the effects of the earth or other weak magnetic fields. The elimination of the magnetic shield often offsets the thicker electrical insulation required by the higher photocathode operating voltage in HPDs.
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detection efficiency and uniformity
Despite the fact that photocathodes are normally quite uniform over their entire surface, most light detectors show response non-uniformity due to local photoelectron collection inefficiencies. The cross-focused HPD has an almost perfectly spherical photoelectron collection field whilst the silicon chip is essentially perpendicular to the photoelectron trajectory. The proximity-focused HPD has a parallel field. Both structures present essentially ideal photoelectron collection geometries and virtually no photoelectron can be lost. As a result, the HPD response is always flat to within a few percent over the entire photocathode surface both in single and multi-pixel versions. The only photoelectron detection inefficiency arises from the energy loss through hard scattering of photoelectrons in the silicon. The virtually 100% efficiency of the HPD for detecting photoelectrons compares very well with PMTs in which photoelectron detection efficiencies of 50% and photocathode response variations of tens of percent are common. Indeed HPDs have been observed to detect as many as twice as many photoelectrons than an equivalent photocathode PMT [S]. A few fringe benefits are also present in HPDs. The higher voltage gradient at the photocathode increases its quantum efficiency by typically 10%. compared with a PMT [5]. The high electric field at the photocathode surface helps to collect those photoelectrons emitted with so low an energy that they may be reattracted to the photocathode by their own image-charge. As most HPDs are built using a transfer technology, as opposed to the in-tube processing of most PMTs, the glass of the photocathode substrate can be chosen almost freely. For example, a high refractive index glass can be used to obtain a better match to scintillation crystals and so achieve a better photon-collection efficiency at the photocathode. Additionally, not having the in-tube processing limitations, better engineered photocathodes may be made.
6. Readout segmentation The photoelectron pattern on the photocathode can be imaged on the silicon chip anode with micron accuracy both in crossed and proximity-focused tubes, with and without magnetic fields. The silicon diode segmentation is not a limit since with appropriate bias voltages very little diffusion can occur within the chip. The HPD position resolution capabilities are in practice limited only by the concentration of the light on the photocathode. Clear windows are most commonly used but fiber input plates are available for high resolution requirements. Presently, only moderately pixelated detectors are commercially available (7-pixel DEP-PP0380 series) but developmental HPDs having 70 X 500 micron pixels with internal readout have already been built and tested [9]. Moderately
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pixelated 19-pixel developmental tubes have been evaluated whilst 20 to 100 pixel tubes are in the industrial developmental phase. The multipixel HPD tubes tested so far have shown the expected position resolution and an excellent response uniformity even across pixel boundaries and cross-talk at the one percent level [lo]. This is a large advantage over segmented PMTs typically plagued by large cross-talk, large detection variations between pixels and large pixel-to-pixel gain variations.
is a problem for mass use. Its price is expected to come down with volume and then it will have a chance to replace other light detectors even in less demanding environments.
References [l] N. S&u, 1957.
7. Conclusions The HPD is a new type of photodetector which is free of the multiplicative gain restrictions of PMTs and with an enormous potential for pixelation. Its unique photoelectron counting capabilities make it the detector of choice for precision photon-counting applications. Its insensitivity to magnetic field make it useful in high and low magnetic field operations whilst the gain stability and response uniformity of this detector may provide a technical solution to monitoring tasks. Because of its superior performances it is expected to start replacing the PMTs in demanding environments such as in magnetic fields and when precision pixelation or precision photon-counting is required. The HPD may represent a valid replacement of PDs as well when the capacitive noise of the PD is a limit. The HPD is presently built in small numbers and its price
Electron Device Conf., Washington
DC, October,
[2] R. DeSalvo, Hybrid Photo Diode tube, CLNS 87-92, Cornell University, Ithaca, NY 14853. [3] G. Anzivino et al., Nucl. Instr. and Meth. A 367 (1995) 384. [4] R. DeSalvo et al., Nucl. Instr. and Meth. A 315 (1992) 375. [5] C. Datema et al., these Proceedings (1st Conf. on New Developments in Photodetection, Beaune, France, 1996) Nucl. Instr. and Meth. A 387 (1997) 100. [6] H. Amaudon et al., Nucl. Instr. and Meth. A 342 (1994) 558. [7] G. Anzivino et al., Nucl. Instr. and Meth. A 365 (1995) 76. [8] G. Anzivino et al., Recent developments of the HPD, Proc. IV Int. Conf. on Calorimetry in High Energy Physics, Isola d’Elba, Italy, 19-25 September 1993. [9] C. D’Ambrosio et al., Nucl. Instr. and Meth. A 332 (1993) 134; A 345 (1994) 279. [lo] L. Waldron et al., these Proceedings (1st Conf. on New Developments in Photodetection, Beaune, France, 1996) Nucl. Instr. and Meth. A 387 (1997) 113; P. Cushman et al, ibid., 107.