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Characteristics of an Optically Scanned SEC Device
Characteristics of an Optically Scanned SEC Device A. CHOUDRY Physicx Department, Univeraity of Hhode Island, Kingston, Rhode Ixland. 1J.S.A .
INTRODUCTION The development of the optically scanned proximity focused SEC device has continued beyond the preliminary feasibility studies. The basic design concepts and the principle of operation of the device have been described by Choudry et aZ.l This paper discusses the changes that have been made in the design of the device and some of its operating characteristics. During this study, the emphasis was placed on evaluating those parameters of the device that affect the electronic data processing of the signal, and thus no attempts were made to generate displays. The fundamental aim of this investigation has been to develop a simple device suitable for spectrophotometric analysis of faint astronomical sources. This calls for a device of large gain and low dark current. One of the parameters limiting the maximum possible gain is the charge deposited on the SEC target during the first step, namely priming. Experimental studies were made to determine the optimum priming parameters. Even a device of relatively low gain would be acceptable provided that it allows a long integration time. I n a device of the type investigated by us, the integration time is limited by all those electronic processes in the background that contribute to discharging the primed SEC target. Most important of these would be the dark photocurrent, e.g. field emission, etc. For this reason, the behaviour of the dark current under various operating conditions was studied to estimate its magnitude and if possible determine its origin. The gain of the device, apart from its dependence on the voltage, is determined by the microscopic morphology of the potassium chloride layer, as has been studied by various a ~ t h o r s ; ~ ~ been shown to it -has be a combination of the generation of secondary electrons and their 253
254
A. CHOUDRY
esca,peand propagation through the voids in the fibrous structure of the layctr. Furthermore, the gain and the target capacity are also related through the SEC layer structure if it is assumed that charge is stored, not only on the surface but also in the body of the layer (as seems to be the case). The target capacity would then be determined by the penetration and behaviour of the low energy (10 to 40 eV) electrons in the target whereas the gain mechanism will also have t,o include the behaviour of the high energy (50 to 5000 elr) electrons as well as the low energy electrons, the latter being secondaries. Apart from purely empirical observations, no attempt has been made in the present study t o arrive at a unified model of the SEC layer mechanics. Phenomenological models of the type described by NcMullan and Towler5 can be used to explain the general features of the device.
EXPERIMENTAL METHOD The modified version of the device is shown in Pig. 1, the basic design of the device being the same an described in Ref. 1. I n our earlier wwrk it was found that the device did not maintain proper vacuum after a period of about six months. I n particular, the dark current tended to increase within four months of the fabrication of the device. It was conjectured that the very coni~iitctdesign of the earlier version hid EL rather large surface to volume ratio and the slow occlusion of gases in such a small volume caused the deterioration in the vacuum.
FIQ.1. Tho REC devicc.
CHARACTERISTICS OF AN OPTICALLY SCANNED SEC DEVICE
255
The design was therefore modified by adding a tubular appendage on the side opposite to the photocathode, as can be seen in Fig. 1 . This has proved to be quite satisfactory and the device has been in operation for over a year without a loss in vacuum as inferred by the stability of the dark current. Figure 2 shows a schematic diagram of the experimental set up used in the measurements. The photocathode of the SEC device is mounted at the focal plane of a conventional 70 mm single lens reflex (SLR) camera. The scene to be projected on the device is first monitored visually through the ground glass screen G and the reflex mirror M2.
/'
f
i-
FIG.2. Schematic diagram of the oxperimental apparatus.
The optical scan generator is assembled together with the SLR camera to form an integral unit. To generate the scanning raster, the laser beam is first passed through a defining pinhole P (20 to 100 pm). Two orthogonally mounted mirrors deflect the beam as shown. These mirrors are driven by two galvanometric drives S1 and S2 to which appropriate sawtooth voltages are applied. The mechanical inertia and the elastic constants of the drivers limit the scan rate to a maximum of 30Hz. Lens L and mirror M1 project the raster on the SEC during the read cycle. During the write cycle, the mirror M1 is retracted into the position denoted by the dotted line. To obtain quantitative measurements of the light incident on the SEC device both the aperture and the shutter speed of the camera were calibrated. The signal from the SEU device is fed into a Keithley 427 current amplifier. This puts a limit on the sensitivity and the bandwidth of all
256
A. CHOUDRY
measurements to about 1 pA a t 1 msec rise time. All our measurements were carried out a t a bandwidth of 300 Hz unless stated otherwise.
RESULTS AND DISCUSSION As described elsewhere,' the operation of the tubeinvolves three steps, namely: (1) priming, (2) writing, and (3) reading. Measurements were taken during these steps to evaluate various parameters influencing the device operation. However, before discussing these measurements, we shall first consider the dark current which limits the long time integration; this is important for astronomical applications. For the measurement of the dark current, the output of the current
0
I
2
Accelerating voltage (kV)
Fro,. 3. Dark current, as a function of accelerating voltage at 1 kHz bandwidth.
amplifier was monitored by a n RMS voltmeter. The bandwidth for all these measurements was 1 kHz. The variation of dark current with the photocathode to SEC target voltage is shown in Fig. 3 . Among other possible sources, the following three would seem to contribute significantly to the observed dark current: (1) photocathode dark current (thermal, field emission, etc), (2) leakage, and (3) amplifier noise (including the noise in cables, etc). To suppress the ever present 60 Hz background, a dry cell battery bank, in a well shielded enclosure, was used as the power supply for both the amplifier and the SEC device. The amplifier noise was estimated to
CHARACTERISTICS
OF AN OPTICALLY SCANNED SEC DEVICE
257
be 0.2 to 0-3pA and thus this also represents the sensitivity limit of our measuring system. The low voltage (i.e. up to about 800 V) dark current seems to have no field emission component and all of it could perhaps be attributed to thermal, amplifier and leakage noise. The significant rise in the dark current after about 1 kV indicates the onset of field emission: thus, for long time integration, it was decided not to exceed this voltage. For estimating various parameters, the nornial operating
4OV
35 v
30 V 25 V
20 v
I
I
I
2
4
I
6
T i m e (min)
FIG.4. Priming ourrent as a fuiict,iori of time at various voltages
conditions would therefore be assumed to be 1 kV with a dark current of 1 pA which corresponds to a flux of 200 photons per pixel with a 10 lp mm -l scanning raster. To prime the SEC’ target, the photocathode is maintained at a negative potent.ia1 (10 to 45 V) and is exposed to a n extended source of uniform illumination. The time variation of the priming current at various priming voltages is monitored on a chart recorder. These results are displayed in Fig. 4. A t t = 0, the priming current is maximum and then decreases to a smaller value. This variation of the priming current I ( t ) can be given approximately by the following relation: (1) I ( t )= I , I, exp( -t/r) where I , is the asymptotic value of the current and T is the relaxation time which depends on various system parameters. If the priming photon flux is n, then
+
258
A. CHOUDRY
I ( 0 ) = cn
(2)
where E is the photocathode quantum efficiency. The total charge q stored on the target is given by
Priming voltage ( V )
FIG.6. SEC layer capacitance variation ea B function of priming voltage.
The relation q = d 2was found to be valid for up to 20 V only and hence the charge stored at various voltages was found by numerically evaluating the integral in Eq. (3). The capacitance of the SEC layer a t various voltages, as found by this method, is shown in Fig. 5. The low voltage limiting value of the target capacitance (-800 pF) agrees with the assumption that, at low voltages, the priming charge resides on the surface and that the SEC layer acts as a parallel plate capacitor. The increase in the capacitance at higher priming voltages could indicate that the photoelectrons have enough energy to penetrate into the layer and then get trapped. At yet higher voltages, the electrons have enough energy to penetrate through the SEC layer and reach the aluminium signal plate directly, and thus no charge is deposited in the layer and its capacitance drops. The priming voltage for maximum charge deposition, as can be seen from Fig. 5, is -25 V, corresponding to a target capacitance of 210 pFcm -,. For a 10 Ip mm-1 scanning raster, this gives 0.3pC (or 3 x lo6 electrons) per pixel with a photocathode efficiency of 10% and a nominal
CHARACTERISTICS OF AN OPTICALLY SCANNED SEC DEVICE
259
gain of 10 (at 1 kV). The maximum integrated photon flux per pixel is 3 x lo6 and this is thus the upper bound of the dynamic range; the lower bound of 200 photons sec per pixel has already been discussed above. Further, the total priming charge q for a photon flux n (photons sec - l ) during an interval t sec can be assumed to follow a reciprocity relation of the form q = cnt. (4) The validity of this relation has been checked experimentally for 5 X < t < 500 sec. I n the write cycle, the voltage is increased to a suitable value (typically a few kilovolts) and the scene is projected on the photocathode. The gain G of the device is realised in this step. Each ~)hotoelectron accelerated by the interelectrode field will generate a number of secondaries in the SEC layer and discharge the pixel, at its landing site, by an amount eG where e is the electronic charge. If the SEC layer is not to be driven positive, the maximum integrated photon flux nt froni the scene is q = Gent (5) p
1
where q is the priming charge. For very low light level scenes, the discharge due to the dark current la becomes important and Eq. ( 5 ) must be modified to q = Gent Idf. (6)
+
It may be remarked here that in Ey. ((i),both Q and Id are functions of the interlectrode voltage V , and thus it would not be possible to increase t arbitrarily for very low light level scenes. The dependence of I d on V has already been discussed (cf. Fig. 3 ) ; for the measurement of G there are many methods available. Since it is easier to interpret the priming current data, we chose a modification of the priming cycle to measure the gain, as shown in Fig. 6. A priming cycle is started and, at a time t , near its completion, it is stopped by releasing the camera shutter. The aperture and the shutter are now adjusted to admit a known amount of light and V is adjusted to the value a t which the gain is to be measured. The shutter is now released to execute the write cycle and discharge the target. After this write cycle, all system parameters are readjusted to the original priming status and the priming cycle is resumed, as indicated by the vertical dotted line a t t , in Fig. 6. This cycle is interrupted at a time 1 , when the value of the priming current has reached the same value as at the previous interruption at t,. These measurements easily give the charge deposited in the second priming and hence the amount of discharge caused during the write cycle. Since voltages higher than 2 kV are not
260
A. CHOUDRY
desirable for long time integration, the gain was measured only in the range of 0.8 to 1-8 kV. In this limited range, the gain was a linear function of the reading voltage and varied from 14 t o 40. For reading, an edge was first projected on the photocathode during the write cycle and the exposed target was completely discharged. The laser spot was now scanned across the edge and the signal output from
I
'I
'2
Time FIG.6. Priming cycle used for measuromcnt of gain.
about 1 mm on either side of the edge was amplified and then processed by a n on-line PDP-9 computer system. The details of this system are described by McCollough et al. in these Pr0ceedings.t Scan rates of up to 100 cm sec -l and voltages of up to 2 kV were utilised; however, a resolution of better than 5 lp mm a t 20% could not be obtained and this aspect of the device operation deserves further scrutiny. I n particular, the following areas should be especially examined: the isolation of vibrations caused by the mechanical optical scanners, the possibility of magnetic focusing, and the introduction of a head mounted preamplifier. It is believed that with a number of such modifications the resolution of thie device could be made t o approach 20 lp mm -l as obtained in an earlier optically scanned SEC device.6 This would then allow simple devices of about 2000 TV lines to be fabricated since the design of the
7 See p. 585.
CHARACTERISTICS OF AN OPTICALLY SPANNED SEC DEVICE
26 1
present device permits scaling up to a format of about 100 mm diameter without encountering fundamental problems. ikKNOWLICl)(: 11iCNTS
It IS a special pleasure to acknowledge the benefits derived by the author 111 varions discussions and other communications with Drs 0. W. Goetze, K . L. Hallam, S. Nudelman and B. Powell. Special thanks are also due to Mr Bachelder under whose supervision the instrument shop rendered exrellent service. This work has been supported by NASA contract NAS5-23173
REFERENCES 1 . Choudry, A., Goetze, G. W., Nudelman, S.and Shen, T. Y., I n “Ad\-. E.E.P.” Vol. 33B, 903 (1972). 2. Goetze, G. W. I n “Adv. E.E.P.” Vol. 22A, 219 (1966). 3. Goetze, G. W., Boerio, A. M. and Green, M., J . Appl. Phys. 35, 482 (1964). 4. Seller, H.and Stark, M.,2.Angew. Phy8. 19,90 (1965). 5. McMullan, D. and Towler, C . 0.. 1 7 ) “Adv. E.E.P.” Vol. 28A, 173 (1969). 6. Beyer, R . R. and Goetze, G. W., I n “Adv. E.E.P.” Vol. 22, 241 (1966).
DISCUSSION What do you mean by scanniiig t,lie target from the opposit’eside? This is indeed just a matter of establishing a convention. Usually, one has an image section and a read sectioii on opposite sides of the target. Even in an earlier optically scanned SEC vidicon, t.he sctmc was written from ono side of the target. and read from the opposite side. and thus one may speak of reading and writing from oppositc sides. In the present, device, both read and write operations are done from the same side. G . 0. TOWLER: Can you describe the method used t o measure the storage cepacit.anae a function of priming voltage? Could not the apparent decrease in capacitance from 25 V to 40 V really be a decrease in voltage across the storage layer? Below the first cross-over potential I do not think there is any doubt that the storage surface will take up t,he photocat,hode potential on priming, but corisider priming when t,he photocathode potential is greater than t,he first cross-over potential. Initially the storage surface will be driven negatively towards cathode potential, because although secoudary einissiori is above unity, the electric fields are such that recombination occurs (external field retarding, internal field zero). However, once the storage surface has been driven somewhat negative t h e internal field increases in a favourable direction for collecting the ret.urned secondaries onto the signal plate. The storage surface will stabilise at a potential at which one secondary is collect.edon the signal plate for each photoelectron impinging on the target,, the other secondaries will recombine. Tho higher tho accelerating voltage between photocathode and the storage surface the higher the secondary emission (in the region of interest) and hence the lower t,he int,ernal field required to provide the necessary collection efficiency for a single secondary t.0 reach the signal plate. Hence when above the first.crossover, the higher the priming voltage, the smaller the voltage developed across the E. KOSTAL:
A. CHOUDRY:
262
A* CHOUDRY
layer. This means less charge is stored and this could be interpreted as a smaller storage capacitance. A. CHOUDRY: An understanding of the charge storage, multiplication and discharge mechanism raises many important questionu of the type p00ed by you. These must be discussed in greater detail than is possible under the present format and thus the following remarks may provide only partial answers. To start with our method of measuring the layer capacity: it may be said that the capacitance itself is not really measured; we measure the chargo stored on the target. The value of geometrical capacitance that one can ascribe to conducting objects is not easily applicable to die1ect)ricssince the surfaces may not be equipotentials and the charge may be stored inside the dielectric. Thus one can only specify the total charge stored for a given photocathode potential (the aluminium anode is held at ground potential). The mechmiism of secondary emission from the surface after the first cross-over is only a part of the picture; we believe that penetration of the SEC layer by the electrons is an important process and plays a significant part even before the first cross-over and continues to so do until it entirely dominates at high electron velocities. With charge stored inside the SEC layer, the field distribution will be different from the one you are assuming and hence the secondaries would behave in a slightly different manner. Further, we believe that an attempt to explain the rise and fall of “capacitance” (i.e. stored charge) through surface secondary emission is not sufficient since the capacity begins to rise even before the first cross-over. Thus, in our opinion, only a unified model, of the type mentioned in the text, which takes into account penetration, charge stored under surface, secondary emission, etc. would offer a satisfactory explanation of the observations.
INTRODUCTION The development of the optically scanned proximity focused SEC device has continued beyond the preliminary feasibility studies. The basic design concepts and the principle of operation of the device have been described by Choudry et aZ.l This paper discusses the changes that have been made in the design of the device and some of its operating characteristics. During this study, the emphasis was placed on evaluating those parameters of the device that affect the electronic data processing of the signal, and thus no attempts were made to generate displays. The fundamental aim of this investigation has been to develop a simple device suitable for spectrophotometric analysis of faint astronomical sources. This calls for a device of large gain and low dark current. One of the parameters limiting the maximum possible gain is the charge deposited on the SEC target during the first step, namely priming. Experimental studies were made to determine the optimum priming parameters. Even a device of relatively low gain would be acceptable provided that it allows a long integration time. I n a device of the type investigated by us, the integration time is limited by all those electronic processes in the background that contribute to discharging the primed SEC target. Most important of these would be the dark photocurrent, e.g. field emission, etc. For this reason, the behaviour of the dark current under various operating conditions was studied to estimate its magnitude and if possible determine its origin. The gain of the device, apart from its dependence on the voltage, is determined by the microscopic morphology of the potassium chloride layer, as has been studied by various a ~ t h o r s ; ~ ~ been shown to it -has be a combination of the generation of secondary electrons and their 253
254
A. CHOUDRY
esca,peand propagation through the voids in the fibrous structure of the layctr. Furthermore, the gain and the target capacity are also related through the SEC layer structure if it is assumed that charge is stored, not only on the surface but also in the body of the layer (as seems to be the case). The target capacity would then be determined by the penetration and behaviour of the low energy (10 to 40 eV) electrons in the target whereas the gain mechanism will also have t,o include the behaviour of the high energy (50 to 5000 elr) electrons as well as the low energy electrons, the latter being secondaries. Apart from purely empirical observations, no attempt has been made in the present study t o arrive at a unified model of the SEC layer mechanics. Phenomenological models of the type described by NcMullan and Towler5 can be used to explain the general features of the device.
EXPERIMENTAL METHOD The modified version of the device is shown in Pig. 1, the basic design of the device being the same an described in Ref. 1. I n our earlier wwrk it was found that the device did not maintain proper vacuum after a period of about six months. I n particular, the dark current tended to increase within four months of the fabrication of the device. It was conjectured that the very coni~iitctdesign of the earlier version hid EL rather large surface to volume ratio and the slow occlusion of gases in such a small volume caused the deterioration in the vacuum.
FIQ.1. Tho REC devicc.
CHARACTERISTICS OF AN OPTICALLY SCANNED SEC DEVICE
255
The design was therefore modified by adding a tubular appendage on the side opposite to the photocathode, as can be seen in Fig. 1 . This has proved to be quite satisfactory and the device has been in operation for over a year without a loss in vacuum as inferred by the stability of the dark current. Figure 2 shows a schematic diagram of the experimental set up used in the measurements. The photocathode of the SEC device is mounted at the focal plane of a conventional 70 mm single lens reflex (SLR) camera. The scene to be projected on the device is first monitored visually through the ground glass screen G and the reflex mirror M2.
/'
f
i-
FIG.2. Schematic diagram of the oxperimental apparatus.
The optical scan generator is assembled together with the SLR camera to form an integral unit. To generate the scanning raster, the laser beam is first passed through a defining pinhole P (20 to 100 pm). Two orthogonally mounted mirrors deflect the beam as shown. These mirrors are driven by two galvanometric drives S1 and S2 to which appropriate sawtooth voltages are applied. The mechanical inertia and the elastic constants of the drivers limit the scan rate to a maximum of 30Hz. Lens L and mirror M1 project the raster on the SEC during the read cycle. During the write cycle, the mirror M1 is retracted into the position denoted by the dotted line. To obtain quantitative measurements of the light incident on the SEC device both the aperture and the shutter speed of the camera were calibrated. The signal from the SEU device is fed into a Keithley 427 current amplifier. This puts a limit on the sensitivity and the bandwidth of all
256
A. CHOUDRY
measurements to about 1 pA a t 1 msec rise time. All our measurements were carried out a t a bandwidth of 300 Hz unless stated otherwise.
RESULTS AND DISCUSSION As described elsewhere,' the operation of the tubeinvolves three steps, namely: (1) priming, (2) writing, and (3) reading. Measurements were taken during these steps to evaluate various parameters influencing the device operation. However, before discussing these measurements, we shall first consider the dark current which limits the long time integration; this is important for astronomical applications. For the measurement of the dark current, the output of the current
0
I
2
Accelerating voltage (kV)
Fro,. 3. Dark current, as a function of accelerating voltage at 1 kHz bandwidth.
amplifier was monitored by a n RMS voltmeter. The bandwidth for all these measurements was 1 kHz. The variation of dark current with the photocathode to SEC target voltage is shown in Fig. 3 . Among other possible sources, the following three would seem to contribute significantly to the observed dark current: (1) photocathode dark current (thermal, field emission, etc), (2) leakage, and (3) amplifier noise (including the noise in cables, etc). To suppress the ever present 60 Hz background, a dry cell battery bank, in a well shielded enclosure, was used as the power supply for both the amplifier and the SEC device. The amplifier noise was estimated to
CHARACTERISTICS
OF AN OPTICALLY SCANNED SEC DEVICE
257
be 0.2 to 0-3pA and thus this also represents the sensitivity limit of our measuring system. The low voltage (i.e. up to about 800 V) dark current seems to have no field emission component and all of it could perhaps be attributed to thermal, amplifier and leakage noise. The significant rise in the dark current after about 1 kV indicates the onset of field emission: thus, for long time integration, it was decided not to exceed this voltage. For estimating various parameters, the nornial operating
4OV
35 v
30 V 25 V
20 v
I
I
I
2
4
I
6
T i m e (min)
FIG.4. Priming ourrent as a fuiict,iori of time at various voltages
conditions would therefore be assumed to be 1 kV with a dark current of 1 pA which corresponds to a flux of 200 photons per pixel with a 10 lp mm -l scanning raster. To prime the SEC’ target, the photocathode is maintained at a negative potent.ia1 (10 to 45 V) and is exposed to a n extended source of uniform illumination. The time variation of the priming current at various priming voltages is monitored on a chart recorder. These results are displayed in Fig. 4. A t t = 0, the priming current is maximum and then decreases to a smaller value. This variation of the priming current I ( t ) can be given approximately by the following relation: (1) I ( t )= I , I, exp( -t/r) where I , is the asymptotic value of the current and T is the relaxation time which depends on various system parameters. If the priming photon flux is n, then
+
258
A. CHOUDRY
I ( 0 ) = cn
(2)
where E is the photocathode quantum efficiency. The total charge q stored on the target is given by
Priming voltage ( V )
FIG.6. SEC layer capacitance variation ea B function of priming voltage.
The relation q = d 2was found to be valid for up to 20 V only and hence the charge stored at various voltages was found by numerically evaluating the integral in Eq. (3). The capacitance of the SEC layer a t various voltages, as found by this method, is shown in Fig. 5. The low voltage limiting value of the target capacitance (-800 pF) agrees with the assumption that, at low voltages, the priming charge resides on the surface and that the SEC layer acts as a parallel plate capacitor. The increase in the capacitance at higher priming voltages could indicate that the photoelectrons have enough energy to penetrate into the layer and then get trapped. At yet higher voltages, the electrons have enough energy to penetrate through the SEC layer and reach the aluminium signal plate directly, and thus no charge is deposited in the layer and its capacitance drops. The priming voltage for maximum charge deposition, as can be seen from Fig. 5, is -25 V, corresponding to a target capacitance of 210 pFcm -,. For a 10 Ip mm-1 scanning raster, this gives 0.3pC (or 3 x lo6 electrons) per pixel with a photocathode efficiency of 10% and a nominal
CHARACTERISTICS OF AN OPTICALLY SCANNED SEC DEVICE
259
gain of 10 (at 1 kV). The maximum integrated photon flux per pixel is 3 x lo6 and this is thus the upper bound of the dynamic range; the lower bound of 200 photons sec per pixel has already been discussed above. Further, the total priming charge q for a photon flux n (photons sec - l ) during an interval t sec can be assumed to follow a reciprocity relation of the form q = cnt. (4) The validity of this relation has been checked experimentally for 5 X < t < 500 sec. I n the write cycle, the voltage is increased to a suitable value (typically a few kilovolts) and the scene is projected on the photocathode. The gain G of the device is realised in this step. Each ~)hotoelectron accelerated by the interelectrode field will generate a number of secondaries in the SEC layer and discharge the pixel, at its landing site, by an amount eG where e is the electronic charge. If the SEC layer is not to be driven positive, the maximum integrated photon flux nt froni the scene is q = Gent (5) p
1
where q is the priming charge. For very low light level scenes, the discharge due to the dark current la becomes important and Eq. ( 5 ) must be modified to q = Gent Idf. (6)
+
It may be remarked here that in Ey. ((i),both Q and Id are functions of the interlectrode voltage V , and thus it would not be possible to increase t arbitrarily for very low light level scenes. The dependence of I d on V has already been discussed (cf. Fig. 3 ) ; for the measurement of G there are many methods available. Since it is easier to interpret the priming current data, we chose a modification of the priming cycle to measure the gain, as shown in Fig. 6. A priming cycle is started and, at a time t , near its completion, it is stopped by releasing the camera shutter. The aperture and the shutter are now adjusted to admit a known amount of light and V is adjusted to the value a t which the gain is to be measured. The shutter is now released to execute the write cycle and discharge the target. After this write cycle, all system parameters are readjusted to the original priming status and the priming cycle is resumed, as indicated by the vertical dotted line a t t , in Fig. 6. This cycle is interrupted at a time 1 , when the value of the priming current has reached the same value as at the previous interruption at t,. These measurements easily give the charge deposited in the second priming and hence the amount of discharge caused during the write cycle. Since voltages higher than 2 kV are not
260
A. CHOUDRY
desirable for long time integration, the gain was measured only in the range of 0.8 to 1-8 kV. In this limited range, the gain was a linear function of the reading voltage and varied from 14 t o 40. For reading, an edge was first projected on the photocathode during the write cycle and the exposed target was completely discharged. The laser spot was now scanned across the edge and the signal output from
I
'I
'2
Time FIG.6. Priming cycle used for measuromcnt of gain.
about 1 mm on either side of the edge was amplified and then processed by a n on-line PDP-9 computer system. The details of this system are described by McCollough et al. in these Pr0ceedings.t Scan rates of up to 100 cm sec -l and voltages of up to 2 kV were utilised; however, a resolution of better than 5 lp mm a t 20% could not be obtained and this aspect of the device operation deserves further scrutiny. I n particular, the following areas should be especially examined: the isolation of vibrations caused by the mechanical optical scanners, the possibility of magnetic focusing, and the introduction of a head mounted preamplifier. It is believed that with a number of such modifications the resolution of thie device could be made t o approach 20 lp mm -l as obtained in an earlier optically scanned SEC device.6 This would then allow simple devices of about 2000 TV lines to be fabricated since the design of the
7 See p. 585.
CHARACTERISTICS OF AN OPTICALLY SPANNED SEC DEVICE
26 1
present device permits scaling up to a format of about 100 mm diameter without encountering fundamental problems. ikKNOWLICl)(: 11iCNTS
It IS a special pleasure to acknowledge the benefits derived by the author 111 varions discussions and other communications with Drs 0. W. Goetze, K . L. Hallam, S. Nudelman and B. Powell. Special thanks are also due to Mr Bachelder under whose supervision the instrument shop rendered exrellent service. This work has been supported by NASA contract NAS5-23173
REFERENCES 1 . Choudry, A., Goetze, G. W., Nudelman, S.and Shen, T. Y., I n “Ad\-. E.E.P.” Vol. 33B, 903 (1972). 2. Goetze, G. W. I n “Adv. E.E.P.” Vol. 22A, 219 (1966). 3. Goetze, G. W., Boerio, A. M. and Green, M., J . Appl. Phys. 35, 482 (1964). 4. Seller, H.and Stark, M.,2.Angew. Phy8. 19,90 (1965). 5. McMullan, D. and Towler, C . 0.. 1 7 ) “Adv. E.E.P.” Vol. 28A, 173 (1969). 6. Beyer, R . R. and Goetze, G. W., I n “Adv. E.E.P.” Vol. 22, 241 (1966).
DISCUSSION What do you mean by scanniiig t,lie target from the opposit’eside? This is indeed just a matter of establishing a convention. Usually, one has an image section and a read sectioii on opposite sides of the target. Even in an earlier optically scanned SEC vidicon, t.he sctmc was written from ono side of the target. and read from the opposite side. and thus one may speak of reading and writing from oppositc sides. In the present, device, both read and write operations are done from the same side. G . 0. TOWLER: Can you describe the method used t o measure the storage cepacit.anae a function of priming voltage? Could not the apparent decrease in capacitance from 25 V to 40 V really be a decrease in voltage across the storage layer? Below the first cross-over potential I do not think there is any doubt that the storage surface will take up t,he photocat,hode potential on priming, but corisider priming when t,he photocathode potential is greater than t,he first cross-over potential. Initially the storage surface will be driven negatively towards cathode potential, because although secoudary einissiori is above unity, the electric fields are such that recombination occurs (external field retarding, internal field zero). However, once the storage surface has been driven somewhat negative t h e internal field increases in a favourable direction for collecting the ret.urned secondaries onto the signal plate. The storage surface will stabilise at a potential at which one secondary is collect.edon the signal plate for each photoelectron impinging on the target,, the other secondaries will recombine. Tho higher tho accelerating voltage between photocathode and the storage surface the higher the secondary emission (in the region of interest) and hence the lower t,he int,ernal field required to provide the necessary collection efficiency for a single secondary t.0 reach the signal plate. Hence when above the first.crossover, the higher the priming voltage, the smaller the voltage developed across the E. KOSTAL:
A. CHOUDRY:
262
A* CHOUDRY
layer. This means less charge is stored and this could be interpreted as a smaller storage capacitance. A. CHOUDRY: An understanding of the charge storage, multiplication and discharge mechanism raises many important questionu of the type p00ed by you. These must be discussed in greater detail than is possible under the present format and thus the following remarks may provide only partial answers. To start with our method of measuring the layer capacity: it may be said that the capacitance itself is not really measured; we measure the chargo stored on the target. The value of geometrical capacitance that one can ascribe to conducting objects is not easily applicable to die1ect)ricssince the surfaces may not be equipotentials and the charge may be stored inside the dielectric. Thus one can only specify the total charge stored for a given photocathode potential (the aluminium anode is held at ground potential). The mechmiism of secondary emission from the surface after the first cross-over is only a part of the picture; we believe that penetration of the SEC layer by the electrons is an important process and plays a significant part even before the first cross-over and continues to so do until it entirely dominates at high electron velocities. With charge stored inside the SEC layer, the field distribution will be different from the one you are assuming and hence the secondaries would behave in a slightly different manner. Further, we believe that an attempt to explain the rise and fall of “capacitance” (i.e. stored charge) through surface secondary emission is not sufficient since the capacity begins to rise even before the first cross-over. Thus, in our opinion, only a unified model, of the type mentioned in the text, which takes into account penetration, charge stored under surface, secondary emission, etc. would offer a satisfactory explanation of the observations.