Abstract
Abstract--We describe the preparation of a sealed, gas-filled photomultiplier (GPMT) for the visible spectral range and present the properties of the first prototypes. They consist of a 50 mm diameter semitransparent bialkali (K-Cs-Sb) photocathode coupled to a 30 mm × 30 mm Kapton multi-GEM electron multiplier. High gain of 2×10 4 in two-GEM mode and a quantum efficiency of 13% at 405 nm have been reached at 700 Torr of Ar/CH4 (95:5). The detector structure and experimental setup are described; results are presented on the GPMT gain, ionfeedback and its suppression, stability, and other critical parameters in various gas mixtures. Hot sealing techniques with In/Sn and In/Bi solders are discussed.

photocathodes are currently operating in gas flow mode. However, bialkali photocathodes for the visible spectral range are extremely chemically reactive, in practice necessitating operation in a baked, sealed, and gettered device. The feasibility of preparation of a sealed GPMT with CsI and a 3-GEM multiplier was recently demonstrated [9, 10] .

In this report, we provide a rather detailed description of the preparation of sealed visible-light GPMTs comprising multi-GEMs and K-Cs-Sb photocathodes and show how their gain, quantum efficiency (QE), and stability depend upon the details of their construction. Finally, we examine the effect of ion-induced feedback on the bialkali photocathodes and propose ways to suppress it.
II. EXPERIMENTAL SYSTEM, MATERIALS AND PROCEDURES

A. Experimental system and detector parts
Due to the high reactivity of bialkali films, the detector package must be baked in UHV and hermetically sealed; leak rates should be less than 10 -12 mbar×l/s. Hot indium sealing could be used, but it is a complex process that relies on careful choices of materials and surface preparation of the window and package.
A. Experimental system and detector parts
Our experience shows that reflective bialkali photocathodes exposed to 10 -5 Torr oxygen are totally degraded after 5 min [11] . Thus, a leak or outgassing even as small as 5×10 -12 mbar×l/s (the detection limit of our system) would give a lifetime of our packages, with volumes of 153 cm 3 , of only about 17 days. This suggests the prudence of incorporating a getter into the package.
A. Experimental system and detector parts
The experimental setup for the production and sealing of K-Cs-Sb photocathode with a gaseous multiplier is shown in Fig. 1 and Fig. 2 . It consists of three ultrahigh vacuum chambers: a load-lock where the glass window for the package is initially baked, an activation chamber in which the semitransparent bialkali photocathode is deposited onto the window and its quantum efficiency measured, and a sealing chamber where the window is sealed to the GPMT package containing the GEM multipliers. They are evacuated by three turbomolecular pumps; an additional titanium sublimation Marcin Balcerzyk, Dirk Mörmann, Amos Breskin, Bhartendu K. Singh, Elisabete D.C. Freitas, Rachel Chechik, Moshe Klin, Michael Rappaport
A. Experimental system and detector parts
Methods of preparation and performance of sealed gas photomultipliers for visible light V pump is mounted on the sealing chamber. All three chambers are baked with internal quartz halogen lamps. The sealing chamber, the magnetic manipulators and the residual gas analyzers (RGA) are also externally baked with heating tapes; the latter are installed on the activation and sealing chambers. The respective base pressures in these chambers are 3×10 -10 Torr and 4×10 -9 Torr at room temperature. A typical composition of the residual gas in the two chambers is shown in Table I . The detector package is made of Kovar; Kovar pins are sealed into the package with glass. The package and pins are electroplated with 10 µm electroless Ni alloy (92-95% Ni, 8-5% P) and 1 µm of Au (with small addition of Co); (Recently it has been found that the gold is superfluous so that future packages will only be nickel plated.)
A. Experimental system and detector parts
The windows are Kovar glass (Schott 8245), ∅64×5 mm with a metal ring (see below) on the circumference, evaporated in a separate vacuum system. A schematic drawing of an assembled multiplier is shown in Fig. 3 . It consists of a cascade of 30 mm × 30 mm Kapton GEMs, 50 µm thick, claded with 5 µm of copper and up to 1 µm of gold. The holes are double conical (surface diameter 70 µm; middle diameter 50 µm) with 140 µm pitch. They are produced at the Printed Circuit Workshop at CERN, Geneva, Switzerland.
A. Experimental system and detector parts
The GEMs are mounted between ceramic spacers, leaving an active area of 20 mm×20 mm. Contacts to feedthroughs are made with 70 µm gold-plated Be-Cu wires, soldered with pure Sn (MP 232 ºC) and minimal amounts of ZnCl 2 -based flux. In the future Sn will be replaced by Au/Sn (80:20) eutectic solder (MP 280 ºC), not requiring the use of flux and providing a more robust contact. The contact to a stainless steel anode, mounted below the last GEM, is made with the same wire but with an HCl-based RS 3 flux; prior to mounting, the anode is thoroughly cleaned in an ultrasonic bath, with standard chemicals. The distance between the photocathode and the top GEM is 5 mm. The distance between successive GEMs and between the last GEM and the anode is 1 mm.
B. Cleaning and preparation procedures
New packages are cleaned in 20% HCl for 2 min, followed by DECONEX 15 PF (10% in double-distilled water), doubledistilled water, and absolute ethanol (30 min each) in an ultrasonic bath; they are blown dry with dry nitrogen and stored under vacuum. Re-used packages are first wiped clean of silicone sealant (see below) with 2-butanone and then opened on a hotplate at 130 ºC. The indium alloy is removed, and the package is cleaned in ethyl alcohol.
B. Cleaning and preparation procedures
Preparing the package for sealing involves careful application of the indium in the package groove. The 10 mm wide metal ring on the window's circumference is deposited while rotating and at normal incidence by e-gun evaporation; it is used both for sealing and for electrical contact to the photocathode. The ring comprises Cr (100 nm) and Cu (200 nm), with an additional 60 nm of Cu deposited with the window tilted at 45 o in order to coat the edge. The Cr/Cu layers replace the previously used Cr/Ni/Au tri-layer; they have been found to give more reliable sealing. All metals have a purity of 5N; just before their evaporation, the glass is cleaned by glow discharge in air. Following evaporation, the windows are either used or stored in vacuum. Windows can be re-used by removing the metal with CR-7 etchant (Cyantek Inc.).
B. Cleaning and preparation procedures
The GEMs are mounted in the packages as delivered from CERN without any cleaning. They are mounted on ceramic frames; the latter are cleaned, prior to mounting, for 12h in DECONEX 15 PF (10% in double-distilled water) in an ultrasonic bath and baked in air for 6 hours at 700 ºC.
C. GPMT production
A low temperature activated getter resistively heated type 172 from SAES Getters) is mounted in the package, followed by the cascaded GEM multiplier (see Fig. 3 ).
C. GPMT production
Prior to sealing, the assembled detector is tested with a semi-transparent CsI photocathode in a dedicated chamber in flowing Ar/CH 4 (95:5). Then the excess solder in the package's groove is machined on a clean lathe and the assembled detector is immediately placed within the sealing chamber for baking.
C. GPMT production
After baking at 220 ºC for 24 h in the load-lock, the glass window is transferred to the activation chamber for photocathode deposition. The photocathode is evaporated through a mask that shields the sealing surface of the metal ring from the evaporants; the deposition overlaps part of the ring for electrical contact. The bialkali photocathode is prepared with the standard successive evaporation method [11] . First, outgassed 5N Sb is evaporated from a boat at 175 ºC to a thickness yielding 70% to 85% transmission, measured with a white incandescent light source and a photodiode. Then K is evaporated at 185 ºC from SAES Getters dispensers and the photocurrent induced by a Hg(Ar) lamp is monitored. When the photocurrent reaches a maximum and then falls by 10%, the evaporation is stopped. Finally, Cs from SAES Getters dispensers is evaporated at 155 ºC. The photocurrent is monitored and evaporation is stopped according to same criterion as for the K evaporation. The K and Cs evaporations may be repeated several times. This process yields photocathodes with up to 25% quantum efficiency (QE) in semitransparent mode. The QE and photocathode transmission are measured in situ using a calibrated Hg(Ar) light source and a monochromator.
C. GPMT production
The sealing chamber is baked with the assembled detector for 3 days at 195 ºC. Then the SAES 172 getter in the detector package is activated by resistive heating to 450 ºC for 10 min. (The getter does not pump methane below 250 ºC.) Argon and methane are then introduced to the sealing chamber through separate mass flow controllers and purified by GateKeeper 35K filters from Aeronex Inc. that purify noble gases, N 2 , and CH 4 to the 1 ppb level at a maximum flow of 1 slm. The pressure is adjusted to give 650 to 700 Torr after cooling to room temperature. The photocathode, currently held in the deposition chamber at the sealing temperature, is moved into the sealing chamber and brought into contact with the melted indium alloy on the package. At this point the package is rotated a few times by about ±30º to break the ineluctable oxide layer on the solder and the package is vibrated via the vertical manipulator of the window. The sealing is effected when the package is cooled to room temperature -a process taking 5 to 6 hours. The sealing chamber is then vented with dry nitrogen, the package is extracted to a glove bag flushed with dry nitrogen and further sealed with silicone-based sealant (Vacseal of Structure Probe, Inc). The sealant is applied in the space between the window and package, for additional protection and sealing of micro-leaks. The absolute QE is measured with a calibrated QE photomultiplier (Photonis XP2020Q), a monochromator and a Hg(Ar) lamp. The gain characteristics in current mode and pulse mode are measured with individual high voltage power supplies for the GEMs and the photocathode. Due to the voltage limit of the present package feedthroughs, we operated the GPMT in a 2-GEM mode. (The package feedthroughs have now been redesigned to handle the voltages required for 3-GEM operation.) For current mode measurements, which can give absolute gain, we chose a 0.4 kV/cm extraction field between the photocathode and the top of GEM1, and 2kV/cm transfer field between GEMs.
D. GPMT evaluation procedures
The ion feedback measurements were carried out with an unsealed detector package inside the sealing chamber, by measuring gain vs. GEM voltage characteristics with bialkali photocathode. They were done with one GEM and an anode. The package and photocathode were grounded and the positive high voltage was applied to the anode interconnected with the bottom of the GEM. The photocurrent was measured by the voltage drop on a resistor in series with the anode.
III. SEARCH FOR OPTIMAL SEALING MATERIALS
Due to persistent problems achieving leak-free sealing, we have systematically investigated various combinations of solders, metal coatings on the sealing surfaces of the window and package, and sealing conditions both in vacuum and in gas. Moreover, tests indicated that the QE is deleteriously effected by prolonged heating during sealing, probably due to loss of Cs from the photocathodes. Typical maximum QE of a semitransparent Cs-K-Sb photocathode in vacuum (measured before sealing) is about 25%. In vacuum, sealing at 126 ºC degrades the QE by 12%; in Ar/CH 4 (95:5), the degradation was 26%. At 143 ºC, the respective losses in QE in vacuum and gas are of 22% and 49%. These findings show the importance of sealing at the lowest possible temperature.
III. SEARCH FOR OPTIMAL SEALING MATERIALS
Wetting tests of In/Sn and In/Bi on Cr/Ni/Au-and Cr/Cuplated stainless steel surfaces showed that the latter produced superior results: On a Ni/Cu-plated surface In/Sn spreads at 118 ºC (i.e., at its melting point), and In/Bi at 112 ºC. On a Ni/Au-plated surface In/Sn spreads at 125 ºC, and In/Bi does not spread even at 156 ºC). Tests performed in vacuum and in 700 Torr Ar/CH 4 (95:5) gave similar results.
III. SEARCH FOR OPTIMAL SEALING MATERIALS
Clearly, the Cr/Cu surface is superior to that of Cr/Ni/Au. We attribute this to the high solubility of Au in In and even higher solubility in Sn. Since the evaporated Au is only 200 nm thick, it practically immediately diffuses into the solder, essentially leaving a clean Ni surface to which the solder must bond. The much higher diffusion rates of Sn and In into Cu than into Ni then accounts for the difference. Although the Sn/In alloy spreads on Cu already at 118 ºC, we preferred to seal at 20 to 30 ºC above its melting point, as recommended by Indium Corporation of America.
III. SEARCH FOR OPTIMAL SEALING MATERIALS
We performed leak tests of Ni/Au-plated test packages sealed with In/Sn at 150 ºC: Cr/Ni/Au-plated windows yielded four successful sealings (i.e., leak rate below 5×10 -12 mbar×l/s) out of ten and Cr/Cu-plated windows gave one successful sealing out of two. Two sealing tests with In/Bi at 123 ºC with a Cr/Cu-plated window resulted in successful sealing.
IV. PERFORMANCE OF THE GPMT IN VARIOUS GASES
Optimization of the visible light GPMT requires systematic investigation in various gases. Though noble gases and their mixtures [5] showed stable multi-GEM operation under high gain, the electron backscattering losses could be of a concern. We investigated the operation of a single GEM coupled to a semitransparent bialkali photocathode, in a non-sealed mode, in Ar, CH 4 , Ar/CH 4 (98:2; 95:5; 93:7; 90:10) and Ar/N 2 (98:2). The gain vs. GEM voltage in these gases is shown in Fig. 4 . The field between photocathode and GEM was balvarez@example.org kV/cm and the pressure at 700 Torr. Surprisingly, all gases except Ar/CH 4 (95:5 and 98:2) showed noticeable feedback, manifested as a deviation from exponential dependence of gain on GEM voltage. Fits at low GEM voltages for purely exponential gain are shown in Fig. 4 . Observation of correlated delayed charge pulses indicated that the source of the deviation is ion feedback. Such ion-induced feedback effects were not observed with the same multiplier coupled to a CsI photocathode, operating with the same gas in similar conditions. The effect is shown more clearly in Fig. 5 , depicting the ratio of the measured gain to the fit to purely exponential gain at low GEM voltages vs. the measured gain.
IV. PERFORMANCE OF THE GPMT IN VARIOUS GASES
From Fig. 5 it can be seen that in Ar, ion-induced feedback starts at a gain of 50, in CH 4 at 25, in Ar/CH 4 (93:7) at 560, in Ar/CH 4 (90:10) at 150, and in Ar/N 2 (98:2) at 40. Because of the ion feedback between the first GEM and the photocathode, the maximum achievable gain in the double-GEM sealed GPMT was strongly limited. However, in Ar/CH 4 (95:5 and 98:2) mixtures, the onset of ion-induced feedback is not seen up to a gain of about 700 in a single-GEM mode. In a double-GEM sealed GPMT with Ar/CH 4 (95:5) we did not see it even at a gain of 10 4 . Such a sensitive dependence of ion feedback on CH 4 concentration in Ar is not fully understood and is under investigation. Nevertheless, some aspects that may be relevant are known. In Ar/CH 4 mixtures with low concentrations of methane, mainly Ar + ions are produced in the GEM avalanche. However, since the ionization potential of CH 4 is lower than that of Ar (12.7 eV compared to 15.8 eV) [5] , Ar + ions undergo electron capture from methane (Penning mixture). We estimate the mean free path for this process as less then 1 mm in our conditions. Therefore, mostly CH 4 + ions reach the photocathode, having kinetic energy below 1 eV. They cannot cause kinematic electron emission from the KCs-Sb photocathode, which is actually a p-type semiconductor with a bandgap of 1eV and activation energy of 2.1 eV [12] . But they can induce potential emission following their neutralization on the surface. This still does not explain the lack of ion-induced feedback in some Ar/CH 4 mixtures and its presence in others. We assume that more complex processes, involving methane-methane and methane-argon interactions, as well as surface-stimulated processes could be responsible. 
SEALED GPMT RESULTS
A sealed GPMT is shown in Fig. 6 . The dark ring on the window surface is the metalized area for sealing and electrical contact. 
A. Quantum efficiency
The highest QE spectrum reached by us in a sealed device is shown in Fig. 7 . It shows a maximum of 13% at 405 nm that falls to about 5% at 546 nm; the fall to 5% at 312 nm is due to the transmission properties of the Kovar-glass window. The statistics of the peak QE values in our sealed devices are shown in Fig. 8 . Included are all sealed devices that were stable during more than one day. It should be noted that part of the reduced QE values are due to photoelectron backscattering on gas molecules [13, 14] . Backscattering-induced QE losses are larger than 50% in noble gases and 34% in Ar/CH 4 (95:5) at the applied field of 0.5 kV/cm. Lower losses will occur at higher fields. But in addition, as discussed above, significant QE reduction occurred during the sealing process. It is expected that the change to In/Bi sealing at 120 ºC will yield superior performance. 
B. Stability
The stability of QE and gain of the sealed devices was monitored over several weeks. A photocathode, with no GEM, was sealed with 700 Torr Ar, using a Cr/Ni/Au-plated window and In/Sn at 150 ºC. Such a photodiode has stable but low QE (peak value 3%) for over seven months (see Fig. 9 ). The QE stability of a device with a getter (SAES type 172), Cr/Ni/Au-plated window, Ar/CH 4 (95:5), and In/Sn sealed at 150 ºC, is shown in Fig. 10 . This package is presumed to have micro-leaks so that the decrease of the QE after 15 days is attributed to the saturation of the getter. 
C. Gain and ion feedback.
The gain of a single-GEM sealed GPMT with 700 Torr Ar, measured in a current mode, is shown in Fig. 11 . The device was checked prior to sealing with a CsI photocathode with the same gas, and it did not show any ion-induced feedback. With the bialkali photocathode, the feedback appears at a gain of 50, as discussed in section IV. The gains measured in current mode, for 1-and 2-GEM operation of a sealed GPMT with approximately 700 Torr Ar/CH 4 (95:5) , are shown in Fig. 12 . Shown for comparison are the gain curves of the device before sealing, measured with a CsI photocathode in the same gas. The onset of ion feedback at a gain of 2×10 4 is seen. This is a significant improvement over earlier devices, generally limited by feedback effects to gains of 10 to 20 [15] . In another device sealed with 600 Torr Ar/CH 4 (95:5), the onset of ioninduced feedback occurred at a gain of 650 in double-GEM mode.
VI. ION FEEDBACK SUPPRESSION
Ion feedback, discussed above, may become an obstacle for operation of the GPMT at higher gains. We have recently confirmed that even a careful adjustment of the electric fields within a multi-GEM structure may reduce the ion feedback at best by a factor of ten [16] . Possibly a more effective solution is the introduction of an active gate electrode between the successive GEM elements to block avalanche-induced ions. Similarly to the solution adopted for ion gating in Time Projection Chambers (TPCs) [17] , we have introduced a gate electrode consisting of alternately biased parallel 50 µm diameter wires 1 mm apart, placed midway between the last and next to last GEMs. The gating signal is a 10 µs-long rectangular pulse, up to 300 V.
VI. ION FEEDBACK SUPPRESSION
In a test operation, the gate was triggered by the anode signal; this is a simple task due to the large difference in drift velocity between ions and electrons. Fig. 13 shows the gate performance in terms of the residual fraction of ions reaching the photocathode vs. the applied gate voltage. We measured an ion suppression factor of 10 4 , the measurement being limited only by the noise of our system [16] . This ion suppression should permit reaching with bialkali photocathodes gains of about 10 6 , currently obtained with CsI photocathodes [8] .
VII. SUMMARY AND FUTURE PROSPECTS
We have built a system and developed techniques for bialkali photocathode production and their sealing in gas in an electron multiplier package. We have prepared a series of sealed bialkali gas photomultipliers with two Kapton GEMs. In the best case, the GPMT had a gain of 2×10 4 with Ar/CH 4 (95:5) at a pressure close to one atmosphere, with no ioninduced feedback. A photocathode sealed in a photodiode mode, with Ar and no GEM, is now stable for more than seven months. Gas sealed GPMTs were stable for up to 5 weeks. The highest QE obtained for a sealed device is 13%. It is consistent with expected QE loss due to backscattering of electrons from the gas and to the high temperature of the sealing, which damages the photocathode. We developed a way to suppress ion feedback by a factor of at least 10 4 with a gating electrode, albeit at the cost of introducing some dead time on a µs scale. However, the sealing process should still be improved. We have examined In/Bi as a lower temperature substitute to In/Sn. We have employed Cr/Cu-plated windows that appear to be superior to the currently used Cr/Ni/Auplated ones. We are currently preparing 3-GEM GPMT devices in an improved package, which should have gains sufficient to detect single photons. If necessary, they will incorporate ion-gating electrodes. The new devices will incorporate 2D readout electrodes; recent investigations having shown that such multi-GEM devices can yield resolutions in the 100 µm range with a very simple delay-line readout system [18] . Though the results reached so far are very promising, the long-term operation of visible-light GPMTs has still to be demonstrated. Important parameters yet to be measured are the behavior at high counting rates and photocathode and GEM aging. While in the short term, bialkali photocathodes operated with Kapton GEMs seem to be stable, GEM production from UHV-compatible insulators such as glass, ceramic and silicon is being investigated.
VIII. ACKNOWLEDGMENTS
We would like to thank Dr. M. Prager for the design of the UHV system. We are indebted to Dr. A.G. Wright and Mr. B. Wild of Electron Tubes Ltd.-UK and Mr. Y. Breslav of ElMul Technologies, Israel, for very useful discussions and to Mr. Edward McEntee of T&E Industries Inc., USA, for his devoted cooperation in the detector package development. Special thanks are addressed to Dr. E. Shefer, presently at Philips-Israel, and to Dr. A. Buzulutskov from BINP, Russia for their long cooperation on the GPMT project. 

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