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Gaseous photomultipliers with solid photocathodes for the detection of sparks, flames and dangerous gases
Nuclear Instruments and Methods in Physics Research A 505 (2003) 207–210
Gaseous photomultipliers with solid photocathodes for the detection of sparks, flames and dangerous gases P. Carlson, T. Francke, B. Lund-Jensen, V. Peskov* Department of Physics, Royal Institute of Technology, Stockholm Physics Center, Stockholm 10691, Sweden
Abstract In many applications, it is necessary to detect sparks or flames in daylight conditions or in illuminated areas. Most flames emit strongly in the ultraviolet spectrum (180–280 nm), and this property can be used for reliable identification of flames. We have developed new spark and flame detectors based on gaseous photomultipliers with CsI, CuI or CsTe photocathodes. A modified version of the detector can also detect smoke and dangerous vapors. These detectors are able to perform complex monitoring and detection functions. Some of their advantages are: low cost, high sensitivity, large output signal and operation under battery power. Gaseous photomultipliers can be position sensitive and, if necessary, be used in combination with various optical systems, for example for monitoring flames from space. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.40 Keywords: Photocathodes; Flame detection
1. Introduction In many applications it is necessary to detect sparks or flames in daylight conditions or in illuminated areas. Relevant examples could be refinery or chemical plants, oil production and drilling platforms, flammable liquid storage and loading facilities, aircraft/vehicle maintenance facilities, textile and paper manufacturing, underground and surface mining. Most flames in air emit very strongly in the ultraviolet (UV) spectrum (180–280 nm) [1], and this can be used for the identification of fire. The aim of this work is to investigate the possibility of using gaseous detectors with solid *Corresponding author. Tel.: +46-86222308. E-mail address: [email protected] (V. Peskov).
photo cathodes for detection of sparks and flames. Gaseous detectors with solid photocathodes were developed by some of us earlier [2,3] for other applications. These detectors have a quantum efficiency comparable to or even higher than the best commercial PMTs, but are also very inexpensive and easy to manufacture, and can be made with either small and compact or with a large sensitive area.
2. Test setups Two test setups were used in this work. The first was oriented towards spark and flame detection, while the second one was intended for studies of the detection of smoke and dangerous vapors.
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01053-2
208
P. Carlson et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 207–210
2.1. Test setup for the detection of fires The first setup is schematically presented in Fig. 1. It consists of a gaseous detector with a solid photocathode, a vacuum pump and a gas system (not shown for simplicity). Sparks and flames produced in air were used as a test source of UV radiation. Two detector designs were used in this work. A first, ‘‘experimental’’ one allowed the detector to be opened in order to make changes, for example installing various photocathodes. This detector was, for simplicity, flushed with gas. The second design was industry/application oriented and thus sealed. 2.1.1. The detector flushed with gas The design follows the schematic drawing of Fig. 1. It was basically a single-wire counter with a CaF2 window assembled from a standard stainless-steel tube cross. The diameter of the tube was 40 mm and the diameter of the gold-coated tungsten wire was 0.5 mm. Compared to traditional single-wire counters, this detector had several modifications: part of the cathode (a disc in this design) was covered with a photosensitive layer, CsI or CuI, and placed as close as possible to the anode wire. The anode wire was, depending on the design, 0.3–1 mm in diameter thus 15–50 times thicker than that in the usual gaseous detectors. In this work we have tested photosensitive layers manufactured by various techniques: vacuum evaporation, spraying technique, or deposition from the alcohol-based solvents. Detailed descrip-
tions of these techniques can be found elsewhere [2,4]. After manufacturing, the disc coated by the photosensitive layer was installed (in air) in the detector. This operation took 5–10 min. The detector was subsequently flushed with the P10 gas at 1 atm. In most of the tests we also heated the photocathode (50–80 C) to remove residual water content. Sparks were produced in air between a sharp anode tip and a metallic cathode plane. As test flames we used a cigarette lighter, flames from matches, and gasoline and alcohol flames, in air. 2.1.2. Sealed detectors This version was based on commercially available single-wire detectors, obtained from the Institute for Vacuum Technique, Moscow.1 They are made of stainless-steel tubes with diameters of 15 and 20 mm, 5 cm in length, and having a 1-mm anode wire. A glass supporting structure (glass interface) exists at the both ends between the cathode and the anode wire. All these detectors had LiF2 windows. For the photocathode manufacturing the glass seals at the interface and the window were opened. The CsI and CuI photocathodes were manufactured by the techniques mentioned above. The detectors were then pumped to a vacuum of 106 Torr, heated to 50 C, filled with gas (P10 with ethyl ferrocene vapors at a total pressure of 1 atm—see Ref. [4] for more details) and sealed. In collaboration with Reagent Research Center we also made some first attempts to develop CsTe photocathodes (see preliminary results in Ref. [5]). Unfortunately, these photocathodes could not be exposed to air, so a more sophisticated technology of detector assembling must be used. 2.2. Setup for evaluation of smoke and vapor detectors The setup for the study of smoke and vapor was similar to the ones presented in Fig. 1. The only difference was that the previous source of the UV photons (sparks and flames in air) was replaced by a vacuum ultraviolet (VUV) source—a corona
Fig. 1. A schematic diagram of the test setups.
1
Prepared by L.S. Sorokin, Types: MST-17, SFM-1, SFM-3.
P. Carlson et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 207–210
discharge in Ar or Xe. The corona discharge was initiated between electrodes of a modified singlewire counter with a LiF2 window and with an anode wire diameter of 50 mm and a cathode diameter of 15 mm. Such a discharge of about 1 mA current emits strong VUV emission of eximer molecules [6]. Any changes in the transmission in the gap between the VUV source and the detector will cause a change of the response in the detector which can be used to identify smoke or dangerous levels of various gases.
3. Results 3.1. Results for open flames
Quantum efficiency (%), Intensity (arb. units)
3.1.1. Results with flushed gas detectors Fig. 2 shows quantum efficiency of our detectors as well as typical emission spectra of flames in air. One can see that the quantum efficiency of CuI, CsI and especially CsTe photocathodes overlaps with the flame emission spectra. At the same time, such detectors are practically insensitive to visible light. The standard way to evaluate the flame detectors is to determine the maximum distance at which some fixed intensity of the flame radiation can be detected. Commercially available detectors
20 18 16 14 12 10 8 6 4 2 0 180
230
280
330
3 80
Wavelength (nm)
Fig. 2. Typical flame spectra in air (open symbols) [1] and quantum efficiency of gaseous detectors with various photocathodes: CsI (diamonds), CuI (triangles), CuI with ethyl ferrocene adsorbed layer (large squares), and CsTe (small squares).
209
are able to record a gasoline fire of 30 30 cm2 at a distance of 30 m [1]. Results of our tests performed with various flames (including a standard one) show that the sensitivity of our detectors was comparable to or better than that of the commercial ones. For example, detectors with CuI and CsI photocathodes (active area of B3 cm2) have a sensitivity that can reliably detect the flame from a match on a distance of a few meters in daylight conditions, with a counting rate produced by the flame of B103 Hz, while the noise counting rate was B2 Hz. A flame from a cigarette lighter was detected at a distance of 7–10 m with a counting rate of B100 Hz. Note also that, since our detectors are very inexpensive and simple, they can be easily manufactured with large sensitive area (up to 1 m2) and the sensitivity will be increased proportionally to the surface area. 3.1.2. Results with sealed gaseous detectors After obtaining encouraging preliminary results from the experimental version we repeated some measurements with the sealed detectors. The response was about half of that from the flushed detectors, and thus scaled well with the photocathode area, which was half the size of the flushed detectors. We also performed a long-term stability test of these detectors. Runs with the detectors having the CsI photocathodes were performed for 2 months. Measurements with the CuI photocathode were performed for almost 1 year. The runs were performed in practical conditions: detectors were continuously exposed to daylight and occasionally to fire radiation. For both detectors the sensitivity variations during these tests were not more than 15%. Unfortunately, only short-term data are available for the CsTe photocathode, and it is premature to make any conclusions about its long-term stability. 3.2. Preliminary results in the detection of smoke and dangerous gases Preliminary measurements from detection of smoke and dangerous vapors were performed with
P. Carlson et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 207–210
210
be achieved. Note that all these measurements were performed in daylight conditions.
Nuber of counts (Hz)
1000000
100000
4. Conclusion
10000
1000 0
1
2
3 4 Time (min)
5
6
7
Fig. 3. Number of counts from the detector when alcohol (diamond), gasoline (squares) and water vapors from human breath (triangles) were introduced to the gap.
the simplified test set described in Section 2.2 and very promising results were obtained. In the case of smoke, the accuracy in the detection of the Xe excimer light attenuation was 2–3% and it was mainly due to the fluctuations in the corona current. Thus, the sensitivity of the detection of the smoke per unit length is inversely proportional to the distance between the light source and the detector, L: DT=Dx ¼ 2 3%=L: In this work, for simplicity, we used only Ar eximer radiation in the detection of dangerous vapors in air. It is known that the VUV light absorption coefficient in air has a minimum around 122.6 nm [7] and this matches rather well with the Ar eximer emission peak. As a result, one can use a 5–10 mm gap between the VUV source and the detector. We also performed measurements of the detectors (with CsI photocathode) response with time when alcohol, gasoline or water vapors were introduced in the gap. Depending on conditions the signal drops by a factor of 1.5–50 (see Fig. 3). Taking into account a typical cross-section of the absorption of 1017 cm2 [7], one can estimate that B1% of the vapor concentration in air can be reliably detected even at distances L below 1 cm. With a stronger VUV source, better sensitivity can
It is demonstrated in this work that gaseous detectors with solid photocathodes are able to perform multiple functionality tasks: flame, smoke and dangerous vapor detection as well as monitoring. Note that, gaseous detectors can be positionsensitive [2,3] and, if necessary, be used in combination with various optical systems. A relevant example could be the monitoring of flames from space (oil pipe flames, forest fires, etc.) (see Ref. [8]). It has been shown that gaseous detectors with solid photocathodes are competitive with the present UV fire detectors.
Acknowledgements We thank J. Kadyk (LBNL, USA) for discussions.
References [1] A Guide for Selection of the Right Flame Detector for Your Application, http://www.spectrex-inc.com/sharpeye/ guide.htm. [2] V. Peskov, Gaseous detector with solid photocathodes, CERN Yellow Report CERN 97-06, July 1997. [3] T. Francke. A ring imaging Cherenkov counter using a solid radiator, Ph.D. Thesis, Royal Institute of Technology, Stockholm, 1991. [4] G. Charpak, et al., Nucl. Instr and Meth A 310 (1991) 128. [5] T. Francke, et al., Novel position-sensitive gaseous detectors with solid photocathodes, IEEE Nucl. Sci., NS-49 (2002) 977. [6] V. Peskov, Journ. Prikl. Spectr. 30 (1979) 860. [7] A.N. Zaidel, E.Ia. Sreider, Vacuum Ultraviolete Spectroscopy, Nayka, Moscow, 1967 (in Rusian). [8] NOAA defense meteorological satellite program at NGDC, http://www.ngdc.noaa.gov/dmsp/fires/globalfires.html.
Gaseous photomultipliers with solid photocathodes for the detection of sparks, flames and dangerous gases P. Carlson, T. Francke, B. Lund-Jensen, V. Peskov* Department of Physics, Royal Institute of Technology, Stockholm Physics Center, Stockholm 10691, Sweden
Abstract In many applications, it is necessary to detect sparks or flames in daylight conditions or in illuminated areas. Most flames emit strongly in the ultraviolet spectrum (180–280 nm), and this property can be used for reliable identification of flames. We have developed new spark and flame detectors based on gaseous photomultipliers with CsI, CuI or CsTe photocathodes. A modified version of the detector can also detect smoke and dangerous vapors. These detectors are able to perform complex monitoring and detection functions. Some of their advantages are: low cost, high sensitivity, large output signal and operation under battery power. Gaseous photomultipliers can be position sensitive and, if necessary, be used in combination with various optical systems, for example for monitoring flames from space. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.40 Keywords: Photocathodes; Flame detection
1. Introduction In many applications it is necessary to detect sparks or flames in daylight conditions or in illuminated areas. Relevant examples could be refinery or chemical plants, oil production and drilling platforms, flammable liquid storage and loading facilities, aircraft/vehicle maintenance facilities, textile and paper manufacturing, underground and surface mining. Most flames in air emit very strongly in the ultraviolet (UV) spectrum (180–280 nm) [1], and this can be used for the identification of fire. The aim of this work is to investigate the possibility of using gaseous detectors with solid *Corresponding author. Tel.: +46-86222308. E-mail address: [email protected] (V. Peskov).
photo cathodes for detection of sparks and flames. Gaseous detectors with solid photocathodes were developed by some of us earlier [2,3] for other applications. These detectors have a quantum efficiency comparable to or even higher than the best commercial PMTs, but are also very inexpensive and easy to manufacture, and can be made with either small and compact or with a large sensitive area.
2. Test setups Two test setups were used in this work. The first was oriented towards spark and flame detection, while the second one was intended for studies of the detection of smoke and dangerous vapors.
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01053-2
208
P. Carlson et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 207–210
2.1. Test setup for the detection of fires The first setup is schematically presented in Fig. 1. It consists of a gaseous detector with a solid photocathode, a vacuum pump and a gas system (not shown for simplicity). Sparks and flames produced in air were used as a test source of UV radiation. Two detector designs were used in this work. A first, ‘‘experimental’’ one allowed the detector to be opened in order to make changes, for example installing various photocathodes. This detector was, for simplicity, flushed with gas. The second design was industry/application oriented and thus sealed. 2.1.1. The detector flushed with gas The design follows the schematic drawing of Fig. 1. It was basically a single-wire counter with a CaF2 window assembled from a standard stainless-steel tube cross. The diameter of the tube was 40 mm and the diameter of the gold-coated tungsten wire was 0.5 mm. Compared to traditional single-wire counters, this detector had several modifications: part of the cathode (a disc in this design) was covered with a photosensitive layer, CsI or CuI, and placed as close as possible to the anode wire. The anode wire was, depending on the design, 0.3–1 mm in diameter thus 15–50 times thicker than that in the usual gaseous detectors. In this work we have tested photosensitive layers manufactured by various techniques: vacuum evaporation, spraying technique, or deposition from the alcohol-based solvents. Detailed descrip-
tions of these techniques can be found elsewhere [2,4]. After manufacturing, the disc coated by the photosensitive layer was installed (in air) in the detector. This operation took 5–10 min. The detector was subsequently flushed with the P10 gas at 1 atm. In most of the tests we also heated the photocathode (50–80 C) to remove residual water content. Sparks were produced in air between a sharp anode tip and a metallic cathode plane. As test flames we used a cigarette lighter, flames from matches, and gasoline and alcohol flames, in air. 2.1.2. Sealed detectors This version was based on commercially available single-wire detectors, obtained from the Institute for Vacuum Technique, Moscow.1 They are made of stainless-steel tubes with diameters of 15 and 20 mm, 5 cm in length, and having a 1-mm anode wire. A glass supporting structure (glass interface) exists at the both ends between the cathode and the anode wire. All these detectors had LiF2 windows. For the photocathode manufacturing the glass seals at the interface and the window were opened. The CsI and CuI photocathodes were manufactured by the techniques mentioned above. The detectors were then pumped to a vacuum of 106 Torr, heated to 50 C, filled with gas (P10 with ethyl ferrocene vapors at a total pressure of 1 atm—see Ref. [4] for more details) and sealed. In collaboration with Reagent Research Center we also made some first attempts to develop CsTe photocathodes (see preliminary results in Ref. [5]). Unfortunately, these photocathodes could not be exposed to air, so a more sophisticated technology of detector assembling must be used. 2.2. Setup for evaluation of smoke and vapor detectors The setup for the study of smoke and vapor was similar to the ones presented in Fig. 1. The only difference was that the previous source of the UV photons (sparks and flames in air) was replaced by a vacuum ultraviolet (VUV) source—a corona
Fig. 1. A schematic diagram of the test setups.
1
Prepared by L.S. Sorokin, Types: MST-17, SFM-1, SFM-3.
P. Carlson et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 207–210
discharge in Ar or Xe. The corona discharge was initiated between electrodes of a modified singlewire counter with a LiF2 window and with an anode wire diameter of 50 mm and a cathode diameter of 15 mm. Such a discharge of about 1 mA current emits strong VUV emission of eximer molecules [6]. Any changes in the transmission in the gap between the VUV source and the detector will cause a change of the response in the detector which can be used to identify smoke or dangerous levels of various gases.
3. Results 3.1. Results for open flames
Quantum efficiency (%), Intensity (arb. units)
3.1.1. Results with flushed gas detectors Fig. 2 shows quantum efficiency of our detectors as well as typical emission spectra of flames in air. One can see that the quantum efficiency of CuI, CsI and especially CsTe photocathodes overlaps with the flame emission spectra. At the same time, such detectors are practically insensitive to visible light. The standard way to evaluate the flame detectors is to determine the maximum distance at which some fixed intensity of the flame radiation can be detected. Commercially available detectors
20 18 16 14 12 10 8 6 4 2 0 180
230
280
330
3 80
Wavelength (nm)
Fig. 2. Typical flame spectra in air (open symbols) [1] and quantum efficiency of gaseous detectors with various photocathodes: CsI (diamonds), CuI (triangles), CuI with ethyl ferrocene adsorbed layer (large squares), and CsTe (small squares).
209
are able to record a gasoline fire of 30 30 cm2 at a distance of 30 m [1]. Results of our tests performed with various flames (including a standard one) show that the sensitivity of our detectors was comparable to or better than that of the commercial ones. For example, detectors with CuI and CsI photocathodes (active area of B3 cm2) have a sensitivity that can reliably detect the flame from a match on a distance of a few meters in daylight conditions, with a counting rate produced by the flame of B103 Hz, while the noise counting rate was B2 Hz. A flame from a cigarette lighter was detected at a distance of 7–10 m with a counting rate of B100 Hz. Note also that, since our detectors are very inexpensive and simple, they can be easily manufactured with large sensitive area (up to 1 m2) and the sensitivity will be increased proportionally to the surface area. 3.1.2. Results with sealed gaseous detectors After obtaining encouraging preliminary results from the experimental version we repeated some measurements with the sealed detectors. The response was about half of that from the flushed detectors, and thus scaled well with the photocathode area, which was half the size of the flushed detectors. We also performed a long-term stability test of these detectors. Runs with the detectors having the CsI photocathodes were performed for 2 months. Measurements with the CuI photocathode were performed for almost 1 year. The runs were performed in practical conditions: detectors were continuously exposed to daylight and occasionally to fire radiation. For both detectors the sensitivity variations during these tests were not more than 15%. Unfortunately, only short-term data are available for the CsTe photocathode, and it is premature to make any conclusions about its long-term stability. 3.2. Preliminary results in the detection of smoke and dangerous gases Preliminary measurements from detection of smoke and dangerous vapors were performed with
P. Carlson et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 207–210
210
be achieved. Note that all these measurements were performed in daylight conditions.
Nuber of counts (Hz)
1000000
100000
4. Conclusion
10000
1000 0
1
2
3 4 Time (min)
5
6
7
Fig. 3. Number of counts from the detector when alcohol (diamond), gasoline (squares) and water vapors from human breath (triangles) were introduced to the gap.
the simplified test set described in Section 2.2 and very promising results were obtained. In the case of smoke, the accuracy in the detection of the Xe excimer light attenuation was 2–3% and it was mainly due to the fluctuations in the corona current. Thus, the sensitivity of the detection of the smoke per unit length is inversely proportional to the distance between the light source and the detector, L: DT=Dx ¼ 2 3%=L: In this work, for simplicity, we used only Ar eximer radiation in the detection of dangerous vapors in air. It is known that the VUV light absorption coefficient in air has a minimum around 122.6 nm [7] and this matches rather well with the Ar eximer emission peak. As a result, one can use a 5–10 mm gap between the VUV source and the detector. We also performed measurements of the detectors (with CsI photocathode) response with time when alcohol, gasoline or water vapors were introduced in the gap. Depending on conditions the signal drops by a factor of 1.5–50 (see Fig. 3). Taking into account a typical cross-section of the absorption of 1017 cm2 [7], one can estimate that B1% of the vapor concentration in air can be reliably detected even at distances L below 1 cm. With a stronger VUV source, better sensitivity can
It is demonstrated in this work that gaseous detectors with solid photocathodes are able to perform multiple functionality tasks: flame, smoke and dangerous vapor detection as well as monitoring. Note that, gaseous detectors can be positionsensitive [2,3] and, if necessary, be used in combination with various optical systems. A relevant example could be the monitoring of flames from space (oil pipe flames, forest fires, etc.) (see Ref. [8]). It has been shown that gaseous detectors with solid photocathodes are competitive with the present UV fire detectors.
Acknowledgements We thank J. Kadyk (LBNL, USA) for discussions.
References [1] A Guide for Selection of the Right Flame Detector for Your Application, http://www.spectrex-inc.com/sharpeye/ guide.htm. [2] V. Peskov, Gaseous detector with solid photocathodes, CERN Yellow Report CERN 97-06, July 1997. [3] T. Francke. A ring imaging Cherenkov counter using a solid radiator, Ph.D. Thesis, Royal Institute of Technology, Stockholm, 1991. [4] G. Charpak, et al., Nucl. Instr and Meth A 310 (1991) 128. [5] T. Francke, et al., Novel position-sensitive gaseous detectors with solid photocathodes, IEEE Nucl. Sci., NS-49 (2002) 977. [6] V. Peskov, Journ. Prikl. Spectr. 30 (1979) 860. [7] A.N. Zaidel, E.Ia. Sreider, Vacuum Ultraviolete Spectroscopy, Nayka, Moscow, 1967 (in Rusian). [8] NOAA defense meteorological satellite program at NGDC, http://www.ngdc.noaa.gov/dmsp/fires/globalfires.html.