ABSOLUTE CALIBRATION OF VACUUM ULTRAVIOLET XENON FLASH LAMPSUSED FOR PHOTOIONIZATION OF METEOR SMOKE PARTICLES

Sebastian M. Ernst (1), Slawomir Skruszewicz (2), Boris Strelnikov (1) and Markus Rapp (1)

(1) Leibniz-Institute of Atmospheric Physics, Schlossstrasse 6, 18225 Kuehlungsborn, Germanyernst@iap-kborn.de, phone +49 (0) 38293 68 331, fax +49 (0) 38293 68 50

(2) Institute of Physics, Rostock University, 18051 Rostock, Germanyslawomir.skruszewicz@uni-rostock.de, phone +49 (0) 381 498 6816, fax +49 (0) 381 498 6802

ABSTRACT

During the years 2006 through 2010, nine ECOMA rocket payloads were launched from the Andøya Rocket Range at 69˚N in northern Norway. The main instrument carried within the payloads was the ECOMA particle detector (PD), which uses active photoionization by xenon flash lamps to detect aero-sols in the mesosphere / lower thermosphere (MLT). A quantitative data analysis of these measurements re-quires an absolute calibration of the utilized xenon flash lamps. In the present paper, we present the design, optimization, operation, data analysis and res-ults of experiment set-ups for an absolute calibration of vacuum ultraviolet (VUV) light sources, in particu-lar the xenon flash lamps used in the ECOMA project.

1 INTRODUCTION

The objective of this paper is to detail the absolute cal-ibration of xenon (Xe) vacuum-ultraviolet (VUV) flash lamps. The lamps are used to detect aerosols from sounding rockets, primarily Meteor Smoke Particles (MSP), which are introduced in the beginning. Follow-ing, a detailed investigation prior the design of our VUV calibration experiment is described. VUV calib-rations have been performed for different purposes since the first half of the 20th century. Nevertheless, a certain optimal strategy does still not exist making this kind of experiment highly dependant on experience values, which have not been perfectly documented in all cases. An appropriate strategy, set-up and work-flow for our purpose had therefore to be determined. Based on this work, first experimental results are presented in the end.

2 METEOR SMOKE PARTICLES

Meteoric material reaching the Earth’s atmosphere ab-lates in the altitude range between 70–100 km due to frictional heating and is believed to re-condense into nanometre-sized so-called “smoke particles” [Hunten et al., 1980]. Fig. 1 illustrates this process. The term “smoke” originates in the space community and refers to particles formed by condensation or chemical reac-

tion from molecularly dispersed matter [Whytlaw-Gray, 1936].

Figure 1. Fate of meteoric material entering the Earth’s atmosphere. The processes are believed to occur at an altitude of 70 to 100 km. After Megner et al. [2006].

MSP are especially important in the middle atmo-sphere where dust sources from below are believed to be neglible. MSP are suspected to play a major role in middle atmospheric phenomena, such as noctilucent clouds (NLC), polar mesospheric summer echoes (PMSE), metal layers, and heterogeneous chemistry controlling key species such as water vapour – see Rapp et al. [2007] for a detailed discussion. MSP are considered the most likely candidate for being con-densation nuclei in related processes making them rel-evant for investigation. For a deeper understanding, simultaneous measurements of particle properties and background properties are important. This includes number densities, size distributions, the ratio of charged particles to neutral particles, as well as the de-termination of their composition [e.g. Megner et al., 2006].

2.1 The ECOMA detector

One recent effort for getting detailed experimental data of those properties is the German-Norwegian “Existence and Charge state Of meteoric dust particles in the Middle Atmosphere” (ECOMA) project [Rapp & Strelnikova, 2009, and Rapp et al., 2010]. During the years 2006 through 2010, nine ECOMA rocket payloads were launched from the Andøya Rocket Range at 69˚N in northern Norway. The main instru-ment carried within the payloads was the ECOMA particle detector (PD). As presented in Fig. 2, it com-

_________________________________________________ Proc. ‘20th ESA Symposium on European Rocket and Balloon Programmes and Related Research’ Hyère, France, 22–26 May 2011 (ESA SP-700, October 2011)

bines active photoionization (and photodetachment) with a sensitive charge measurement by means of a Faraday cup to measure the concentration of MSP. This fundamental design avoids limitations from aero-dynamics and secondary charging effects and does not depend on the a-priori charges of the particles.

Figure 2. Schematic principle of operation of the ECOMA detector. Within the presented figure, the direction of flight would be to the right. See text for details.

Figure 3. Latest generation of the ECOMA detector. A prominent fea-ture are the three numbered Xe flash lamps in the centre. Around them, in the front, the shielding grids can be seen, in the back the gold-plated and perforated electrode.

The detecting element of the PD is a gold-plated elec-trode at the bottom of a conducting cylindrical hous-ing. The opening of this structure is shielded by two grids operated at +3 V and -3 V, respectively, prevent-ing free electrons and positive ions from entering the detector. It is therefore referred to as a Faraday cup.

Particles with a certain altitude dependant cut-off size (e.g. 2/5 nm at 85/75 km) are energetic enough to enter the detector [Rapp et al., 2005, and Hedin et al., 2007] and can be detected if charged. The interest in also detecting uncharged as well as smaller particles leads to the introduction of the three ultraviolet (UV) Xe flash lamps, which are combined in this kind of in-strument for the first time.

The flash-photons photoionize (and/or photodetach) electrons from particles in front of the detector. Hav-ing a rather high kinetic energy, these photo-electrons can easily penetrate the shielding of the Faraday cup and therefore be detected. The lamps flash in a se-quence one after another and are supposed to emit light in different sections of the UV spectrum. This al-lows the ionization of particles having different ioniza-tion potentials. It gives us the opportunity for a select-ive study of various kinds of particles – having differ-ent ionization potentials – which gives further clues on their overall composition. Examples for the expec-ted ionization thresholds of particles consisting of Fe2O3 or SiO2 are 5,5 or 9,8 eV, respectively. The measured physical property is the current at the elec-trode of the PD.

Figure 4. Raw-data samples of flash currents recorded at altitudes between 85 and 86 km on the upleg part of the trajectory of sounding rocket flight ECOMA07. Coloured diamonds mark data samples which were recorded immediately after the cyclic firing of the flash lights. Red refers to the effect of the lamp with the highest photon energy, green to medium and blue to weakest.

The current shows dominant peaks when one of the Xe lamps flashes, which are considered as photo-elec-tron current measurements – see Fig. 4 for a raw data sample illustrating this behaviour. Since the flash lamps were fired in a sequence starting with the smal-lest maximum photon energy over the medium one to the one with largest photon energy, it is no surprise that the maximum photoelectron currents increase with increasing altitude (i.e., flight time). Notably, photoelectron peaks from the weakest flash lamp can hardly be discerned from instrument noise indicating

that the corresponding photon energy range can hardly photoionize the particles. For a more quantitat-ive interpretation of these data, however, an absolute characterization of the flash lamps becomes essential.

3 VACUUM-UV

This chapter deals with the calibration of the Xe flash lamps used in the ECOMA detector. It is explained step by step detailing the overall strategy.

3.1 Introduction to the VUV wavelength-range

Mainly, two SI-based units are used: nano meter (nm) and electron volt (eV). They can be converted as fol-lows

E= h∗cλ

(1)

where E is the kinetic energy of a photon in eV, h is the Planck constant (4,1 10-15 eV s), c is the speed of light (3,0 108 m/s) and λ the corresponding wavelength in m. For wavelengths in the context of light and spectroscopy, measures in nm are widely used, while for photoionization and -detachment measures in eV make more sense.

The nomenclature for UV light and its divisions is not perfectly unique and varies throughout literature. Within this paper, UV is defined as light in a range from 1 to 400 nm (1240 to 3,1 eV). VUV, sometimes also referred to as UV-C, is considered to be the light between 100 to 200 nm (12,4 to 6,2 eV). The name is based on the fact, that light below 200 nm is blocked by air, so any work with light of shorter wavelengths has to be performed under vacuum. The wavelength, at which a material starts to block light, is also re-ferred to as cut-off-wavelength or just cut-off. Most types of quartzite-glass optics have cut-offs at wavelengths shorter than 180 nm (6,9 eV), although special purified quartzite-glass lenses for laser applica-tions do exist, which permit light of wavelengths down to 160 nm (7,7 eV) to pass. Notable optical ma-terials permitting almost the entire range of VUV to pass are LiF (cut-off at about 105 nm / 11,8 eV) and MgF2 (nominal cut-off at about 115 nm / 10,8 eV).

Working in the VUV range poses problems with win-dow materials of any kind. It is a good idea to avoid using lenses, which have to be substituted with mir-rors as optical elements instead. Optical set-ups have to be built either in an environment, which can be evacuated or in a helium (He) atmosphere, which can theoretically be used as an alternative, as it permits VUV to pass even at room pressure.

3.2 The xenon flash lamps

The Xe flash lamps are unmodified Perkin Elmer FX-1160 series products. The lamps are designed to flash at a variable frequency. A single flash emits light with an energy of up to about 0,5 J according to the data sheet provided by the manufacturer. The series has three types, which differ in the lamps' window materi-al only. According to the data sheet, the FX-1162 has a magnesium fluorite (MgF2) window, the FX-1161 has a window based on quartzite (+SiO2) and the FX-1160 based on borosilicate (+B2O3). Nominal transmittance curves of the respective materials as provided in the data sheet are shown in Fig. 5. The opening angle of the lamp is given as 30°. Parameters like the exact composition of the gas inside the lamps (with Xe being probably just a major component), its pressure, details on the ignition, thermal loss and the time, which a single flash lasts, are not included the provided data.

Figure 5. The cut-off wavelengths and nominal transmissions of the three Perkin Elmer FX-1160 series lamps are show. For a comparison, the cut-off of air is marked at the top.

A detailed analysis of the lamps prior to investigating the spectra was conducted. It primarily showed, that a single flash lasts for about 1,5 µs and has some kind of an afterglow (less than 10% of a flash's peak intensity) of more than 10 µs. Measurements have been per-formed with a radiation-hard Si photo diode in the near-visible range (300 to 400 nm / 4,1 to 3,1 eV). The lamp is powered by the discharge of a capacitor having a capacitance of 0,5 µF and is being charged at about 700 V. The voltage is adjustable in the lamps' power supplies and was kept in the same configuration as im-plemented in the ECOMA's electronic's flight hard-ware. Based on the discharge characteristics of the ca-pacitor, an energy per flash (including thermal loss) in the range of 0,2 to 0,3 J can be estimated resulting in a peak power emission of roughly 0,4 MW. This estima-tion became the major reason to develop our own cal-ibration set-up. The light's intensity was by orders of magnitude higher than what most detectors, which we

could find during an initial investigation, could handle before running into e.g. saturation effects.

3.3 Absolute calibration strategy in VUV

The basic idea is to perform a cross-calibration by comparing light sources. Therefore, a light source with known characteristics – a radiometric standard – is required, to compare the Xe lamps with. There are two fundamental groups of light sources, window-less lamps and lamps with windows. The following section just gives a brief overview, while a good conclusive overview and comparison of radiometric standards can be found in Klose et al. [1987].

Window-less lamps can for example be based on noble gas such as Ar, which is injected into a capillary, ig-nited by a high voltage discharge and removed by powerful vacuum pumps immediately after ignition. To prevent any of the gas from entering the vacuum nearby, there is usually a series of chambers connected only by tiny apertures. Each of those chambers is evacuated separately permitting a high pressure-gradi-ent between the discharge capillary and the vacuum chamber. Spectra generated with such lamps are al-most perfectly reproducible, do not suffer from the in-fluence of varying transmissions of any window ma-terials and can theoretically even be used as an irradi-ance source standard. The major disadvantages of these lamps are the high level of complexity and the need for various pumps and intense cooling of the discharge capillary.

The most prominent examples for VUV lamps with front windows are deuterium lamps. Due to varying properties of window materials, such lamps do not give reproducible spectra by design. They must be cal-ibrated against an absolute source, usually a synchro-tron, which results in the terminology of VUV lamps with window being irradiance transfer-standards. An absolute radiometric (source) standard is transferred with the help of another VUV lamp. Deuterium lamps are widely considered the only portable transfer stand-ards available. Based on their low degree of complexity and preferable portability, they become the usual first choice as a radiometric standard if the rather complic-ated construction of a window-less lamp is not inten-ded. We decided to go for a cross-calibration with a deuterium lamp as a transfer-standard instead of imple-menting a window-less lamp. Having our own (trans-fer-) standard also allowed us to set up and design our own calibration experiment. Thus, we can investigate the Xe lamps in more detail. The two facilities, which are capable of conducting an absolute calibration of light sources (usually deuterium lamps) against syn-

chrotron radiation on a regular basis, are run by the National Institute of Standards and Technology (NIST) in the United States and by the Physikalisch-Technische Bundesanstalt (PTB) – the German nation-al metrology institute. Consequently, we arranged a calibration of a deuterium lamp at the PTB. For a de-tailed description of the calibration set-up used at PTB, the reader may be referred to Paustian et al. [2005].

Deuterium is known for its smooth spectrum above 165 nm, but shows complex molecular lines between 115 and 165 nm (10,8 and 7,5 eV) making this range difficult to apply. It requires to take special care when designing a spectrometer set-up [Nettleton & Preston, 1981]. Besides the molecular lines, most problems are caused by the window material, which is commonly used for VUV deuterium lamps, i.e. MgF2. Cracked hydrocarbon deposits from within the vacuum cham-ber tend to accumulate on MgF2 surfaces under vacu-um conditions when being excited due to the lamp's operation. This generally causes a decrease of transmit-tance, while the process is not quantitatively predict-able. Systematic measurements by Key & Preston [1980] showed “ageing rates” ranging from +0,03 to -0,05 % per hour, depending on the wavelength. Ein-feld & Stuck [1976] observed shifts of the maximum radiance towards longer wavelengths after 10 to 100 hours of operation of deuterium lamps varying even for lamps of the same type. Deuterium lamps can be considered stable within reasonably short time inter-vals, about tens of hours, with time remaining an is-sue. It can be dealt with by precise planning and log-ging the history of operation.

3.4 VUV detectors

The basic options to detect VUV light range from photo plates, various photo diodes, photo multiplier or tubes, respectively, to ionization chambers.

The most traditional method in detecting VUV is us-ing photographic plates or films based on Schumann emulsion, which can be used in a vacuum [VanHoosi-er et al., 1977]. Although most films of this type are not produced any more on an industrial basis and al-though the manufacturing and handling poses enorm-ous challenges, they are still under development and used due to their preferable characteristics in terms of good resolution and variable geometries [Ben-Kish et al., 2000].

A more recent approach is to use photo diodes. De-pending on the materials and principles, which they can be based on, they are capable of detecting various sections of UV and VUV light. While the near-visible

range of UV can be covered with Si based photo di-odes, the VUV range requires more advanced materi-als. An overview by Monroy at al. [2003] lists com-mercially available diodes based on (artificial) dia-mond, GaN and AlGaN. With this field being under intense research, we compiled the characteristics of ex-amples of those materials from latest results. For Al-GaN, Saito et al. [2009] stated, that depending on the ratio of Al vs. Ga, such diodes can be sensitive from about 300 nm to at least 100 nm (4,1 to 12,4 eV) or even less. BenMoussa et al. [2008a] documented, that pure AlN shows even better results between 100 and 200 nm (12,4 and 6,2 eV) and is blind above that range. Another paper by BenMoussa et al. [2008b] also shows good results for diamond in this range. It is notable, that depending on the kind of manufacturing of those diodes, they can easily handle high intensities of light without degradation and still show a linear behaviour.

The probably most important option is the Mi-crochannel Plate Detector (MCP). MCPs are arrays of 104 to 107 or even more miniature electron photo-mul-tipliers [Ladislas Wiza, 1979]. First suggested in the 1930s and first actually produced in the 1960s, they have become the de-facto standard for imaging from the UV to the soft x-ray range. MCPs are space-re-solved detectors, so they literally allow taking photo-graphs within their range of sensitivity. Various types and configurations do exist, some of which can also be used for VIS spectroscopy. For a good overview, the reader is referred to Timothy [1983].

We decided to use an initial configuration based on a MCP combined with a phosphor screen with the in-tended option of later comparing the results to meas-urements conducted with AlN and AlGaN photo di-odes. The MCP allowed us to investigate the Xe lamps with relative ease in the beginning, as they can handle the high intensities from the Xe lamps' light without problems and permits the optical optimization of a calibration set-up. VUV photons produce electrons at the MCP's surface, which are then multiplied inside the channels of the MCP. Electrons coming from the MCP are then accelerated towards the phosphor screen, where they cause the emission of light in the visible range, which can be recorded with e.g. normal CCD cameras.

3.5 Experimental set-up

The experimental set-up or spectrometer consists of one lamp at a time, a vertical slit aperture, a monochro-mator and a detector. All parts are implemented in a way, that they can be operated under vacuum condi-tions.

Figure 6. Diagram of the principle of operation and the the actual exper -imental calibration set-up as seen from the top.

The lamps are exchangeable, so either one Xe lamp or the deuterium calibration lamp is mounted in the spec-trometer. Each lamp exchange requires a re-pressuriza-tion and opening of the vacuum chamber and evacu-ation afterwards. The Xe lamps are operated at 10 Hz for calibrations, while they have been operated at about 17 Hz during flight. Further tests within the cal-ibration have been conducted between 1 and 20 Hz to analyse performance and behaviour. Xe lamps are mounted inside the vacuum chamber while the entire power supply and trigger chain remains outside. Therefore, a modified feed-through was constructed and tested. The mounting of the Xe lamps in the calib-ration setup does not allow much of the generated heat to be removed, which causes the Xe lamps to con-tinuously heat up during operation.

The deuterium lamp used is a V01 by Heraeus Noble-light, emitting about 30 W of CW light. Is has a pre-heating phase of about 30 minutes. Its light is con-sidered stable after about another 30 minutes of opera-tion. The lamp is half mounted inside the vacuum chamber and half outside, while the actual vacuum seal is provided by a Viton ring around its glass body. The mounting construction was derived from the original construction used at PTB for calibrations of the same type of lamps.

Both the FX-1162 Xe lamps and the deuterium lamp have MgF2 windows, which show degradation effects as previously described. For reversing or minimizing them, their windows are polished with cotton swaps and a mixture of aluminium hydroxide powder and distilled water before each use. The FX-1161 and FX-

1160 Xe lamps' windows, quartzite and borosilicate, are cleaned with isopropyl alcohol (IPA) instead.

As a dispersive element, an Acton Research VM-502X normal-incidence vacuum monochomator has been implemented. It is a Seya-Namioka-type monochro-mator having only a concave grating mirror without further optical elements. The mirror has a 1200 L/mm grating and its focal length is given with 0,2 m. The device is rated for measurements between 50 and 300 nm. The wavelength projected on the exit of the monochromator can be adjusted by rotating the grat-ing mirror inside the spectrometer using an external controller. Originally, the VM-502X was shipped with entrance and exit slit apertures, but the exit slit aper-ture has been removed in favour of the MCP thus al-lowing images of sections of the spectra with a width of bit less than 100 nm to be taken. In order to obtain one “complete” spectrum, the grating mirror is rotated in a way that the spectrum shifts 12,5 nm per step for a general analysis and 5 nm per step for calibration – from 100 to 300 nm being in the middle of the pic-tures at the position of the former exit slit aperture. The width of the remaining entrance slit was varied in a range of near-zero to 5 µm.

The detector assembly consists of a MCP with a phos-phor screen as mentioned before. The phosphor screen emits light in the green section of the visible spectrum of light. We then take photographs of the screen with a monochromatic digital camera. For cal-ibration, the MCP is operated at 1,8 kV; the phosphor screen is operated at 3,9 kV. Although other configur-ations of operating voltages are possible, this proved to be the best one within technical limits to suppress background noise in the pictures. For the Xe lamps, an exposure time of the digital camera of 10 s proved to be optimal. It is realized by automatically accumu-lating 10 shots of 1 s each. The procedure is repeated 12 times per grating mirror orientation in order to re-ceive data with a satisfactory statistical error.

As stated before, the calibration is performed under vacuum conditions. The feasible maximum pressure for working with VUV light is about 10-4 mbar, but the safe operation of the used MCP requires a pressure of at least 10-6 mbar. Therefore, during the calibrations, an operational pressure inside the detector / mono-chromator section of the spectrometer of 10 -6 mbar is kept. Independently, we varied the operational pres-sure of the lamps' section of the spectrometer from10-4 to almost 10-8 mbar for tests, but keep it at10-5 mbar for calibration. It is notable, that for reduc-tion of background noise of the MCP, the Pirani gauges used for pressure measurements have to be switched off.

4 DATA-ANALYSIS

In the following chapter, we describe the data pro-cessing step by step from the raw data to actual spec-tra. Monochromatic digital images are taken from the phosphor screen. It is assumed, that their colour value per pixel is proportional to the actual brightness of the respective parts of the spectra. As previously men-tioned, a series of images is taken for a sequence of grating mirror positions. The middle sections of the pictures are then combined to “panoramic shots” of entire spectra as they can be seen in Fig. 7.

Figure 7. Examples for “panoramic shots” of entire spectra assembled from about 200 raw images each.

The images have to be further processed before the data can be used. First of all, the background has to be eliminated. This is achieved by the subtraction of im-ages taken with the light source switched off. Second, bright spots in the images or areas showing variations that are caused by the MCP have to be identified and either filtered out or excluded from further data pro-cessing. This is done both manually and automatically with experimental software filters.

Figure 8. Example for extracted data before the application of correc-tions or filters.

After this, the rows of pixels, which contain the spec-tra, are extracted. By stacking them, we receive a spec-

trum. An example is shown in Fig. 8. Once the data is extracted, it needs to be corrected for optical effects such as the wavelength- and orientation-dependant re-flectivity of the used grating mirror. One can see the related effects before correction in the spectrum in Fig. 8, as is shows “steps” between its sections.

Figure 9. Examples for extracted and improved data.

After the corrections are done, we get a first impres-sion of the not-yet-calibrated spectra presented in Fig. 9. Features like maxima and minima become vis-ible depending on the achieved degree of images sharp-ness, probably indicating molecular lines. To finally get calibrated Xe spectra, the spectrum of the deuteri-um calibration lamp is recorded the same way as the Xe lamps' spectra previously.

Figure 10. Deuterium spectra. The red curve shows the original spec-trum as provided by the PTB. The blue curve shows the same spectrum but recorded with our experimental set-up. Dotted lines indicate, that there is yet no conclusive data for these sections available.

We finally have two versions of the deuterium spec-trum as we show in Fig. 10. One being absolute as provided by the PTB and one measured with our ex-perimental calibration set-up. Consequently, we can compute a relative sensitivity as a function of wavelength of our set-up by dividing the original deu-terium spectrum by our deuterium spectrum. This is

then applied to the data of the Xe lamps.

Figure 11. An example for a calibrated spectrum of a Xe lamp with a MgF2 window. The data is proportional to by an yet to be determined factor to absolute values in photons per wavelength.

We receive a calibrated spectrum as presented in Fig. 11 in the end. Note, that the attribute calibrated in this context still implies, that it is relative. It does not give any information on how many photons per wavelength are emitted. To get this information, a yet unknown factor has to be determined due to e.g. the not perfectly known opening angle of the Xe lamps. Nevertheless, it makes possible to compare the differ-ent Xe lamps to each other and second, it allows for comparing different sections of the spectra to each other.

4.1 Observations

A significantly reduced sensitivity of the MCP detect-or above 200 nm can be observed, so data of wavelengths greater than 220 nm is neither reliable nor feasible anymore.

A degrease of transmission of MgF2 can barely be re-cognized in the data during less than 10 hours of oper-ation per lamp calibration. Besides the new lamps which have not been used on-board sounding rockets, the flown and recovered lamps are also analysed in de-tail. All three types of window materials show notable degradation effects after having been exposed during the payloads' re-entries and following splash-downs into salt water of the North Atlantic. MgF2 can be re-stored to an acceptable quality by intense polishing as previously described. Although, identical lamps show differences between each other even after intense cleaning, which may be caused by mechanical vari-ations of e.g. the Xe ignition electrodes. Self-cleaning of MgF2 by heating [Preston et al., 1980] can be ob-served with the Xe lamps after several hours of con-tinuous operation. We conclude, that this happens due to the mounting of the Xe lamps in the calibration set-

up, which allows only little of the produced heat to flow away from the Xe lamps. The quartzite windows show an increased opacity, which cannot be reversed by just cleaning or polishing. Varying the pressure in one section of the spectrometer does not show any sig-nificant effect. A specific short-term temperature de-pendence of the spectra was not observed. The flashes are stable in intensity with a tolerance of only 0,05 %. Varying the frequency of the Xe flashes shows a non-linear frequency-depending variation of the overall in-tensity of the light but does also not manifest in vari-ations of characteristics of spectra.

5 CONCLUSIONS AND FUTURE

In general, the cut-off-wavelengths of the respective window materials could be determined. The spectra of all Xe lamps do not show obvious lines, but dominant wide peaks, which indicates that it is e.g. either a mix-ture of Xe with other gases and / or Xe under higher pressure. This issue is under further investigation. Therefore, we plan to modify one of the original Xe lamps so that its gas pressure can be varied. For getting data for wavelengths greater than 220 nm, initial meas-urements with a visible and near visible light (VIS) spectrometer are under way. The implementation of the mentioned AlGaN and AlN photo diodes into the VUV spectrometer has yet to be finished.

This work was supported by the German Space Agency (DLR) under grants 50 OE 0301 and 50 OE 0801 (Project ECOMA). The authors would like to acknowledge and thank: The Clusters and Nanostructures research group led by Karl-Heinz Meiwes-Broer, Rostock University, Germany, for providing basic equipment, infrastructures, space and good advice. Mikhail Khaplanov of the Department of Meteorology (MISU), Stockholm University, Sweden, for very useful advice based on experience with similar calibrations. Jean-François Hochedez of the Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), France, formerly at the Solar Physics Department of the Royal Observatory of Belgium (ROB), Belgium, for providing various con-tacts and initial introductions to people within the material science com -munity related to detectors for vacuum-uv. Jean-Luc Reverchon of THALES Research and Technology, Palaiseau, Paris, France, for providing AlGaN-based detector samples and related data. Hongxing Jiang, Jingyu Lin and Ra-jendra Dahal of the Nanophotonics Center, Texas Tech University, Lubbock, Texas, US, for providing an AlN-based detector sample and related data. Reiner Thornagel of the department “Radiometry with Synchrotron Radi-ation”, Physikalisch-Technische Bundesanstalt (PTB) – the German national metrology institute, Berlin, Germany, for providing helpful advice especially on the construction of the calibration set-up.

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