Lefeuvre, G

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29th International Cosmic Ray Conference Pune (2005) 8, 3740Measurement of the absolute fluorescence yield in Nitrogen between0.5 and 2.3 MeVG. Lefeuvre , P. Gorodetzky and P. Salin (a) APC, 11 place Marcelin Berthelot, 75231 Paris cedex 05, France Presenter: G. Lefeuvre (lefeuvre@cdf.in2p3.fr), fra-lefeuvre-G-abs1-he15-posterDetecting UHECR showers using fluorescence to determine energy requires a precise knowledge of the fluo-rescence yield. If the fluorescence mechanism itself is well understood, as well as the number of excited linesin N  and their precise wavelengths, the yield is known to only about 25% (even worse for each line). So, dif-ferent collaborations (Auger, EUSO, HiRes...) decided to create a consortium of about ten experiments, calledAirLight, where the fluorescence yield is measured accurately for different electron energies and with differentsetups. In APC (Paris, France), the EUSO group uses aSr source (0.5 - 2.3 MeV electron energy, differentenergies being selected) to produce the fluorescence and a grating spectrophotometer to count the number ofphotons produced per cm of gas traversed by the electron. Pressure, temperature and gas composition can bevaried. The aim is to obtain an accuracy better than 10 %. A great attention is paid to the determination ofthe different efficiencies of the setup. In particular, the weak point of most experiments: the PMT efficiencies(quantum and collection) will be measured by comparison to a NIST photodiode. This delicate point will bedetailed.1. IntroductionIn an extensive air shower, a fraction of the total energy is spent in the electromagnetic part, which gives riseto atmospheric fluorescence. This fluorescence produces photons mainly between 300 and 430 nm.The phenomenon of de-excitation of andin the near UV is well known : there are 13 lines, the mostintense being 337, 316 and 357 nm for , and 391 nm for(see the spectrum in fig1).Figure 1. Fluorescence spectrum, see Ulrich in [2]The fluorescence yield 	 , defined as the number of photons emitted by a charged particle per meter of travel,has not yet been quantified accurately enough : the overall precision is around 25 %, reaching 14 % only when38 G.Lefeuvre, P. Gorodetzky and P. Salinthe electron energy is integrated in the range 0.5 - 2.3 MeV [1]. Still several questions remain unsatisfactorilyanswered, such as : what is the efficiency of each line ? does the production of fluorescence really follow the electron in the gas ? what are the pressure, temperature and gas composition dependences ?In order to improve the precision on 	  by at least a factor 2, about ten experiments are running in manycountries (AirLight consortium) [2, 3], either at low energy with a radioactive source, or at high energy inaccelerators.In Paris, the aim is to measure the fluorescence yield with a precision around 10 % in different conditionsof pressure, temperature, gas composition (  ratio), and relative humidity. The use of asourcelimits this work to relatively low energy (0.5 - 2.3 MeV), still much higher than the electron gun experiment(E  10 keV) and around minimum ionisation energy.The experiment is based on photon counting and spectral analysis. A schematic view of the bench test isshowed in fig 2.2. Description of the fluorescence bench testFigure 2. Schema of the uorescence benchTheﬀsource provides electrons between 0 and 2.28 MeV with an activity of 370 MBq. The electrons arecollimated by a lead collimator and strike a conical scintillator ( ﬁ = 20 mm) read by the photomultiplier (PMT1), after they have traversed a volume of gas in which they excite Nitrogen molecules. The useful volume is aportion of a cone of heigth 40 mm and mean radius 13 mm. The solid angle of the useful volume is 6.4 10 ﬂﬃ sr.On each side of the cone, the emitted light goes to the photodetectors through quartz windows. The electronsinside this cone are counted and their energy measured by the PMT 1 placed below the scintillator. The othertwo PMT detect the photons : one with a BG3 filter, which select the photons between 300 and 400 nm(PMT 2), the other behind a monochromator (PMT 3). Both of them work in single photoelectron mode.At atmospheric pressure, the counting rate is of order of 1 MHz in PMT 1, some kHz in PMT 2 and a few Hzin the PMT 3.Measurement of the absolute uorescence ... 39ADC channel0 100 200 300 400 5000100200300400500600700800electron spectrum1st discriminatorlevelTDC channel2900 3000 3100 3200 3300 3400 3500 3600 3700102103TDC spectrumlifetimes of the Nitrogen lines at 1 atmFigure 3. On the left, the electron spectrum. The slope between the channels 50 and 100 is due to the threshold of 0.5 MeVapplied to the signal. On the right, a TDC spectrum in log scale, showing the integrated lifetimes of the Nitrogen levelsbetween 300 and 400 nm. One can see the randoms on the left of the spectrum.The PMT 2 and 3 are Photonis XP2020Q, with special single photoelectron voltage dividers for two of them.The monochromator is a Jobin-Yvon H25, with a resolution of 6 nm per mm, and a spectral width of 1 nm.We make sure that we get all the photons emitted toward the monochromator by inserting a converging lens(f = 150 cm) between the window and the entrance slit of the monochromator. The lengths are such that theentrance slit is the optical image of the fluorescence volume and the numerical aperture is respected. Themagnification, given the focal length of the lens, is 7, which means that the dimensions of the image are 5.7mm by 1.8 mm : the image can be entirely included in the slit.The pressure, temperature and gas flow are controlled inside and outside the volume. We use pure Nitrogengas and dry Air. A mixer also allows to introduce impurities, mainly water vapor.Data taking is based on coincidences between electron and each photon signal. The electron signal is about100 ns wide to take into account the lifetimes of the Nitrogen levels. An energy binning is made by applyingthree different thresholds to the electron energy spectrum (0.5, 0.8 and 1.2 MeV). For each of these thresholds,ADC and TDC spectra are recorded. Moreover, electrons, photons and coincidences are counted at each stepof the data acquisition, which allow to take into account the dead times of the electronics.3. Calibrations3.1 calibration of the experimentThe solid angles and the different absorptions (windows, grating of the monochromator...) have been preciselymeasured. The effective counting rates have been compared to those expected. A great attention is paid to gainvariations. These variations can be compensated detecting the position of the peak of the ADC spectra andsending a feedback to the high voltage power supplies of the PMT.3.2 calibration of the photomultipliersThe difficult point of this kind of experiment is the systematic error on the quantum and collection efficienciesof the PMT. The manufacturer rarely gives a better precision than 15 %. But if we aim at obtaining a totalsystematic error lower than 10 %, it is necessary to measure the PMT efficiency.40 G.Lefeuvre, P. Gorodetzky and P. SalinThe overall PMT efficiency is proportional to the product of the photocathode efficiency ! #" by the collectionefficiency %$ (defined as the ratio between the number of photoelectrons reaching the first dynode and thenumber leaving the cathode).& #" is not totally flat along the surface of the entrance window, due to the way the bialkali photocathode isdeposited by flash in situ in the glass envelope. However, it does not depend on the applied high voltage. Onthe other hand, '$ is highly non-uniform for the first dynode is not axially symmetric. ($ also strongly varieswith the high voltage. Because what is of interest is how many electrons are collected on the anode for onephoton falling on the photocathode (which is proportional to ! #"*)+%$ ), we first make a relative mapping of thephotocathode by sending light through a fiber mounted on an X-Y movement. The anode pulses are recordedfor each X-Y position at the single photoelectron gain, which interests us (around 2200 V).Then the absolute efficiency is measured at one position. For that, we use a NIST photodiode, accurate to1 %. The gain of this diode is around 1, hence we have to reduce by some ,#-!. the light which enters the PMT(whose gain is around ,#-0/ ). A factor ,#- . is enough since the important point is to have a level in the diode andsingle photoelectron pulses to count in the PMT. To reduce the background, the light is pulsed. We use either apulsed LED or a halogen lamp whose light goes through a rotating blade linked to a locking amplifier. In thatcase, the light wavelength is selected by the monochromator.The setup is the following (fig.4): exiting the monochromator, the light enters an integrating sphere with twoother ports : one for the diode and another one, much smaller, leading to a second integrating sphere. Anotherport with a small hole (1 mm  ) on the second sphere is connected to the PMT. Then both the light in the diodeand in the PMT are measured at the same time.Figure 4. Calibration of the PMT4. AcknowledgementsWe thank F. Lelong and J.P. Reny for their help in the construction of the fluorescence bench.References[1] M.Nagano et al., Astr.Phys. 20 (2003) 293-309[2] http://www.auger.de/events/air-light-03/index.en.html[3] http://lappweb.in2p3.fr/IWFM05/[4] Photomultiplier tubes, principles and applications, Photonis, 2002

Measurement of the absolute fluorescence yield in Nitrogen between 0.5 and 2.3 MeV

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