1 5-9 October th ICATPP, Como, Italy S. Maltezos NITROGEN MOLECULAR SPECTRA OF AIR FLUORESCENCE EMULATOR USING A LN 2 COOLED CCD S. Maltezos, E. Fokitis, V. Gika, P. Fetfatzis, G. Koutelieris and E. Galanis G. Koutelieris and E. Galanis NTUAVILLA OLMO Physics Department, National Technical University of Athens

2 5-9 October th ICATPP, Como, Italy S. Maltezos Outline  The Air Fluorescence Yield in UHECRs  Emulator Setup of the Nitrogen Fluorescence  Noise Tests of a LN 2 Cooled CCD  Nitrogen Molecular Spectra  Conclusions and Prospects

3 5-9 October th ICATPP, Como, Italy S. Maltezos Fluorescence radiation  During an EAS event: the produced secondary charged particles (electromagnetic component) ionize or excite the atmospheric air molecules producing fluorescence radiation after their de- excitation (N 2 and N 2 + ).  The total energy of the emitted radiation of air fluorescence during the EAS event is about 90 % of the energy of the UHECR primary particle  The main source of the experimental uncertainty of the energy determination of the primary particle comes from our limited accuracy in the air fluorescence yield Τhe motivation for our recent efforts and also for this work, comes from relevant results of experimental and theoretical studies for the determination of air fluorescence yield.

4 5-9 October th ICATPP, Como, Italy S. Maltezos Air fluorescence yield An emulator setup we present below gave us the possibility to develop a high resolution and high sensitivity UV spectrograph which should be appropriate to record the air fluorescence in electron beam accelerators. The spectrum of the molecular spectrum is composed of several band heads of the 2P(v‘,v“) - second positive) and 1N(v‘,v“) - first negative band systems of N 2 and N 2 + respectively. The number of photons (mostly in UV range) detected by a fluorescence telescope is proportional to the deposited energy of ionizing particles inside the field of view at atmospheric depth X, is given by the integral: Where,  denotes the transmittance of the atmosphere,  D  the efficiency of the detector and Y the fluorescence Yield which depends on the wavelength, temperature and pressure.

5 5-9 October th ICATPP, Como, Italy S. Maltezos LN 2 cooled CCD assembly The high-vacuum pumping system for the CCD cryogenic chamber (in final setup the CCD was installed) Internal chamber (1) with Xcam CCD40-11 attached (in final setup the CCD was installed) A Xcam CCD40-11 used for set-up purposes, and window fitted Exploded assembly A Vacuum of about 2x10 -5 mbar (3) has been achieved using a turbo-pump operating at RPM (2) and a LN 2 trap (4). This procedure lasts typically 3 days The CCD, type CCD30-11 (AIMO) from E2V has 1024x  m square pixels. The read noise is 6 e - rms (equivalence: 7.63 e - /ch)

6 5-9 October th ICATPP, Como, Italy S. Maltezos Emulator Setup: lamp & spectrograph Initial setup using a thermoelectrically cooled CCD from SBIG Modified setup with a LN 2 cooled CCD from E2V and electronics/cryogenics from XCAM Τhe source of the emulator setup is a spectral lamp filled with low pressure atmospheric air made by Meltz Electrolamps Moscow. In the CCD windows one or more spectral lines can be recorded at the same time with a resolution  =0.04 nm

7 5-9 October th ICATPP, Como, Italy S. Maltezos Emulator setup: CCD moving mechanism The CCD can move along a pre-defined focal curve (in plane) achieving the optimal position at each spectral window (~5 nm for the SBIG CCD and ~ 15 nm for the E2V CCD) It can be shown by geometrical analysis that the position vector of the focal curve is given by: where, R is the mirror radius and d is the distance between curvature center and grating. A i is the possible grating position while O is the center of curvature of the spherical mirror. This study is very accurate for small active area of the optical grating improving the resolution at the same time.

8 5-9 October th ICATPP, Como, Italy S. Maltezos Read noise tests of the LN 2 cooled CCD Noise distribution with exposure time 1 s. Lamp HV power supply “on”. Read noise (sigma): N R =39 channels Noise distribution with with exposure time 1 s. Lamp HV power supply “off”. Read noise (sigma): N R =13 channels

9 5-9 October th ICATPP, Como, Italy S. Maltezos Expected signal to noise ratio  The improvement of the dark signal during cooling is limited by the read noise and this is reached around -83 o C (190 K).  Operating at this temperature range, a signal-to-noise ratio around 20 for 1 h exposure time per vertical strip 222  m wide and 6656 mm height (it covers the whole structure of the line) in a typical accelerator beam such of the AIRFLY experiment could be achieved. Where, I s is the signal current, I d (as a function of operation temperature) is the dark current, N r is the read noise and t is the exposure (integration) time. The signal-to-noise ratio is a crucial index of merit for the molecular spectrum recording at the electron beam accelarators environment

October th ICATPP, Como, Italy S. Maltezos The air fluorescence spectrum NoLine wavelength (nm) Linear dispersion 1/R d (nm/mm) R d (mm/nm) Δx (cm) In our emulator setup we can record the spectral lines in 10 pre-selected spectral windows-positions. A typical low resolution air fluorescence spectrum (recorded by AIRFLY) S. W. for SBIG CCD  nm S. W. for E2V CCD  nm

October th ICATPP, Como, Italy S. Maltezos Nitrogen molecular spectra Two successive pictures of the line nm of 1N system obtained with E2V CCD at -124 o C and 300 s exposure time. The diagram below is produced by integration of the pixel channels (counts) along the y-axis. The weak line at 394 nm is just visible covered by the total noise nm

October th ICATPP, Como, Italy S. Maltezos Nitrogen molecular spectra Two previous plots with zoom in narrower spectral window. The external noise is superimposed to the signal, nevertheless the rotational spectrum (lines from R and P branches) with Boltzmann envelop is not hard to identify.

October th ICATPP, Como, Italy S. Maltezos Conclusions and Prospects  We investigated the possibility to maximize the sensitivity of a emulator setup for the air fluorescence study using a CCD sensor cooled at LN 2 temperature range.  The emulator setup has been tested in conjunction with the 1 m focal length UV Spectrograph in a modified configuration.  The tests, by means of recording the nitrogen molecular spectral transitions, were very promising apart the external electronic noise.  In our future plans is to improve the mechanical support of the cryogenic chamber/CCD and also fixing the problem of the external noise. A new set of measurements at very low light levels has to be scheduled for studying the detection limits will be obtained.