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Cerenkov Radiation contribution to Stray Light in NIRCam

Cerenkov Radiation contribution to Stray Light in NIRCam. Peter McCullough 19 Jan 2006 STScI TIPS. History.

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Cerenkov Radiation contribution to Stray Light in NIRCam

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  1. Cerenkov Radiation contribution to Stray Light in NIRCam Peter McCullough 19 Jan 2006STScI TIPS

  2. History Cerenkov radiation was discovered by 30-year old Pavel Cerenkov in 1934 in the USSR. Cerenkov observed the emission of blue light from a bottle of water subjected to gamma rays*. His parents, Aleksei and Mariya Cerenkov, were peasants. He received the Nobel prize in 1958. He died in 1990. *Gamma rays accelerate electrons via the Compton effect. The relativistic electrons then create the Cerenkov effect. nobelprize.org and Google image search

  3. Some experiences with Cerenkov light Fly’s eye (George Rieke): detected Cerenkov light in atmosphere NIRCam pre-Phase A: Cerenkov glow from lenses not a problem HST FOS: stray light HST STIS near-UV channel: stray light HST STIS opto-isolators: miscommunication

  4. NIRCam Shortwave Imaging Path

  5. Physics The number of photons per nm of wavelength per cm of pathlength of the relativistic particle is 0.46E6 Z2l-2 (1-(b n)-2) Z=1 for the most common cosmic rays (protons) l is the wavelength in nm, b = v/c, n=1.5 for the glasses within NIRCam. The coefficient 0.46E6 equals 2 p a times 1E7, the number of nm per cm, with the fine structure constant a = 1/137. The quantity (1-(b n)-2) ~ 0.5 for v = 0.9c and n=1.5.

  6. Cone of light nobelprize.org

  7. Physics applied: The integrated Cerenkov photon flux within a bandpass filter is proportional to l-1 for a filter of a given resolution, such as R=4. The integral of all Cerenkov photons longer than a given l (here in microns) is approximately equal to 230/l photons per proton per cm of pathlength. To be both simple and conservative, we assume that every cosmic ray induces Cerenkov radiation, and the flux equals 5 protons/cm2/s inside NIRCam’s optics located in L2 orbit (cf. 0.25 protons/cm2/s at HST’s orbit outside SAA). Including Helium cosmic rays will increase Cerenkov luminosities by no more than a factor of 2.

  8. NIRCam Imaging Triplet

  9. Imaging triplet: NIRCam’s imaging triplet glows at l > 0.5 microns with a luminosity of L = p/4 * (7 cm)2 * 4 cm * 5 ions/cm2/s * 460 photons/ion/cm L = 350,000 photons per second. If they escape isotropically, then the flux at the detector is 400 photons/s/SCA area, or ~1% of the maximum dark current or ~1% of the zodiacal light thru an R=100 filter.

  10. NIRCam FPA Fold Mirror

  11. Luminosity may not be isotropic: In a special case of a long bar or cylinder, nearly all of the Cerenkov radiation can be made to escape its ends due to either mirror coating or total internal reflection. The latter principle is exploited by Mack (2002) as shown below: Cosmic Ray path Glass Cerenkov photons Detectors

  12. Final Fold mirror: NIRCam’s SW final fold mirror, if it were not light-weighted, would glow at l > 0.5 microns with a luminosity of L = (10 cm)2 * 2 cm * 5 ions/cm2/s * 460 photons/ion/cm L = 460,000 photons per second, which is comparable to the zodiacal light collected by one SCA through a F200W (R=4) filter. However the mirror is light-weighted and it appears to be contained within its mount such that glow from its back or sides cannot reach the detectors.

  13. Other luminescence not addressed here! “Hence we were inclined to think that this light produced by the gamma rays was one of the many luminescence phenomena. Pierre and Marie Curie thought so and they were incontestably among the first to observe this kind of light, at any rate under conditions where it was fairly heavily masked by the ordinary luminescence.”(Cerenkov’s Nobel lecture; P.R.M.’s emphasis)

  14. Summary • The contribution to stray light in NIRCam due to Cerenkov radiation is … • negligible from lenses. • (in principle) significant from the final fold mirrors’ substrates but (in practice) can be blocked. • Caveat: this author cannot adequately judge the effectiveness of that blocking from drawings available to him. • Therefore: designers should be made aware of the need to contain the Cerenkov glow emitted by transparent materials within NIRCam and other instruments on JWST.

  15. See McCullough’s Technical Report, JWST-STScI-000558, and/or the following:

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