Cerenkov Radiation contribution to Stray Light in

NIRCamPeter McCulloughPeter McCullough

19 Jan 200619 Jan 2006STScI TIPSSTScI TIPS

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.

History

nobelprize.org and Google image search

*Gamma rays accelerate electrons via the Compton effect. The relativistic electrons then create the Cerenkov effect.

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

Some experiences with Cerenkov light

NIRCam Shortwave Imaging Path

The number of photons per nm of wavelength per cm of pathlength of the relativistic particle is

0.46E6 Z2 (1-( n)-2)

Z=1 for the most common cosmic rays (protons)

is the wavelength in nm,

= v/c,

n=1.5 for the glasses within NIRCam.

The coefficient 0.46E6 equals 2 times 1E7, the number of nm per cm, with the fine structure constant = 1/137. The quantity (1-( n)-2) ~ 0.5 for v = 0.9c and n=1.5.

Physics

Cone of light

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The integrated Cerenkov photon flux within a bandpass filter is proportional to for a filter of a given resolution, such as R=4. The integral of all Cerenkov photons longer than a given (here in microns) is approximately equal to

230/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.

Physics applied:

NIRCam Imaging Triplet

NIRCam’s imaging triplet glows at > 0.5 microns with a luminosity of

L = /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.

Imaging triplet:

NIRCam FPA Fold Mirror

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:

Luminosity may not be isotropic:

Cosmic Ray pathGlassCerenkov photonsDetectors

NIRCam’s SW final fold mirror, if it were not light-weighted, would glow at > 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.

Final Fold mirror:

“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)

Other luminescence not addressed here!

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.

Summary

1.0 REFERENCES

Jackson, J. D. 1975, “Classical Electrodynamics,” 2nd edition Long, K. 2003, “[SRR version of the JWST] Mission Operations Concept Document,” Fig

3.2.1.-1. Mack, D. J. 2002, “A Quartz Cerenkov Detector: How Complicated Could That Be?”

http://www.jlab.org/~mack/QWEAK/transp_Qweak_quartzcerenkov.ps Moe, H. J. 1991, “Advanced Photon Source: Radiological Design Considerations,” LS-141,

http://www.aps.anl.gov/Facility/Technical_Publications/lsnotes/ Nagano, M, Kobayakawa, K, Sakaki, N., and Ando, K. 2003, astroph/0303193 Pawlowski, B 2003, “Study of Fluorescence Properties of Lithium Fluoride,” NIRcam

Engineering Memo, NIR-EM-0040 Rosenblatt, E. I., W. A. Baity, E. A. Beaver, R. D. Cohen, V. T. Junkkarinen, J. B. Linsky, and

R. Lyons 1992 “An Analysis of FOS Background Dark Noise,” FOS ISR 071, http://www.stecf.org/poa/FOS/fos_bib.html

Rybicki, G. B. & Lightman, A. P. 1979, “Radiative Processes in Astrophysics”

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