Cerenkov Radiation contribution
to Stray Light in NIRCam
Peter McCullough
19 Jan 2006
STScI TIPS
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
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
NIRCam Shortwave Imaging Path
Physics
The number of photons per nm of wavelength per cm of
pathlength of the relativistic particle is
0.46E6 Z2 l-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.
nobelprize.org
Cone of light
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.
NIRCam Imaging Triplet
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.
NIRCam FPA Fold Mirror
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
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.
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)
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.
See McCullough’s Technical Report, JWST-STScI-000558, and/or the following:
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”