From: utzoo!decvax!cca!ima!ism780!mark
Newsgroups: net.physics
Title: Re: (Usenet) two questions on REMs/RADs/RBEs and xrays and crts
Article-I.D.: ism780.5
Posted: Fri Feb  4 09:16:00 1983
Received: Sun Feb  6 01:04:51 1983

It has been a long time since I hung around a nuke lab, but I once had
to know what all of those were.  This is all from memory (I don't have
my health-physics text anymore).

Measures such as Curies and Roentgens are not directly relevent to
radiation dosage - since they merely count ionization events without
considering the damaging potential of those events.

RAD     -       Measure of Ionizing energy absorbed

    This is the basic measure of radiation absorbtion.  One RAD corresponds
    to the absorbtion 100 ergs of ionizing enerby per gram of tissue.  This
    measure is misleading however ...


RBE     -       Relative Biological Effect (occasionally R B Potency)

    Different forms of radiation have a differing ability to harm
    animals.  Alpha radiation is EXTREMELY damaging to the extent that
    it is absorbed.  Fortunately a few feet of air or good piece of paper
    will considerably attenuate it.  Beta radiation will penetrate into
    skin, doing a little damage - but is stopped by a piece of metal foil.
    Gamma radiation (photons of nuclear origin) is highly energetic and will
    penetrate much protective clothing - but will not do as much damage per
    absorbed erg as would alpha radiation.

    In order to compensate for the different degrees of havoc wrought by
    different types of absorbed radiation, the RBE is used as an adjustment
    which attempts to map received energy into a more-or-less linear measure
    of absorbed damage.  For a simple source of radiation, RBE is a number
    between zero and one.  For real sources, it is computed as the weighted
    average of the RBE's of the individual radiation components.


REM     -       Roentgen Equivalent Man

    This is an attempt to measure the actual degree of damage to which a
    subject has been exposed.  It is the dosage multiplied by the RBE.
    This is the more meaningful number when considering the potential harm
    that a radiation field might do.  A Geiger counter measures ionization
    incidents, and thus gives a measure of the ionizing particle flux.  This
    is useful in making simple determinations such as this area is hot and
    this area is clean.  For measuring actual radiation hazard, there are
    more complex devices (such as Nemo spheres) which attempt to simulate
    the absorbtion of particles in the body.


As to your real question, a little more data is required to ascertain the
likely threat to terminal users.

	(1) You estimate the energy of the particles hitting the screen,
	    but not their flux.  I understand that you needed to know the
	    energy per particle to figure out the wavelength of the
	    emitted radiation, but the actual hazard comes from the total
	    energy absorbed, not the energy per particle.

	(2) The absorbing, scattering and reradiation properties of the
	    CRT are crucial to the question.  The user is not struck by
	    the beam from the CRT, but rather by particles which pass
	    through the thick screen and photons which are emitted from
	    the collision.  This computation could be non-trivial:

	    There is probably a 15-20 degree variation in incidence angle
	    between the electron stream and the screen.

	    The incomming stream of particles does not have a velocity, but
	    rather a distribution.  The likelihood of capture is probably
	    highly dependent on the velocity of the incidental particle.
	    A computation based on mean velocity may yield a good
	    approximation of the energy absorbed by the screen - but a
	    meaningless statement about the spectrum of re-radiated energy.

	    The screen is not perfectly uniform, geometrically or compos-
	    itionally.  The phosphor is probably a combination of more and
	    less exotic materials, each of which has different absorbtion
	    cross-section and re-radiation properties.


I never got beyond basic cross-section/absorbtion computations and always
ignored second-order emissions.  I suspect that the whole of your problem
deals with second-order emissions.  I'm not even competent to guess how
important the above considerations are.  I would suggest that unless you
are looking for a thesis topic, that you would do better to:

	a) Contact OSHA.  They have been preparing standards for
	   radiation emission of EDP equipment for the last few
	   years.  They can probably send you reams of repors and
	   the relevent standards to boot.

	b) If you have an "in" to someone who works in highly-hush-hush
	   military work or (dare I say it) "No Such Agency", you might
	   try to find out if the "Tempest" requirements for terminals
	   mention radiation passing through the screen of a CRT.  They
	   weren't really interested in biological hazzards, but you can
	   bet that they know everything about ALL forms of radiation which
	   might be emitted from a terminal.

	c) Don't bother trying to measure the radiation with equipment in
	   your physics department.  I am fairly certain that the basic
	   radiation safety equipment won't tell you much at all.  The
	   really sensitive equipment (like gamma spectrometers) won't
	   work unless you can put the CRT into the bottom of a small
	   lead cylinder.

	d) If you want to try a simple "Mr. Wizzard" experiment get some
	   fast b&w film (ASA 600-1000) and a few beta-gamma badges.
	   Make about a dozen test packets with each and set them up
	   in contact with the screen, atop and behind the CRT, below
	   and around the HV transformer and suspended 12-24 inches
	   away from the terminal.  Then put another few under lead
	   bricks and a few more on top of a desk, five feet from the
	   terminal (with lead foil between them and the terminal).
	   Expose them all for a month, have the badges processed and
	   develope the film in boiling DEKTOL.  Compare them and see
	   if you mesured anything significant.

---mark---

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