[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: Response of RS111 and TLDs to cosmic rays
Sandy, and others
First, congratulations on your grandson!
Second, I will try to describe the setup that could help with the algorithm.
Please bear with me, this was around 1987, and I do not remember all the
details. My co-worker, Mr. Peter Carny presented this paper on a radiation
protection conference held, I think, in Dresden (East Germany) in 1988.
TLDs were placed as follows:
1) at 1m above the ground at elevation ~ 60m above the see level
2) at the same location with a 10cm Pb platform to shield off the
terrestrial gammas
3) at ~ 800m above the see level
4) at the same location with a 10cm Pb platform
5) at ~ 1600m on the ground (1m)
6) at the same location with a 10cm Pb platform.
The exposure time was ~ 4-7 months. The RS111 measurements were taken at
the beginning and at the end of the exposure.
Controls (for fading and calibration) and freshly annealed TLDs were placed
at the above locations. We transported the 'field TLDs' to those locations
in Pb shield and 'controlled' the transport exposure too.
At our lab, we had a 'low background' cave where we kept background TLDs
(annealed at the same time as the field TLDs). The 'low background' in the
cave was measured with RS111 and we also unfolded a 3x3" NaI photon spectra
(<3MeV). Both cave measurements were in a good agreement and confirmed
minimal cosmic rays. (The lab itself was in a old building with a lot of
concrete above). I think the readings in the cave were below 1microR/h.
One other thing maybe of interest to you, I seem to recall, is that an
'extrapolated' RS111 own background response was ~0.1microR/h, a pretty low
number.
As to the algorithm, in my previous post, I mentioned that the TLD cosmic
readings where compared against the RS111. Here it is how and why we did
not compare the TLD or RS111 cosmic ray responses against the external dose
to humans.
At each location 'n' the average TLD dose rate Tn is written: Tn = TGn +
TCn
(with the following assumptions/corrections made: the average exposure rate
from gammas 'G' and cosmics 'C'; corrections on uniform fading; TLDs
background from the cave, for example the MgSiO4 TLDs, manufactured by
Kyokko, are wrapped in glass (K-40 ???); transport dose; TLDs and the RS111
were calibrated with a Ra source, etc.)
The RS111 response at locations n=1, 3 and 5 is written rn = rgn + rcn.
The rcn values were taken from the RS111 manual since we knew the average
barometric pressure. (All locations were weather stations in the former
Czechoslovakia.)
So r1 = rg1 + rc1. The rc1 value was measured/confirmed on a large deep
lake at the approximately same elevation. In the preceding post I noted
that we had verified the RS111 cosmic ray response in a plane.
For locations 1&2: The TLD responses are written: T1 = TG1 + TC1 and T2 =
TG2 + TC2. Since TG1 > TG2 and TC1 = TC2 we have T1 - T2 = TG1 - TG2.
Additionally, with rg1 ~ TG1 we found that TG2 ~ 0 and TC1/rc1 ~ 0.8.
The same goes for the locations 3&4 and 5&6.
Additionally, since rc1 (~4microR/h) < rc3 (~5microR/h) < rc5 (~7microR/h)
the TCn/rCn ratios had been confirmed at slightly different levels.
The mutual ratio of the terrestrial gamma response for TLDs (TGn) and RS111
(rgn) was found close to unity, as expected. Both used the same Ra
calibration and the TLD photon energy response was not deviating a lot from
the RS111.
The TLD's filtration were various, depending on the materials. The ones
with higher Zeff (CaF2 and CaSO4) were shielded with Cu and Al. LiF were in
plastics, and the Kyokko TLDs used their own filtration (a perforated Zn
filter with a plastic cover). The low energy photon responses were tested
previously with X-rays. All TLDs were additionally wrapped in a few mm of
plastics to provide for the secondary charge equilibrium and build-up. Thin
Al containers were used to avoid water/humidity and to minimise heating from
the sun.
One may question the cosmic ray (predominantly neutron) distribution at
different elevations and a possible exposure of the TLD's from air may have
been accounted for as from the cosmic rays.
However, when we are to relate the dose (equivalent) to humans from cosmic
rays based on the TLD or RS111 response, we have to be more careful. This
is because the energy deposition (or dose) from neutrons depends on the
materials, e.g. tissue, Ar/Fe and the TLD material. Additionally, we may
have to apply the radiation weighting, formerly, quality factors. (The
primary interaction of the neutrons in tissue are scattering or absorption
on H atoms. The RS111 neutron response = ionisation in the chamber, is due
to recoiled Ar or Fe (in the wall). And the TLD main response is due to
recoiled H in the plastic wraps or from recoiled nuclei in the TLD
material.)
That is why we did not (dare to) relate the cosmic ray TLD responses with
the tissue dose equivalent by applying the TLD (gamma) calibration factor.
I do not dispute the fact that the TLD response and the dose equivalent may
have similar neutron energy dependence for vast orders of neutron energies
and that the gamma dose calibration factor fits the picture. However, I
would only do that if I had the energy distribution of the cosmic rays and
independently confirm that it is not a coincidence or have tested the TLD
response for those neutron energies.
Hope this helps. If you want further information, please do not hesitate to
contact me. I share the test outcomes with Peter, who deserves full credit.
Regards,
Miroslav Lieskovský
Health Physicist
NB POWER - PLGS
P.O. BOX 600
Lepreau, N.B.
Canada, E5J 2S6
tel. (506) 659 7421
fax. (506) 659 6507
mlieskovsky@nbpower.com <mailto:mlieskovsky@nbpower.com>
-----Original Message-----
From: Sandy Perle [mailto:]
Sent: December 2, 1999 5:11 PM
To: Multiple recipients of list
Subject: Re: Response of RS111 and TLDs to cosmic rays (Was: Dose Rates
vs. A
Miro,
Perhaps you can provide additional information?
(1) What type if algorithm, or, elemental responses did you use to
determine the exposure from the TLDs?
(2) If an algorithm was used, what path did the algorithm follow to
determine the exposure?
(3) What type of filtration was over the elements you did use?
I agree with your comment regarding the ratio of RS111 and TLDs.
My experience has been that the TLDs derive excellent correlation
to expected exposure when used as an environmental dosimeter.
The 0.75 to 0.85 seems significantly lower than I would expect.
Thanks...
> 2) TLD cosmic ray response.
>
> The TLDs will respond to the cosmic rays. We have exposed various TLDs
> (LiF, CaF2, CaSO4:Dy, CaSO4:Tm, AlP glass and MgSiO4) at different
> elevations. The components of TLD response were analysed against the
> response of the RS111. We established that the cosmic ray readings of the
> TLDs were approximately from 0.75 to 0.85 of the RS111 cosmic rays
response.
>
> Yet, interestingly, the environmental TLDs readings are often expressed as
> the external gamma doses and do not subtract/correct for the cosmic ray
> response.
----------------------------------------------------------------------------
------------
Sandy Perle Tel:(714) 545-0100 / (800)
548-5100
Director, Technical Extension 2306
ICN Worldwide Dosimetry Division Fax:(714) 668-3149
ICN Biomedicals, Inc. E-Mail:
sandyfl@earthlink.net
ICN Plaza, 3300 Hyland Avenue E-Mail: sperle@icnpharm.com
Costa Mesa, CA 92626
Personal Website: http://www.geocities.com/scperle
ICN Worldwide Dosimetry Website: http://www.dosimetry.com
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html