Environmental Health and Safety UG RSOG A Guide to Nuclear

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					                            Environmental Health and Safety
UG-RSOG -025-2004

A Guide to Nuclear Radiation Shielding
 at U of Guelph

Prepared November 11th, 2005

1. Introduction to the revision 001 Document

This Guide was written in order to address the need for information on the appropriate shielding
of personnel against external ionizing radiation fields during the conduct of their work with
nuclear substances and other radiation sources. This Document focusses on ionizing radiation
from “nuclear sources”, meaning radioactive and artificial radiation sources such as accelerators,
but is NOT directly intended for X-ray system users. A separate guide, RSOG-026 (in
preparation) addresses protection from radiation fields arising from X-ray system operation.
Further, with the exception of thermalized neutrons, protection from radiations of such high
energy or character that may induce nuclear transmutations is also not addressed here: see
RSOG-027, (in preparation.)

Policy and regulation are also NOT explicitly addressed in this document. However, various
threshold values indicated by statute or regulation may be mentioned for expository purposes.
Further, one is always expected to follow and respect the ALARA principle in implementing and
radiation protection measure, such as shielding. What this may mean in practice is that, in some
circumstances, shielding as a method alone may not be sufficient for protection of personnel, and
that one must also not disregard or overlook any possible INTERNAL exposure (i.e.,
contamination) concerns.

While trustworthy shielding values are presented for many common situations in this Document,
the best test of shielding effect is ultimately a survey with a trustworthy exposure meter. Double
checking any estimated shielding for appropriate protective value on an actual system rounds out
a diligent radiation protection design. Such testing is deemed a necessity, and will, as a side
benefit, reveal any initial contamination in an experimental setup.
  2. Radiation Characteristics

  This Guide will not provide an in-depth review of radiation physics. Listed here for summary
  purposes only are the major characteristics of the radiations commonly encountered in academic
  teaching and research environments not involving Medium and High Energy Physics
  experiments.

  Table 2.1 Main Radiobiological Characteristics of the Common Radiations

common name actual identity                  generically low           “canonical” shielding materials and
            and energy limit                 to high LET† ?            important comments
            of applicability
            here
alpha               helium-4 nucleus,        very high                 un-necessary for external sources BUT
                    10 MeV                                             very high contamination danger if loose
                                                                       source
beta                electron,                medium at most            penetrating power varies greatly with E;
                    2 MeV                    energies                  tritium $s stoped by microns of plastic -
                                                                       32-P $s require more than 1 cm PMMA‡
                                                                       or PVC*
gamma               high energy              low                       generically high penetrating power, even
                    photon,                                            at presumably low energies
                    3 MeV                                              may require “significant” thicknesses of
                                                                       high-Z material to effectively attenuate
neutrons            neutrons                 moderate to high          low-Z and intrinsically high cross-section
                    100 keV                  based on                  materials such as light water, boron and
                                             absorber: results         cadmium. Consult RSO to be sure.
                                             from secondaries

  †Linear Energy Transfer rate - measure of the ability of the radiation to ionize, and thereby deposit energy in matter;
  per unit length in a given substance and usually normalized for density.

  ‡PolyMethylMethAcrylate = plexiglass = Lucite = Perspex

  *PolyVinyl Chloride




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3.1 Shielding from Alpha Emitters

As noted in the Table of Section 2, alpha emitters are so highly ionizing (possessing a high LET,
Linear Energy Transfer rate) that they will not as a rule penetrate even the dead horny layer of
skin, except when initial or decay energies start significantly exceeding 5 MeV. It is a happy
coincidence that the majority of common alpha sources have alpha lines within the 4 - 6 MeV
“band”, with many of these at or below about 5 MeV. Thus the greatest risk from alpha radiation
in particular, disregarding any accompany gamma radiation, is in regard to potential internal
uptake of source material. In these cases, the opposite in hazard level holds, that is, a very high
level of carcingenicity for most of the known alpha emitters. This is due to the conjugate
radiobiological aspect of a high LET radiation - severe local damage and mutation induction
rates to somatic as well as germ cells.

No more needs to be said here as far as external shielding from alphas as the issue is mainly
containment- for information on protection from the internal contamination risk, see the
appropriate Guide.



3.2 Shielding from Beta Emitters

Beta radiations are emitted in a very wide range of energies, from the MeV range down to the
few keV range for most tritium emanations. Thus there is no prospect of offering a generic
minimal shielding value for many common materials, for all commonly used beta emitters.
However, beta radiations as a group are none-the-less shielded against with relative ease even
when decay energies get as high as the 1.7 MeV Emax / 600 keV Emed for 32-P. A 3 cm thickness
of plexiglass will shield with 99.9% efficiency all “common” decay betas, and even 1 cm of such
plastic is a good shield for most lab work.

The user IS however cautioned against the error of using certain, seemingly good shielding
substances in an inappropriate way. The most common error is to employ relatively thin sheets of
lead in order to attenuate high energy betas, especially from 32-P source, thinking that electrons
will be simply stopped by such a high density substance. Below 4mm of lead, such “shielding”
actually makes a rather efficient source of X-rays! The high-Z nuclei of lead in particular make
excellent bremsstahlung converters for electrons, as in an X-ray tube anode. Without yet another
few mm of lead to stop these self-generated photons, a beta emitter now winds up inadvertently
becoming a gamma source. For this reason, rely on low-Z materials such as plexiglass and other
polymers for shielding. Not only are they bound to be less toxic than lead, but are generally light,
cheap and conveniently transparent! Fortunately too, the source-vendors’ practice of selling beta
emitters in “lead pigs” is steeply on the decrease, thus helping to rid users of this inclination to
use lead themselves.

The following table will serve as a selection guide for most users at the University of Guelph.
The reader will surmise that for tritium (as was the case for most alpha emitters) shielding is
actually not an issue, but rather, internal contamination becomes a concern. For nuclides with
greater decay energies than tritium, BOTH internal and external sources are of concern. Gloves

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      are generically useful in all cases for the contamination concern, while also being very good
      shielding, when doubled-up, against the medium-energy emitters.

      Table 3.1 Shielding data for some common used beta emitters.

Nuclide       Max beta                    Distance to stop in air …in water ~ plastic
              energy
3-H           18.6 keV                    0.19 inches !                     few microns

14-C          156 keV                     8.6 inches                        0.012 inches

35-S          167 keV                     9.6 inches                        0.015 inches

32-P          1.71 MeV                    20 feet !                         0.3 inches



      3.3 Shielding against Gamma Emitters

      Except at the very lowest energies (i.e., below 1 keV), gamma radiation is difficult to attenuate in
      comparison to the charged particle or directly-ionizing radiations mentioned above. Its
      intrinsically low-LET makes most gamma radiation a good “X-raying” radiation. Compounding
      this is the fact that lower-E gammas, as produced in many nuclear transitions, and hence
      formally still “gamma radiation” by strict definition, are more efficiently absorbed by tissue.1
      This unfortunate mix of factors is what makes gamma radiation more intrinsically hazardous in
      most circumstances than other external radiations, despite its lower LET.

      The reader should realize that the attenuation of any radiation involves much more than the
      stopping of a photon in a “bullet-in-sandbag” fashion, though in this analogy too, the conversion
      of the bullet’s linear kinetic energy into the random energy of spattered sand and heat is
      admittedly very complex when watched in detail. For the high-LET case, this detailed “shower
      creation” is generally of little concern, as it is over and done with on a micron-sized level in most
      shielding materials of interest. For gammas of any appreciable energy, however, one is in effect
      working to “prune” or defeat a growing shower of secondary electrons and X-rays generated
      when the primary gamma hits the first shielding nucleus. Physicists call this complex secondary
      radiation development an “electromagnetic shower”, which can propagate through many inches
      of lead in the case of high-energy experiments. There is a tremendous literature on this important
      phenomenon which the reader may consult, but for our purposes one can get by quite
      conveniently with a concept known as the “half-value layer”.

      In order to summarize the complex attenuation events attending the stopping of gamma radiation,


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                              Here, photoelectric and coherent scattering cross sections rise quickly.
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the “half-value layer” or HVL was invented in order to provide a practical measure of the
shielding requirement for realistic situations involving potential exposure of personnel. The HVL
is defined essentially as:

that thickness of particular shielding substance, that for a particular initial energy of gamma
radiation, will provide sufficient attenuation to reduce exposure (or dose) to tissue by one half.

The complicated nature of any “spray” of secondary radiation emerging from a slab of shielding
is not explicitly mentioned for a BIOLOGICAL or EFFECTIVE HVL - with such data someone
has already done the hard job of converting such details into the best approximation to the
attenuation factor in what is, to first order, an exponential attenuation law.

Be advised that this is NOT the case for what is termed the PHYSICAL or ATTENUATION
HVL, and indeed that this nomenclature is not entirely consistent across information sources. In
this later case, most useful to computational experts, the HVL defined relates to the number of
photons which have had their first interaction within a given thickness of material, i.e., what
thickness of a given substance, with a given inbound energy, is sufficient to render a 50%
probability that the initial gamma will undergo a first scatter event. In most cases there is no
simple way to estimate a biological or effective protective layer from this later, more theoretical
quantity, which will realistically have a complex event-versus-energy dependency).

Provided with this document is a table below giving some overall shielding parameters for open-
source nuclides commonly used by investigators at the University, as well as more detailed HVL
data as extracted from The Health Physics and Radiological Health Handbook (Nucleon Lectern
Associates, 1984).

If working with gammas of 0.5 MeV energy or higher be aware of the possible need for build-up
factor correction - see below.


Table 3.2 Some practical shielding and source exposure factors for some commonly used
gamma sources.




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3.3.1 Sample Calculation to Demonstrate Use of the HVL

       We’ve got a field of 25 uSv/hr due to gammas on an experiment with Na-22
           tracer;
       We’d like to get this down to at least 0.1 uSv/hr, even before using distancing
           measures;
       The radiation characteristics are 1.28 MeV principle gammas , HVL = 0.96 cm in
           lead at this energy;
       So…
              25/ 0.1 = a factor of 250
              Note that 2 to the power of 8 = 256 (slightly better than x 250)

       ~8 HLVs x 0.96 cm = 8 cm of lead (presumably brick) to achieve this level of
           protection



3.3.2 The Build-up issue

Notwithstanding remarks about the simplification of gamma shielding estimation using HVLs,
the reader is cautioned that for incident gamma photons of more than about 0.5 MeV in energy,
an additional POSITIVE correction factor may be required to account explicitly for a
phenomenon known as build-up. Build-up is a dose/exposure issue, and can be defined as the
creation of an exposure field which is higher than would be naively estimated by a simple
exponential law. It originates with the shielding itself, from a combination of the complex
electromagnetic shower phenomenon mentioned before and a higher tissue attenuation factors at
lower and also higher energies. More fundamentally, the exponential attenuation law assumed
for lower energies fails all by itself, partly due to new interaction processes at higher energies,
and sometimes because of geometric consideration. Indeed, build-up correction tables will


  Nuclide Max gamma                  Thickness ofPb to                 1 cm dist, 1 mCi
          energy                     shield to half-intensity          source activity
  125-I        31 keV            0.02 inches             1.4 r/hr
                but 27 keV is in (a steel garbage can is
               high proportion sometime recommended)
  59-Fe        1.292 MeV             0.38 inches                       6.18 R/hr

  22-Na        1.25 MeV              0.26 inches                       11.9 R/hr
                 (has beta too)

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indicate the applicable geometry for any given data.

Therefore, in cases where anticipating the generation of high energy gammas, one needs to
examine whether build-up is a serious effect. The best way to accomplish this is to reference
build-up factor tables alluded to above, for the material and energies of interest. The reader may
find such information in the Health Physics Handbook mentioned previously, or simply consult
the RSO for more guidance.


3.4 Protection from (Low Energy) Neutrons

High energy neutrons are liable to engage in complex nuclear reactions with various shielding
materials, thus the shielding of such radiation will not be discussed in a generic guide such as
this. However, when higher energy neutrons eventually reach epi-thermal and thermal energies,
as most neutrons outside of HEP2 experiments will, certain defensive measures that resemble
shielding from decay radiations become applicable. One must think in terms of selecting
materials with efficient moderating properties, and with large absorption cross sections -
preferably both in one substance. These materials differ radically from, say, efficient gamma
absorbers.

As a common example, a cheap and relatively effective shielding choice is light hydrogen, in the
form of ordinary water, paraffin or polyethylene, for instance. At the very least, several inches of
any of these substances will moderate the initial spectrum from sources like Am-Be to a large
degree, and make the more “ravenous” neutron absorbers such as borax (boron, thermal cross
section ~ 755 barns) or cadmium ( 2450 barns, but chemically toxic) far more effective.
Otherwise, light water by itself will eventually absorb the neutrons at a much lower efficiency
represented by an absorption cross section of about 0.66 barns. The figures mentioned should be
regarded as optimistic, as some fraction of an impinging neutron flux will still have a higher
energy. Notwithstanding, the “ability” of some materials to make good absorbers of epi-thermal
neutrons due to resonance absorption should be explored (by referencing applicable cross section
tables or databases).

If the calculations related this type of shielding estimation are unfamiliar or appear daunting to
the prospective investigator using a neutron source, he or she is urged to seek out the RSO for
guidance.

The reader should also be made aware of potential hazard due to activation of materials as a
result of long term and/or intense exposure to neutrons. Activation is the phenomenon of
inducing radioactivity in some previously stable substances through neutron absorption. Again
the magnitude of this effect requires careful checking of material cross-sections data while
planning an experiment, and the RSO should be consulted if the reader is unfamiliar with this.

In order to encourage the prospective, but non-expert investigator to confer with the RSO on
such occasions, no attempt will be made to provide cross-section data here. Rather, the


                    2
                        High Energy Physics, as in multi-GeV particle beams.
                                                                                                     7
investigator is strongly urged to engage the RSO’s assistance in the planning of radiobiological
protection measures for experiments involving neutrons.




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