22.
05 Reactor Physics - Part Eight – Supplemental
Why Radiation Is Dangerous
1. Background: Nuclear engineers and radiation workers all recognize that
exposure to radiation can be dangerous. But surprisingly few can explain why. A brief,
mostly qualitative, explanation is provided here.
2. Quantification: One of the difficulties in explaining radiation, be it nuclear
power or nuclear medicine, to the public is that few people are cognizant of the units that
are employed to measure radiation. Instead, the assumption is often that there are only
two levels: 1) none which implies safety and 2) any which implies lethal. We note here
the actual units, of which there are three:
a) Exposure: This is what a radiation detector measures. Radiation may
consist of either charged (alpha, beta, protons, fission products) or neutral
(gamma, neutron) particles. For charged particles, a detector directs the
beam onto a gas. Each particle ionizes the gas to create an electron/ion
pair. An electric field is imposed across the gas and that field causes the
electron and ion to move in opposite directions to the anode and cathode
where they are registered as a signal. Neutral particles are similarly
detected except that they first interact with the detector wall thereby
creating charged particles that move into the gas.
The SI unit of exposure is the X-unit (not yet named after a person). It is
equal to one Coulomb per kilogram (1C/kg) and is a measure of the
amount of ionization that is created.
b) bsorbed Dose: This is the amount of energy deposited in tissue as a
A
result of the radiation that is incident on the tissue. Its SI unit is the Gray
which equals one joule per kilogram. It should be noted that energy
released is not necessarily equal to energy absorbed. When radiation
passes through tissue, it interacts. For example, a photon may undergo a
Compton scatter to produce an electron and a lower energy photon.
Energy has been lost by the incident photon. But it may not be deposited
locally unless both the scattered electron and the newly emitted low
energy photon attenuate locally.
To relate exposure to absorbed dose one needs to know the energy needed
to create an ion pair. In tissue, it is 25 eV. In air, it is 34 eV. Thus, for
air:
⎛ 1C ⎞⎛⎜ 1 ion ⎞⎛ 34 eV ⎞⎛ 1.6 x 10 −19 J ⎞⎛ 1 Gy ⎞
⎜ ⎟ ⎟⎜ ⎟⎜ ⎟⎜
1 X unit = ⎜
kg air ⎟⎜ −19 ⎟⎝ ion ⎠⎜ 1 eV ⎟⎜ 1J / kg ⎟⎟
⎝ ⎠⎝ 1.6 x10 C⎠ ⎝ ⎠⎝ ⎠
= 34 Gray
c. D
ose Equivalent: This is the amount of biological damage done by the
radiation. It is related to the absorbed dose by the relation:
H=QD.
Where H is the dose equivalent, Q is the quality factor, and D is the
absorbed dose. The quality factor allows for the LET (linear energy
transfer) of the radiation. Photons, which interact sparsely, have a LET of
unity. Protons, which interact densely (i.e., many nearby interactions as
they slow down) have a LET of ten.
The SI unit of the dose equivalent is the Sievert. It is also equal to a
joule/kilogram because quality factor is dimensionless.
4. Lethal Dose: The LD 50/30 is the dose that is lethal to 50% of the population in
30 days. It is usually cited as 4.5 Sv.
5. Energy Equivalent to a Lethal Dose: Assume (for ease of arithmetic) that a
typical person has a mass of 100 kg. So, the energy equivalent to a lethal dose is
450 joules. 1 J equals 1 Watt-second. So, the lethal dose is 450 Watt-seconds. In
other words, if light from a typical reading lamp (100 W bulb) were ionizing
radiation, you would receive a lethal dose after 4.5 seconds. The amount of
energy involved in a lethal dose is vanishingly small. Why then is radiation so
harmful?
6. Interaction Mechanisms: Neutral particles generate charged particles as they
attenuate. For example, photons produces electrons via pair production, Compton
scattering, or the photoelectric effect while neutrons knock hydrogen nuclei
(protons) loose. So, we need to consider how charged particles slow down. The
energy lost per unit distance of travel is the stopping power. The figure below
shows it as a function of particle energy:
Atomic
Charge
Collision
Pickup
dE Radiative Loss
−
dx
Energy
There are three mechanisms for energy loss. At very high (relativistic) energies,
the particle radiates energy as Bremsstrahlung. At energies that are typically
encountered in most nuclear applications (1-10 MeV), the dominant mechanism is
collisions with atomic electrons. At low energies (keV and below) charge pickup
occurs – the particle becomes neutral.
How much energy can be transferred in an atomic collision? For a head-on
interaction, it is:
4 mM
Q max = E
(m + M )2
Where
Q max is the energy transferred,
m is the electron mass,
M is the mass of the incident particle, and
E is the energy of the incident particle.
Consider a 1 MeV proton that strikes an electron. In this case, m<<M, and
we obtain
4m
Q≅ E
M
≅ 50 eV
So, only 50 eV is lost per collision. Hence, for a single 1MeV proton to
attenuate, 20,000 collisions are required. This is one reason why ionizing
radiation is so dangerous. The amplification factor is enormous. There is
another, equally important reason though. The energy lost per collision is
on the order of 50 eV. The energy required to ionize an electron, which is
the process whereby chemical bonds in molecule, such as DNA are
broken, is 25 eV. Thus, the energy lost in each of the 20000 collisions that
it takes to slow down one proton is almost exactly that needed to break a
chemical bond.
7. Effect of Other Types of Energy Sources: It remains to note why other
energy (visible light, heat) do not have a lethal effect. The energy
associated with visible light is typically a few eV – less than that required
to break a chemical bond. Similarly, the energy distribution associated
with a high temperature is the quantity kT where k is Boltzmann’s
constant.
8. ummary: To summarize, the reason that ionizing radiation is lethal if
S
absorbed even in seemingly small quantities is that: 1) neutral particles
generate charged ones; 2) many thousands of atomic collisions are
required to slow down each charged particle; and 3) the energy released
per atomic collision is almost exactly that needed to ionize an electron in
tissue.
