Linear Energy Transfer (LET)
Measurement of the number of ionisations which radiation causes per
unit distance as it traverses the living cell or tissue is called the
linear energy transfer of the radiation. The concept involves lateral
damage along the path, in contrast to path length or penetration
capability. Medical X-rays and most natural background radiation are
low LET radiation, while alpha particles have high LET. On the
average, fission fragments have high LET.
density of ionisation causes special problems in sperm and ova
because the damage (protein breakage) is concentrated within a few
cells. The two-year sterility of Japanese fishermen exposed to fallout
from the 1954 hydrogen bomb test is probably an example of this
effect. Sperm and the cells which produce sperm were damaged
beyond their capability of prompt repair.
a young girl in St George, Utah, USA, Elizabeth Catalan used
to stand outdoors and watch the mushroom clouds raised by the
Nevada nuclear tests float overhead. She has never been able to have
children. She, like some other women in St George, is unable to carry
a foetus to birth. Elizabeth's father, president of a local college, died
prematurely of leukaemia. He used to go horse-riding with three
friends and was frequently outside when the grey clouds laden with
radioactive chemicals went over. Three of the four men are now dead
sister died in her late twenties of a thyroid disease which
may have been caused by the radioactive iodine released in the
atomic blasts. Elizabeth and her mother attribute many of their
abnormal health problems, and those of family and friends, to the
atomic fallout. No government studies have been undertaken to
confirm or deny these claims. However, the situation was so widely
recognised as abnormal by the local population that the Governor of
Utah has filed a court claim against the US Federal Government for
wrongful deaths of the people of Utah. About a thousand individual
damage claims have entered the courts in the USA, and as part of the
trial preparations Dr Carl Johnson undertook a detailed study of the
Mormon population of Utah exposed to the fallout. It is reasonable to
conclude that the health problems reported by the people of Utah are
typical of what could be expected on the basis of theoretical
10 May 1984, US District Court Judge Bruce S. Jenkins ruled on
the first twenty-four claims of US government negligence in its
conduct of nuclear testing. He has awarded $2.6 million in damages to
ten claimants. This landmark, 489-page, carefully worded decision is
expected to be appealed against by the US Federal Government.
order to have a quantitative sense of the frequency of the
different cell effects caused by radiation exposure, imagine a colony
of 1,000 living cells exposed to a 1 rad X-ray (about the dose for one
X-ray spinal examination). There would be two or three cell deaths,
two or three mutations or irreparable changes in cell DNA and about
100,000 ionisations in the whole colony of cells -- ranging from 11 to
460 ionisations per cell.
While cells can repair some damage, no one claims that there is
perfect repair even after only one such X-ray.
comparable 1 rad exposure to neutrons which have higher linear
energy transfer (LET) would be expected to cause more cell deaths
and more mutations. The ionisations caused would range
from 145 to 1,100 per cell.
particles which occur naturally would cause roughly 10
times as many cell deaths and mutations, and 3,700 to 4,500
ionisations per cell. Alpha particles have high linear energy transfer.
average number of cell deaths and mutations caused by fresh
fission particles (i.e. those present soon after detonation of a nuclear
bomb) would be even greater, with the ionisations as frequent as
130,000 per cell.
In nuclear reactors, most of these
extremely high-energy early fission fragments are enclosed within the
fuel rod. In a nuclear bomb blast, they are all released but they
decay very quickly and do not persist long in the environment.
instead of thinking of a colony of living cells, we think of a
person exposed to 1 rad (again about the skin dose from one spinal
X-ray) of 1 MeV (million electron volts) energy, this corresponds to 2.2
billion (US) photons per cm^2 acting on the body. In the words of Karl
Morgan, `It is inconceivable that all the billions of irradiated and
damaged cells would be completely
This unrepaired damage accumulates,
eventually causing a reduction in the level of health that is normal
for a particular age.
very simply, ionising radiation seriously disrupts the chemistry
of the cell. It can also kill or permanently change the cell. Every
exposure to ionising radiation has this effect, and it is not
possible for the body to perfectly repair all of the damage. Whether or
not the residual unrepaired damage is of concern to the individual
exposed is a personal value judgment. It is not at all clear that
ordinary people find the damage `acceptable" unless it initiates a fatal
cancer, and yet this is the basis on which radiological safety
standards are set in all nations of the world.
M. Sievert, the famous radiologist, who had supervised radiation
therapy since 1926 at the Karolinska Institute in Stockholm, pointed
out at an international meeting in 1950 that `there is no known
tolerance level for radiation'.
A tolerance level is a level below
which there is no damage (sometimes called a threshold). A safety
level is ordinarily a fraction (one-tenth) of the tolerance