8.12 Rate of Loss After Isotope Ingestion
309
8.12 Rate of Loss of an Ingested Radioactive Isotope
Because a radioisotope can be expelled from the body, its effective lifetime in the
body, τ eff , may differ from its physical lifetime, τ phys . The rate of loss must be the
sum of the rate of expelling biologically plus the disintegration rate. Thus
1
τ eff
=
1
τ phys
+
1
τ bio
,
(8.6)
where τ bio is the biological lifetime, i.e. the time that 1 − 1/e of the substance would
be eliminated if the isotope itself were stable. The same relation applies to the halflives.
As an example, consider polonium ingestion, which is easily lethal. Polonium210 is an alpha particle emitter, with a physical half-life of 138.38 days, and an
approximate 50 day biological half-life. The emitted alpha particle has an energy of
5.30438 MeV. There is also likely a gamma ray emission with energy 803 keV but
with low probability (less than 1%). After ingestion, 50% or more is eliminated
in feces. 45% gets into the spleen, kidneys, and liver, muscles, and 10% into
the bone marrow. Liver and kidney damage occur from alpha particle damage to
cells. Victims often experience vomiting, diarrhea and hair loss, just as in radiation
sickness. Polonium-210 in a dose of 5 Sv is lethal. This dose would come from the
ingestion of just under a tenth of a microgram of Polonium.
It is the alpha particle emission that makes Polonium-210 highly radiotoxic
if ingested or taken into the lungs. The ‘radiotoxicity’ (as opposed to chemical
toxicity) of a substance refers to its potential capacity to cause damage to living
tissue due to its radioactive emissions. To establish the radiation dose due to
exposure to Po-210, one must differentiate between external exposure and internal
exposure. Since Po-210 is mainly an alpha emitter, and these alphas do not penetrate
the skin, the main damage is from internal exposure either by ingestion or by
inhalation. Once in the blood stream, polonium disperses through the entire body
giving rise to a whole-body radiation dose which kills or damages cells, tissues, and
organs. To determine the biological hazard, the so-called effective dose coefficients
should be used. These relate the biological hazard (measured in sievert, Sv) to
the activity of at intake (in becquerel, Bq) of the radionuclide. The effective dose
coefficients for intake of Po-210 (ICRP 72) are as follows: The effective dose
coefficient for ingestion is 1.2 × 10 −6 Sv/Bq; The effective dose coefficient for
inhalation is 4.3 × 10 −6 Sv/Bq. To obtain the dose from intake of Po-210 from
ingestion of this material, the activity of intake (in Bq) is multiplied by the effective
dose coefficient for ingestion. For a mass of Po-210 of 0.1 μg (corresponding to an
activity of 1.7 × 10 7 Bq), the dose is given by: 1.7 × 10 7 Bq×1.2 × 10 −6 Sv/Bq=
20 Sv/0.1 μg or 200 Sv/μg. This is a very high dose. To put this into perspective, a
radiation dose of 5 Sv received over a short period will cause death in 50% of cases
within 30 days (Lethal Dose, LD 50/30).
309
8.12 Rate of Loss of an Ingested Radioactive Isotope
Because a radioisotope can be expelled from the body, its effective lifetime in the
body, τ eff , may differ from its physical lifetime, τ phys . The rate of loss must be the
sum of the rate of expelling biologically plus the disintegration rate. Thus
1
τ eff
=
1
τ phys
+
1
τ bio
,
(8.6)
where τ bio is the biological lifetime, i.e. the time that 1 − 1/e of the substance would
be eliminated if the isotope itself were stable. The same relation applies to the halflives.
As an example, consider polonium ingestion, which is easily lethal. Polonium210 is an alpha particle emitter, with a physical half-life of 138.38 days, and an
approximate 50 day biological half-life. The emitted alpha particle has an energy of
5.30438 MeV. There is also likely a gamma ray emission with energy 803 keV but
with low probability (less than 1%). After ingestion, 50% or more is eliminated
in feces. 45% gets into the spleen, kidneys, and liver, muscles, and 10% into
the bone marrow. Liver and kidney damage occur from alpha particle damage to
cells. Victims often experience vomiting, diarrhea and hair loss, just as in radiation
sickness. Polonium-210 in a dose of 5 Sv is lethal. This dose would come from the
ingestion of just under a tenth of a microgram of Polonium.
It is the alpha particle emission that makes Polonium-210 highly radiotoxic
if ingested or taken into the lungs. The ‘radiotoxicity’ (as opposed to chemical
toxicity) of a substance refers to its potential capacity to cause damage to living
tissue due to its radioactive emissions. To establish the radiation dose due to
exposure to Po-210, one must differentiate between external exposure and internal
exposure. Since Po-210 is mainly an alpha emitter, and these alphas do not penetrate
the skin, the main damage is from internal exposure either by ingestion or by
inhalation. Once in the blood stream, polonium disperses through the entire body
giving rise to a whole-body radiation dose which kills or damages cells, tissues, and
organs. To determine the biological hazard, the so-called effective dose coefficients
should be used. These relate the biological hazard (measured in sievert, Sv) to
the activity of at intake (in becquerel, Bq) of the radionuclide. The effective dose
coefficients for intake of Po-210 (ICRP 72) are as follows: The effective dose
coefficient for ingestion is 1.2 × 10 −6 Sv/Bq; The effective dose coefficient for
inhalation is 4.3 × 10 −6 Sv/Bq. To obtain the dose from intake of Po-210 from
ingestion of this material, the activity of intake (in Bq) is multiplied by the effective
dose coefficient for ingestion. For a mass of Po-210 of 0.1 μg (corresponding to an
activity of 1.7 × 10 7 Bq), the dose is given by: 1.7 × 10 7 Bq×1.2 × 10 −6 Sv/Bq=
20 Sv/0.1 μg or 200 Sv/μg. This is a very high dose. To put this into perspective, a
radiation dose of 5 Sv received over a short period will cause death in 50% of cases
within 30 days (Lethal Dose, LD 50/30).