Radiation Units, Health, and Applications of Radioactivity

Radiation Units and Health

Introduction to Radiation Units

Radioactivity is quantified using several units, each describing different aspects of radioactive decay and its effects. Understanding these units is essential for interpreting measurements and assessing health risks.

• Curie (Ci): The original unit of radioactivity, defined as the number of nuclear disintegrations per second in 1 gram of radium.

Equation:

$1 ext{ Ci} = 3.7 imes 10^{10} ext{ disintegrations/s}$

• Becquerel (Bq): The SI unit of radioactivity, defined as one disintegration per second.

Equation:

$1 ext{ Bq} = 1 ext{ disintegration/s}$

Absorbed Dose and Biological Effects

Some units measure the energy transferred from radiation to living tissue, which is crucial for understanding biological effects.

• Rad (radiation absorbed dose): The amount of radiation that results in the absorption of 0.01 joules of energy per gram of tissue.

Equation:

$1 ext{ rad} = 0.01 ext{ J/g}$

• Gray (Gy): The SI unit for absorbed dose, where 1 gray equals 100 rad or 1 joule per kilogram of tissue.

Equation:

$1 ext{ Gy} = 100 ext{ rad} = 1 ext{ J/kg tissue}$

• Absorbing 1 rad by 70,000 g of water (about a 150 lb person) increases the temperature by only 0.002°C, but this energy is sufficient to break approximately $1 imes 10^{21}$ molecular C–C bonds in the body, which can cause significant biological damage.

Predicting Effects of Radiation

The biological impact of radiation depends on the type of emission and the tissue affected. To account for these differences, the rem (roentgen equivalent man) is used.

• Rem: Calculated as the absorbed dose (in rad) multiplied by a quality factor that reflects the type of radiation and tissue sensitivity.

Equation:

$ext{rem} = ext{rad} imes ext{factor}$

• For beta particles, the factor is 1.

• For alpha particles, the factor is 10 for most tissues and 30 for eye tissue.

• Most common exposures are a few dozen millirems (mrem) or less; a medical X-ray is about 20 mrem.

• Sievert (Sv): The SI unit related to rem, where 1 Sv = 100 rem.

Average Annual Exposure to Radiation

Humans are exposed to radiation from both natural and artificial sources. The majority of exposure comes from natural sources.

Source	Amount (mrem)
Radon gas	200
Medical sources	53
Radioactive atoms in the body naturally	39
Terrestrial sources	28
Cosmic sources	28*
Consumer products	10
Nuclear energy	0.05
Total	358

*Additional info: Flying in an airplane increases exposure due to reduced atmospheric shielding from cosmic rays.

• Key Point: About 82% of annual radiation exposure is from unavoidable natural sources.

Effects of Short-Term Exposure to Radioactivity and Radiation

The health effects of radiation depend on the dose, duration, and tissues exposed. Effects range from no detectable impact to severe illness or death at high doses.

Exposure (rem)	Effect
1 (over a full year)	No detectable effect
~20	Increased risk of some cancers
~100	Damage to bone marrow and other tissues; possible internal bleeding; decrease in white blood cell count
200–300	Visible “burns” in skin, nausea, vomiting, fatigue
>300	Loss of white blood cells; hair loss
>600	Death

Applications of Radioactivity

Radioactive Elements in the Body

Several radioactive isotopes are naturally present in the human body, each with a characteristic half-life, mass, and activity.

Radioactive Isotope	Half-Life (y)	Isotope Mass in the Body (g)	Activity in the Body (decays/s)
$^{40}$K	$1.26 imes 10^9$	0.0164	4,340
$^{14}$C	5,730	$1.6 imes 10^{-6}$	3,800
$^{87}$Rb	$4.9 imes 10^{10}$	0.19	600
$^{210}$Pb	22.3	$5.4 imes 10^{-10}$	15
$^3$H	12.3	$2.0 imes 10^{-14}$	7
$^{238}$U	$4.47 imes 10^9$	$4.6 imes 10^{-8}$	5
$^{226}$Ra	1,620	$3.6 imes 10^{-11}$	3

Irradiation of Food

Radiation can be used to kill microorganisms in food, extending shelf life and reducing spoilage. Commonly irradiated foods include tomatoes, mushrooms, sprouts, berries, eggs, and meats. The process uses emissions from isotopes such as cobalt-60 or cesium-137.

• Key Point: Irradiation does not make the food itself radioactive.

Medical Imaging

Radioactive isotopes are widely used in medical diagnostics and treatment. For example, radioactive iodine ($^{131}$I) is used to image the thyroid gland and, in higher doses, to treat thyroid tumors.

• Diagnostic doses are typically less than 40 rem, while therapeutic doses can be thousands of rem.

PET Scans (Positron Emission Tomography)

PET scans are advanced imaging techniques that use positron-emitting isotopes, such as fluorine-18, to visualize metabolic activity in tissues.

• Fluorodeoxyglucose (FDG), a glucose analog labeled with $^{18}$F, is injected into the patient.

• Metabolically active cells absorb FDG, which undergoes positron decay.

• Positrons annihilate with electrons, producing two gamma rays that are detected to create a 3D image of tissue activity.

Equation (Positron Emission):

$^{18}_9 ext{F} ightarrow ^{0}_{+1}eta + ^{18}_8 ext{O}$

Radioactive Isotopes with Medical Applications

Various radioactive isotopes are used for diagnosis and treatment in medicine. Their applications depend on their chemical properties and the type of radiation they emit.

Isotope	Use
$^{32}$P	Cancer detection and treatment, especially in eyes and skin
$^{59}$Fe	Anemia diagnosis
$^{60}$Co	Gamma ray irradiation of tumors
$^{99m}$Tc	Brain, thyroid, liver, bone marrow, lung, heart, and intestinal scanning; blood volume determination
$^{131}$I	Diagnosis and treatment of thyroid function
$^{133}$Xe	Lung imaging
$^{198}$Au	Liver disease diagnosis

Additional info: The "m" in $^{99m}$Tc indicates a metastable (excited) nuclear state, which is useful for imaging due to its short half-life and gamma emission.