Units of Radiation Measurement and Biological Effects

Measuring Radioactivity

Introduction to Radiation Measurement

Radioactivity is the process by which unstable atomic nuclei lose energy by emitting radiation. Measuring radioactivity is essential in nuclear chemistry, medical applications, and radiation safety. Different units and properties are used to quantify radiation exposure, intensity, and biological effects.

• Radioactivity refers to the number of nuclear decay events per unit time.

• Exposure measures the intensity of radiation in the air.

• Absorbed dose quantifies the energy deposited in tissue.

• Biological effect considers the impact of radiation on living organisms.

Units of Radiation Measurement

The following table summarizes the common units, SI units, and the properties they measure in radiation science.

Common Unit	SI Unit	Property Measured
Curie (Ci) 1 Ci = 3.7 × 1010 disintegrations/sec	Becquerel (Bq) 1 Bq = 1 disintegration/sec	Activity: Decay events
Roentgen (R)	Gray (Gy) 1 Gy = 1 J/kg	Exposure: Ionizing intensity
Rad 1 Rad = 0.01 Gy	Sievert (Sv) 1 Sv = 1 J/kg	Energy absorbed by tissue
Rem 1 Rem = 0.01 Sv	Sievert (Sv)	Tissue damage (biological effect)

Relative Biological Effectiveness (RBE)

Not all types of radiation cause the same biological damage, even if the absorbed dose is the same. Relative Biological Effectiveness (RBE) factors account for differences in tissue sensitivity and radiation type.

• RBE is used to convert absorbed dose (in rad or Gy) to effective dose (in rem or Sv).

• Formula: $\text{Effective Dose (rem)} = \text{Absorbed Dose (rad)} \times \text{RBE}$

• Common RBE values: X-rays (1), alpha particles (20), neutrons (10).

Example Calculation

Example: The initial responders to the Chernobyl nuclear disaster were exposed to 2.75 Sv of radiation. Convert this value to rem.

• Conversion: $1 \text{ Sv} = 100 \text{ rem}$

• Calculation: $2.75 \text{ Sv} \times 100 = 275 \text{ rem}$

Practice Problems

• Practice 1: A typical chest X-ray exposes a patient to an effective dose of 0.09 mSv. How many Rad is this? Solution: $1 \text{ Sv} = 100 \text{ rad}$, so $0.09 \text{ mSv} = 0.00009 \text{ Sv} = 0.009 \text{ rad}$

• Practice 2: Two technicians in a nuclear laboratory were accidentally exposed to radiation. If one was exposed to 5 mGy and the other to 8 mrad, which technician received more radiation? Solution: $1 \text{ mGy} = 0.1 \text{ rad}$, $8 \text{ mrad} = 0.008 \text{ rad}$, so the technician exposed to 5 mGy received more radiation.

• Practice 3: A solution of iodine-131, a radionuclide used in the diagnosis and treatment of thyroid disease, is found just prior to administration to have an activity of $0.08 \times 10^9$ Bq/mL. If 2.57 mL were delivered intravenously to the patient, what dose of I-131 (in μCi) did the patient receive? Solution: $1 \text{ Ci} = 3.7 \times 10^{10} \text{ Bq}$, so $0.08 \times 10^9 \text{ Bq/mL} \times 2.57 \text{ mL} = 2.056 \times 10^9 \text{ Bq}$; $2.056 \times 10^9 \text{ Bq} \div 3.7 \times 10^{4} = 55.6 \mu\text{Ci}$

Summary Table: Radiation Units and Conversions

Unit	Symbol	SI Equivalent	Property
Curie	Ci	1 Ci = $3.7 \times 10^{10}$ Bq	Activity
Becquerel	Bq	1 Bq = 1 disintegration/sec	Activity
Rad	rad	1 rad = 0.01 Gy	Absorbed dose
Gray	Gy	1 Gy = 1 J/kg	Absorbed dose
Rem	rem	1 rem = 0.01 Sv	Biological effect
Sievert	Sv	1 Sv = 1 J/kg	Biological effect

Additional info: Radiation measurement is a key topic in nuclear chemistry (Ch.21), and understanding unit conversions and biological effects is essential for both scientific and medical applications.