Dosimetry

Abstract

Radiation interacts with matter by exciting and ionising the atoms that form the molecules or crystal structure of the material encountered. The physics of atomic excitation and ionisation is discussed in some detail in Part II of this book, particularly in the context of exploiting such mechanisms for detection and research purposes. This chapter focuses instead on the extent of measuring radiation for the purpose of quantifying its effects, including those on biological tissue. The chapter introduces the basic quantities to describe radiation and its impact, with the latter sections dedicated to the biological effects and the recommended radiological dose limits.

Glossary

Absorbed dose See Energy dose

Activity For a radioactive source is defined as the number of disintegrations per second and is measured (in the SI) in becquerel (Bq)

$$\begin{aligned} \mathrm{1~Bq = 1~decay/s} \end{aligned}$$

Average total dose See Effective dose

Becquerel (Bq) The SI unit of measurement for radioactive disintegration (activity), corresponding to one disintegration per second

Curie (Ci) Non-SI unit of measurement for radioactive disintegration (activity), corresponding to $3.7\times 10^{10}$ Bq

Deterministic effects (also Tissue reactions) Reaction of biological tissue to an acute radiation dose, owing to the death or malfunction of a critical number of cells

Effective dose ( ${\varvec{E}}$ ) (also Average total dose) Tissue-weighted sum of tissue equivalent doses:

$$\begin{aligned} E=\sum _T{w_T~ H_T}, \end{aligned}$$

where $w_T$ is the tissue-weighting coefficients and $H_T$ is the per-tissue equivalent dose. Effective dose is measured in sievert (Sv)

Energy dose (also Absorbed dose) Mean energy (W) absorbed from radiation by a material per unit volume (V) per unit density ($\rho$), or otherwise per unit mass m

$$\begin{aligned} D=\frac{W}{V\rho } =\frac{W}{m}. \end{aligned}$$

The energy dose is the fundamental dose quantity and in the SI is measured in gray (Gy), defined as 1 Gy=J/kg

Equivalent radiation dose ( ${{\varvec{H}}_{\varvec{T}}}$ ) The SI unit of measurement for the absorbed dose in a specific tissue or organ. It is defined by means of tissue-specific weight coefficients $w_T$:

$$\begin{aligned} H_T=w_T ~ D_T, \end{aligned}$$

and measured in sieverts (Sv)

Exposure See Ion dose

Exposure rate Time derivative of the exposure

$$\begin{aligned} \dot{D}_I (t)=dD_I/dt \end{aligned}$$

Flux, $\varvec{\phi }$ The number of particles ($N_{inc}$) incident onto a surface per unit area (A) per unit time interval ($\varDelta T$):

$$\begin{aligned} \phi =\frac{N_{inc}}{A~\varDelta T} \end{aligned}$$

Fluence rate See Flux

Fluence, $\varvec{\varPhi }$ The number of particles ($N_{inc}$) incident onto a surface per unit area (A)

$$\begin{aligned} \varPhi =\frac{N_{inc}}{A} \end{aligned}$$

Gray (Gy) The SI unit of measurement for the absorbed dose, corresponding to 1 J/kg

Half-life In a radioactive decay, the time necessary for the number of decay centres to drop by a factor of two. Equals the mean lifetime $\tau$ multiplied by the natural logarithm of 2, or approximately 0.7$\tau$

Ion dose, ${{\varvec{D}}_{\varvec{I}}}$ (also Exposure) The electric charge Q liberated by radiation in a volume V of air per unit density $\rho _{air}$

$$\begin{aligned} D_I=\frac{Q}{V\rho _{air}}. \end{aligned}$$

The unit of measurement is the roentgen (R)

LNT (linear-non-threshold) hypothesis Assumption that relates linearly the incidence of stochastic effects in biological tissue to the amount of absorbed effective dose

Mean lifetime, $\varvec{\tau }$ In a radioactive decay, the time necessary for the number of decay centres to drop by a factor of e. Related to the decay probability $\lambda$ by $\tau =1/\lambda$

Rad Non-SI unit of measurement for the absorbed dose, corresponding to 0.01 Gy

Radioactive decay law Exponential mathematical expression that describes the number of decay centres N(t) at time t following a disintegration process whose characteristic probability is $\lambda$:

$$\begin{aligned} N(t)=N_0 e^{-\lambda t}, \end{aligned}$$

where $N_0$ is the number of decay centers at time $t=0$

Radon Radioactive gas accounting for nearly 80% of the ubiquitous radioactive background dose to humans

Roentgen, R Non-SI unit of measurement for the Ion dose, or Exposure,

1 R=2.58$~ 10^{-4}$ C/kg-air

Rem (roentgen equivalent man) Non-SI unit of measurement for the equivalent radiation dose. 100 rem = 1 Sv

Sievert (Sv) The SI unit of measurement for the equivalent radiation dose. One Sv corresponds to 1 J/kg

Stochastic effects Reaction of biological tissue to a radiation dose causing permanent damage to the DNA of cells. Such reaction can take the form of cancer or heritable mutations

Threshold dose Effective radiation dose above which tissue reactions are expected to take place in biological tissue

Tissue reactions See Deterministic effects

Ubiquitous background radiation Natural radiation responsible for non-negligible dose to all humans. It consists of cosmic and terrestrial external radiation and radionuclides present in the body