Definitions

Standard

Measurement of a variable is determined by comparison with a primary standard, which is designated or widely acknowledged as having the highest metrological qualities, and whose value is established without reference to other standards for the same variable. All measurements with respect to the manufacturing or monitoring of LEA products are assessed via comparison with this standard. Standards manufacturing by LEA are used for calibration of customers' instruments.

The International System of Units (SI unit system) is universally acknowledged and applied as the reference system within scientific communications.

Radioactivity

Natural radioactivity, discovered by Henri Becquerel in 1896, is often contrasted with artificial radioactivity. In fact, there is no difference between the two phenomena: quite simply, radioactive isotopes are either natural, having been present in the soil and atmosphere since the formation of the Earth, or artificial, having been created in nuclear reactors or particle accelerators since the 1930's. However, natural sources generally exhibit only low levels of activity.

In physical terms, radioactivity arises when the atoms which make up a given material are unstable. Some of these atoms undergo spontaneous transformations, thereby emitting radiation.

There are three main types of radiation, corresponding to the three forms of radioactivity:

• Alpha radioactivity, which consists in the emission of two protons and two neutrons.
• Beta radioactivity, whereby, in the nucleus of the atom, a neutron is converted into a proton (known as beta- radioactivity, characterized by the emission of an electron, e-), or a proton is converted into a neutron (known as beta+ radioactivity, characterized by the emission of an anti-electron or positron, e+).
• Gamma radioactivity, unlike the other two types outlined above, is not linked to any transformationin the nucleus. It consists in emission of electromagnetic radiation, like visible light or Xrays, generating higher-energy phenomena.

A standard for radioactivity

The unit of radioactivity adopted under the SI system is the becquerel (Bq). This unit measures the transformation (or disintegration) of nuclei, with associated emission of ionizing radiation.
The activity of a radioactive body is quantified on the basis of the number of disintegrations per second occurring in the nuclei of its atoms, measured in becquerels.

One becquerel (Bq) is equal to one disintegration per second.

Variables and units

The definitions given below are taken from the standard NF ISO 31-10:

• Activity
The activity A of a quantity of a radionuclide in a given energy state at a given time is the quotient of dN by dt, where dN is the mathematical expectation of the number of spontaneous nuclear transitions from that energy state during the time period dt.

A = dN/dt • Unit : Bq (Becquerel). 1Bq = 1s-1.

• Decay constant
The decay constant ë, of a radionuclide in a given energy state is the quotient of dP by dt, where dP is the probability of a given nucleus undergoing a spontaneous nuclear transition from that energy stateduring the time period dt.

= dP/dt
Unité : s^-1.

• Radioactive half-life
The radioactive half-life T1/2 of a radionuclide is the time required for the activity of a quantity of that radionuclide to fall to half its initial value. It is related to the decay constant ë via the following equation :

T1/2 = (ln2)/
Unité : s.

• Specific activity
The specific activity Am of a radionuclide present in a mass m of material is the quotient of the activity A of the radionuclide by m.

Am = A/m
Unité : Bq.g^-1.

• Activity concentration
The activity concentration A_v, of a radionuclide present in a volume V of material is the quotient of the activity A of the radionuclide by V.

Av = A/V
Unité : Bq.cm^-3.

Note : Note: this variable is mainly used to characterize gaseous standards. It is considered under STP reference conditions (T = 273.16 K, P = 101.325 kPa).

• Particles flux
The particle flux F, is the quotient of dN by dt, where dN is the increment in the number of particles emitted during the time period dt:

F = dN/dt
Unité : s^-1.

Remarque : Note: this variable is identical to the “emission rate”. The particles considered must be specified. The solid angle within which emission is considered must also be stated.

For example : Flux particle flux within 2 pi sr : F = 3,5 x 103 s^-1.

• Particle mass flux

Unité : s^-1.kg^-1, s^-1.g^-1.

Notes :

• This variable is identical to “mass emission rate”.
• The particles considered must be specified.
• The solid angle within which emission is considered must be equal to 4 pi sr:
Fm = 2,5 x 104 s^-1.g^-1.

Another unit, the curie (Ci), corresponds to the number of nuclei that disintegrate in a gram of radium in one second. The curie was formerly the standard unit, and is still in use.
1 Ci = 3,7 x 10¹⁰ Bq, (37 billion disintegrations per second).

Tableau de conversion Becquerel/Curie

T = Téra (10¹²) m = milli (10^-3)
G = Giga (10⁹) µ = micro (10^-6)
M = Méga (10⁶) n = nano (10-9)
k = kilo (10³) p = pico (10^-12)