2.2.4 The positron decay β<sup>+</sup>

In case of radioactive decay by positron emission, a proton in the nucleus is transformed into a neutron and a positively charged electron (positron β<sup>+</sup> ) then a proton ejected from the nucleus. A positron is an antiparticle of an ordinary electron:

$$\mathbf{p}^+ \to \mathbf{n} + \mathfrak{k}^+ + \mathfrak{v} + \text{energy} \tag{4}$$

A ¼ λN (5)

After ejection from the nucleus, it loses its kinetic energy in collision with atoms of the surrounding matter and comes to rest; this usually happens within a few millimeters from the site of its origin in body tissue [6].

#### 2.2.5 Alpha decay

Radionuclide that decayed by a particle emission or by nuclear fission has relatively little importance for direct usage as tracers in nuclear medicine.

Both of these decay modes occur primarily among very heavy elements that are of a little interest as physiological tracers [7].

The particles, which are released with kinetic energy, are usually found between 4 and 8 MeV.

Decay by alpha particle emission results in transmission of elements, but it is not isobaric.

Activity: It is the total number of nuclei that are decaying per second. It is the probability that any individual atom will undergo decay during the same period:

Figure 9. The nucleus captures one of its orbital electrons and X-ray.

where A = activity; N = the number of decay nuclei in the sample; λ = decay constant.

The decay factor (e�λ<sup>t</sup> ) is an exponential function of time (t). Exponential decay is characterized by disappearance of a constant function of activity or number of atoms prevented per unit time interval:

$$\mathbf{A} = \mathbf{A}\_0 \ e^{-\mathbf{\hat{x}}} \tag{6}$$

where A is the activity of radionuclide at a given time t; A0 is the activity of radionuclide at time t = 0; decay constant (λ).

The decay constant (λ) is the probability that a nucleus will decay per second, so its unit is (s�<sup>1</sup> ). Activity can be determined by direct measurement.
