1.1. Introduction

We are living in a naturally radioactive world. Our universe developed 1.5 10 ¹⁰ years ago from a high energy density singularity, the so-called "Big Bang". It then expanded and was cooled by this expansion, the current 3-K background radiation being remnant radiation from the first seconds after the Big Bang (Fig. 1.1).

Figure 1.1: Schematic of universe expansion, t = time, T = temperature and k = Boltzmanns constant

Everything we see around us is the substance from a supernova star which exploded 4.6 10 ⁹ years ago. At that moment all possible isotopes of all elements entered the composition of the supernova. The shortest-lived radionuclides decayed quickly, still in the plasma and gaseous phase as revealed by the presence of these radionuclides in the radiation spectrum of newer super-novae. Cooled gas condensed and formed solid dust particles that agglomerated to form the first universal bodies, which collided with each other to form the first protoplanets. Long-lived radionuclides such as ⁹⁹Tc, ²³⁹Pu and ²⁴²Pu, which became part of the composition of the protoplanets, gradually decayed although longer lived radionuclides such as ²³⁵U, ²³⁸U and ²³²Th did not decay completely. These are left as a reminder of the natural radioactivity evolution over the 4.6 billion year lifetime of the Earth.

Until the invention of the X-ray tube by Roentgen in 1885 natural radiation was the only ionising radiation in existence. In 1896 natural radioactivity was discovered by Becquerel...