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From radium to luminova -part 1

here is the part 1 of a work written by me around 2000 for the Italian magazine " Orologi & Market " ( at that time owned by Pucci ).
footnotes :
1) part 2 coming ASAP ;
2) the translation from italian to english was not mine , so I'm not responsible for all kind of mistakes/typos  and so on .

Full text below.

The need to read time even under inadequate conditions or in the dark was first felt during the first decades of the last century, with the development of military watches (essentially wrist watches), which had to guarantee accurate reading along 24 hours. Both hands and indexes had to be provided with luminescent materials, that is to say, capable of emitting visible light, meaning that their wave length had to be included in a range between 690 and 400 nanometers, corresponding to a band width between dark red and dark violet. Light emission, however, requires for sources of energy (included between 40 and 80 kilocalories per volume of material), which can be either external (heat) or radiating. In other words, luminescence requires for “exciting” substances emitting continuous radiations and for a *”scintillator”, that is to say a phosphorescent material emitting light as long as possible at the end of each excitation. “Phosphors” are materials capable of emitting light uninterruptedly (a phenomenon called *afterglow”) even when the radiation has ceased for a variable period of time - from a millisecond to several days or even years, according to their chemical composition or other “secondary ” conditions. *Fluorescent” materials, instead, emit light spontaneously for about one hundred-millionth of a second at the end of each excitation. These differences in “attitude” can be explained as follows: in both cases, light emission is the result of an external excitation transferred to the electrons present in the material, which pass from a “standard” energy level to a higher level; once this state of excitation is achieved, two main reactions may occur.

1)            In fluorescent materials, the electrons’ initial level of energy is instantaneously restored and light emission is therefore very short (as indicated above);

2)            In phosphorescent materials, instead, at the end of excitation an intermediate energy status is immediately restored in electrons. Such phenomenon is called “electron trap”. It is statistically improbable that an electron abandons such a status if no other stresses intervene in the process (meaning that it can also stand this status for a very long period of time): the time spent under the electron trap status is directly proportional to fluorescence that is to say to light emission. ?

< Radium was the first material to be employed as an exciter (from the 10s until the beginning of the 50s): discovered in 1902 and obtained from uranium refining processes, it essentially releases alpha particles (isotope Ra 228), and to a lower degree, beta particles and gamma radiations (isotope Ra 226) during its radioactive decay. As far as its dangerousness is concerned, it is important to note that the penetration capacity of alpha particles is very limited. A simply sheet of paper, in fact, is sufficient to block them. This means that they do not even pass through the skin most external layer and, therefore, not even through the glass or the back of metallic watches. Therefore the penetration capacity of beta particles is also rather limited (for ex. 6mm in the air), meaning that they also have difficulty to pass through the skin most external layer or through the back of metallic watches. It can only be dangerous in case of ingestion or absorption through many skin layers as it happened to workers in the period between the two world wars that entered in contact with uranium by wetting the brush sunk in radioactive paint to polish dials.

3) The penetration capacity of gamma radiations, instead, is very high. It can be measured from the outside by means of a Geiger counter: starting from the “basic" radiation value (that is to say the natural one), which is equal to 200 millirad/year, tests carried out in the United States on several military radium-painted watches of the 40s, have showed that the emission average values are equal to 20 millirad/hour, meaning that the watch owner was absorbing in ten hours the same quantity of radiations as we all naturally absorb in one year.

To this purpose, it is important to note that risks deriving from radiations are generally considered as directly proportional to the amount of radiations absorbed. This depends on:

a)            the time of exposition

b)            the mass (total quantity) of radiating material

c)            the radiation dose emitted within the time unit by the mass unit of the radiating material

d)            the natural decay speed.

The aforesaid values are to be considered as limit values: according to other studies, the average dose of emission for a radium watch is 4 millirad/year only, that is to say, one fiftieth of natural dose, equal to an hour emission of 0.0013 millirad/hour (supposing that the owner wears the watch for 3000 hours a year). The second “historical” material (employed for the construction of some military watches as from the 40s) is tritium (H3). It is a little radioactive isotope which only releases beta particles during its decays and transforms into helium (He 3) in % equal to 5.5 a year. This means that, every 12.3 years, it transforms half of its initial quantity and that, in this period of time, it contemporaneously loses half of its radioactivity. The years necessary to reduce 50% of materials radioactivity is defined as “half cycle”: as far as tritium is concerned, such a period of time is very short if compared to that of one of the two components of radium, isotope 226, whose half cycle is equal to 1600 years. Apart from the fact that beta particles are not particularly dangerous, it is important to consider that the level of radiation absorption in metallic tritium-painted watches is inferior to 0.03 millirad a year (in any case, much inferior to radiums radioactivity). Absorption in watches equipped with plastic cases is slightly higher: it can be estimated in the range of one five-hundredth with respect to natural radiation, and therefore, in this case too, the risk is irrelevant. The writing “T<25” stamped on many watches as from the 60s (it is important to remember that a law issued by the American Food and Drug Administration in 1961 allowed for the free marketability of unlicensed tritium-painted watches), means that tritium radioactive emission is inferior to 25 milliCurie. As the Curie is a measure representing the quantity of radioactivity contained in one gram of radium, T<25 means that each gram of tritium present in paint, produces a level of radioactivity inferior than 25:1000 of one gram of radium, that is to say, 40 times less.