Particle physics in everyday life

The search for the nature of the fun­da­ment­al com­pon­ents of the Uni­verse really began at the start of the 20^th Cen­tury. The “atoms” ima­gined by Demo­crit­us 300 years before Christ really began to be under­stood when it was real­ised that con­trary to their ety­mo­logy (a‑tomos = unbreak­able, that which can­not be divided into smal­ler ele­ments), atoms were them­selves com­posed of even smal­ler elements. 

Particles, which, as things stand today, are the most basic ele­ments of mat­ter. But as is often the case in the evol­u­tion of sci­ence, research that ini­tially served no pur­pose oth­er than to gain a detailed under­stand­ing of the laws of nature and its com­pon­ents has led to applic­a­tions that have pro­foundly changed our daily lives. How has particle phys­ics found its way into our daily lives? 

The gen­er­al prin­ciple is often the same: dir­ect a beam of particles at a tar­get and study or use its effects. Depend­ing on the type of particle used and the tar­get chosen, the con­sequences (and uses) will be very different.

Let’s start with some­thing that this art­icle could not have been writ­ten without: electronics.

The basic prin­ciple of all mod­ern elec­tron­ics is the use of sil­ic­on, which belongs to the class of “semi­con­duct­ors”. A semi­con­duct­or is char­ac­ter­ised by the num­ber of charge car­ri­ers it has (elec­trons or elec­tron gaps called “holes”). To increase this num­ber of charge car­ri­ers, « impur­it­ies » are intro­duced into the sil­ic­on crys­tal, atoms that add or remove elec­trons and thus loc­ally modi­fy the elec­tric­al prop­er­ties of the medi­um. This is called doping.

Dop­ing must be car­ried out in an extremely pre­cise man­ner: one part of the sil­ic­on crys­tal must be doped with an excess of elec­trons while, a few micro­metres deep­er, anoth­er part must be doped with atoms that remove these electrons.

The arti­fi­cial inser­tion of these dop­ing atoms can be done by “ion implant­a­tion”: they are accel­er­ated by an elec­tric field that gives them a great­er or less­er energy, which allows them to pen­et­rate more or less deeply into the sub­strate to dope cer­tain lay­ers at pre­cisely determ­ined depths.

The irra­di­ation of mater­i­als can be vol­un­tary or invol­un­tary. But in all cases, it mod­i­fies their micro­struc­ture, and this is why it will be used or stud­ied in order to bet­ter under­stand the prop­er­ties of these mater­i­als and their evol­u­tion over time.

Ion implant­a­tion is a sur­face treat­ment pro­cess that can also be applied in many situ­ations oth­er than elec­tron­ics. It allows the chem­ic­al com­pos­i­tion and sur­face struc­ture of a mater­i­al to be mod­i­fied. Depend­ing on the nature of the sub­strate and the implanted ion, cer­tain mech­an­ic­al or chem­ic­al prop­er­ties of the sur­face (hard­ness, wear res­ist­ance, fatigue, cor­ro­sion res­ist­ance, etc.) can thus be optim­ised without chan­ging its main properties.

The phe­nomen­on of age­ing under irra­di­ation is mainly stud­ied in the nuc­le­ar sec­tor. At the heart of today’s nuc­le­ar power plants, steel is sub­jec­ted to intense irra­di­ation from the radio­act­ive fuel rods used to power the install­a­tion. The react­or ves­sel, for example, is a non-replace­able com­pon­ent. It is vital to know and anti­cip­ate the age­ing of its struc­ture over dec­ades of use.

But this work is also use­ful for the next gen­er­a­tions of react­ors, whose tem­per­at­ure and irra­di­ation con­di­tions will be even more demand­ing than today, not to men­tion future ther­mo­nuc­lear fusion react­ors, such as ITER, whose mater­i­als in con­tact with the plasma under­go intense neut­ron irradiation.

In the food industry, food irra­di­ation is one of the meth­ods used to extend the shelf life of foods. This tech­nique makes it pos­sible to stop the ger­min­a­tion pro­cess (pota­toes, seeds, etc.) and to kill the para­sites, moulds, and micro-organ­isms respons­ible for the deteri­or­a­tion and/or rot­ting of food.

To do this, three types of radi­ation are used: X‑rays or gamma (ɣ) rays (which are two types of elec­tro­mag­net­ic radi­ation, like light, but whose energy is much high­er than the part vis­ible to the eye), or elec­tron accelerators.

How­ever, this tech­nique does not com­pletely ster­il­ise food (which still needs to be pack­aged and cooked prop­erly), but it slows down spoil­age and allows it to be stored for longer. It also pre­vents insects and oth­er pests from lay­ing eggs in fresh pro­duce and des­troy­ing them.

Gamma irra­di­ation is also used in agri­cul­ture. It is called muta­gen­es­is by gamma irra­di­ation. The prin­ciple is to sim­u­late (and accel­er­ate) the pro­cess of genet­ic muta­tion that occurs nat­ur­ally in the liv­ing world. This tech­nique, which has been used since the 1950s, makes it pos­sible to select new plant strains with favour­able muta­tions (taste, col­our, growth, fruit size, etc.).

In France, for example, in 2014, 370 000 hec­tares of sun­flower crops – i.e. 56% of pro­duc­tion – were grown from seed­lings obtained by muta­gen­es­is. In Texas, 75% of the grapefruits grown are of the Rio Star vari­ety (red­der and sweeter), also pro­duced by the muta­gen­es­is process.

The med­ic­al com­munity also bene­fits from the advant­ages of elec­tron accel­er­at­ors in terms of ster­il­ising equip­ment. The use of radio­act­ive sources of Cae­si­um 137 also makes it pos­sible to treat blood bags using the gamma rays emit­ted, in order to elim­in­ate cer­tain cells that could cause a fatal dis­ease in patients requir­ing a trans­fu­sion. Saline solu­tions used to clean and store con­tact lenses are also ster­il­ised by irradiation.

In nuc­le­ar medi­cine, the use of nuc­le­ar react­ors or particle accel­er­at­ors allows the cre­ation of radio­act­ive com­pounds that do not exist nat­ur­ally on Earth (as they decay in times ran­ging from minutes to days). How­ever, these ele­ments are very import­ant, both in terms of dia­gnost­ic ima­ging (e.g. Positron Emis­sion Tomo­graphy which uses a radio­act­ive ele­ment: Flu­or­ine-18 or scin­ti­graphy with Tech­ne­tium-99) and also in terms of ther­apy (Iod­ine 131 for the treat­ment of thyroid cancer).

Cur­rently, a new tech­nique for irra­di­at­ing can­cer­ous tumours is being developed: had­ron­ther­apy. This tech­nique uses a particle accel­er­at­or to tar­get tumours inside the patient’s body that are dif­fi­cult to treat with oth­er con­ven­tion­al tech­niques (often brain tumours). This is an extremely tar­geted radio­ther­apy tech­nique, the advant­ages of which, in terms of pre­ci­sion and patient radiopro­tec­tion, give reas­on to hope that, in addi­tion to Ger­many and Italy, a centre may be built in France in the com­ing years.