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.