DOZIMETRIE• 1. Tipuri de radiatii nucleare si caracteristici

• 2. Interactia radiatiei cu substanta

• 3. Actiunea biologica a radiatiilor

• 4. Detectori de radiatii

• TIPURI DE RADIATII NUCLEARE SI CARACTERISTICI• Dozimetria sau dozimetria radiatiilor nucleare este un capitol al metrologiei radiatiilor

nucleare care se ocupa cu masurarea dozei de radiatii nucleare.• Aplicatiile radionuclizilor se bazeaza in principal pe folosirea radiatiilor emise in

procesul dezintegrarii radioactive.• Radiatie-un fascicul de particule in miscare.• Termenul de particula, folosit aici in sensul cel mai general, cuprinde atat fotonii

(particule cu masa de repaus nula) cat si corpusculele (particule cu masa de repaus diferita de zero).

• • Fasciculele de fotoni constituie radiatiile electromagnetice, iar fasciculele de

corpuscule (radiatiile alfa, deuteronii, electronii, protonii etc.) constituie radiatii corpusculare.

• Radiatiile nucleare cuprind o parte din radiatiile electromagnetice si anume cele penetrante: radiatiile de franare, radiatiile Rontgen (sau X), radiatiile de anihilare si radiatiile gamma.

• Radiatiile corpusculare sunt formate din particule elementare (electroni, mezoni, protoni, neutroni etc) si din nuclee de atomi (deuteroni, helioni etc.) in miscare.

• Caracteristica principala a radiatiilor nucleare este faptul ca, prin interactiunea cu substanta ele produc, direct sau indirect, ionizarea acesteia.

Radiatiile alfaRadiatiile alfa, fiind nuclee de heliu, au masa de repaus mare, ceea ce le permite sa se deplaseze rectiliniu si cu parcurs mic. In functie de energie, parcursul in aer al radiatiilor alfa este cuprins intre 2 si 10 cm, iar energiile emise de acestea sunt cuprinse in intervalul 2÷9 MeV.Intr-un camp magnetic, radiatiile alfa emise de o substanta radioactiva sunt deviate sub forma unui fascicul ingust, ceea ce inseamna ca ele practic sunt, emise cu aceeasi energie.Acest tip de radiatii provoaca o puternica ionizare specifica. In medie, o particula cu energia de 2 MeV produce in aer 60 000 perechi de ioni pe cm.Radiatiile betaRadiatiile beta sunt compuse din particule cu sarcina electrica negativa sau pozitiva. In literatura sunt cunoscute sub numele de radiatii beta minus (negatroni), respectiv radiatii beta plus (pozitroni). Aceste particule au o masa de repaus de peste 7300 ori mai mica decat a particulelor alfa.

Intr-un camp magnetic, radiatiile beta emise de o substanta radioactiva sunt deviate sub forma unui fascicul larg, distributia energiilor fiind continua, de la valori foarte mici (aproape zero) pana la o valoare maxima, bine determinata. Aceasta este numita limita superioara a spectrului continuu beta, iar energia medie a unui spectru de radiatii beta emis de o anumita substanta radioactiva, reprezinta aproximativ 40% din valoarea acesteia. Spectrul energetic continuu al radiatiilor beta determina un parcurs care variaza in limite foarte largi. Parcursul lor in aer este de la cativa mm pana la cativa m si chiar zeci de metri.

Radiatiile gamma si RontgenRadiatiile gamma si Rontgen sunt radiatii de natura electromagnetica care se caracterizeaza printr-un parcurs foarte mare si o ionizare specifica mica. In functie de energia lor, pot strabate in aer un parcurs cuprins intre metri si sute de metri.Cele doua tipuri de radiatii electromagnetice nu se diferentiaza prin natura. Radiatiile gamma au in general o energie mai mare implicit o lungime de unda mai mica fata de radiatiile Rontgen si o putere mai mare de patrundere printr-un mediu material.

NeutroniiNeutronii sunt particule lipsite de sarcina electrica, cu masa de repaus egala cu a protonilor (nuclee de hidrogen) mai mica de patru ori decat a radiatiilor alfa si aproape de 2 000 ori mai mare decat a radiatiilor beta. Parcursul lor printr-un mediu material este in forma de zig-zag datorita interactiunii lor cu nucleele si electronii atomilor acestuia. Materialele usoare (parafina, grafit, apa, apa grea, beriliu metalic, oxid de beriliu etc.) sunt material care incetinesc neutronii si se numesc moderatori.Borul si cadmiul absoarbe neutronii si materialele grele ii incetinesc mult mai putin.Functie de energia lor, neutronii sunt calasificati astfel:- neutroni termici cu energie < 1eV- neutroni de rezonanta cu energie 100 eV ÷ 1000 eV- neutroni intermediari (epitermici) cu energie 103 eV ÷ 106 eV- neutroni rapizi cu energie > 106 eV

INTERACTIA RADIATIILOR CU SUBSTANTAProcesul de interactiune a radiatiilor cu substanta este important in cunoasterea efectului acestora asupra detectorilor cat si a organismului. Procesele de interactiune ale radiatiilor gammaPentru energiile pe care le au radiatiile gamma emise de surse radioactive uzuale, principalele procese de interactiune sunt:a) efectul fotoelectricb) efectul Comptonc) formarea perechilor de electroni

Efectul fotoelectric - are loc la intalnirea fotonului gamma incident cu un electron de pe una din paturile electronice ale atomului substantei cu care interactioneaza, process prin care fotonul cedeaza electronului intreaga sa energie , acesta din urma fiind smuls din atom. Efectul Compton - are loc la intalnirea fotonului gamma incident de energie hν, cu un electron liber sau usor legat. Formarea de perechi (efectul de materializare electron-pozitron) – apare ca urmare a interactiunii dintre radiatia gama sau Rontgen cu o energie mai mare de 1.022 MeV si campul nucleului. Atenuarea radiatiilor Rontgen si gammaLa trecerea radiatiilor gama printr-o substanta, acestea sunt atenuate treptat, ca urmare a proceselor de interactiune care au loc. Pentru grosimi mari strabatute, intensitatea lor scade mult, atenuarea fiind exponentiala ca si in cazul radiatiilor beta. In acest caz insa se poate vorbi de o valoare determinata a parcursului lor maxim; atenuarea creste pentru materialele cu densitate mare.

Procesele de interactiune ale neutronilor

Neutronii interactioneaza numai cu nucleele atomilor. Ei nu au sarcina electrica si nu este necesar sa aiba o energie cinetica ridicata pentru a strabate campul coulumbian al atomului pentru a ajunge la nucleu. Cu nucleul interactiona si neutronii cu energii foarte joase. Probabilitatea de patrundere a neutronilor in nucleu este ridicata, in special cei cu energie cinetica scazuta (neutroni termici).

La trecerea neutronilor prin materie sunt posibile trei tipuri de interactiuni: imprastiere elastica, imprastiere neelastica si absorbtie (captura neutronica).

Interactia radiatiilor alfa cu substanta

La trecerea radiatiilor alfa printr-o substanta, ele pot suferi trei tipuri de interactiuni: ciocnire, franare in camp electric si captura de catre nucleu. Ciocnirea este tipul de interactiune care se produce cu probabilitatea cea mai mare. In urma ciocnirii unei particule alfa in miscare cu un atom, se poate produce o excitare a acestuia ca urmare a ridicarii unui electron pe un nivel de energie superior. Excitarea are loc ca urmare a actiunii campului electric creat de particula alfa respectiva asupra electronilor orbitali. La revenirea electronilor pe nivelele fundamentale, atomii vor emite radiatii electromagnetice, unele in spectrul vizibil. Prin interactiuni succesive, radiatia alfa isi pierde energia sa si se incetineste pana cand energia ei scade sub o anumita limita la care nu mai poate produce ionizari. In acest stadiu, particulele alfa capteaza cate doi electroni de la atomii mediului strabatut si se transforma in atomi de heliu.

Atenuarea fasciculului de radiatiiIn urma proceselor de interactiune, particulele alfa isi pierd treptat energia si numai in final sunt imprastiate. Numarul de particule dintr-un fascicul incident nu scade pe masura ce acestea strabat o substanta oarecare. La o anumita grosime a materialului ele sunt total absorbite, procesul numindu-se atenuare cu parcurs.

Interactia radiatiilor beta cu substanta

Mecanismul interactiunilor este asemanator celui intalnit in cazul radiatiilor alfa. Exista deosebirea ca, pentru radiatiile beta minus, fortele din atomii materialelor care actioneaza aupra electronilor sunt de respingere. Spectrul energetic al radiatiei de franare este continuu; energia maxima a limitei superioare a spectrului radiatiei Rontgen de franare este socotita egala cu energia maxima a radiatiei beta incidenta.Atenuarea fasciculului de radiatiiFascicolul de radiatii beta isi pierde energia fie interactionand cu electronii atomilor mediului strabatut, fie interactionand cu nucleul. La fiecare ciocnire, radiatia beta sufera mari fluctuatii statice in ceea ce priveste pierderea de energie.

In general se foloseste parcursul masic maxim Rm max=Rm·ρ (ρ-densitatea materialului, g/cm3). Calcularea parcursului maxim, pentru o anumita substanta se face impartind valoarea parcursului masic maxim la densitatea absorbantului ρ [g/cm3]. In practica, valorile parcursului maxim al radiatiilor beta in aluminiu functie de energia radiatiilor si se recomanda a fi citit din diagrama.

Actiunea biologica a radiatiilor

La interactiunile radiatiilor ionizante cu substanta vie au loc aceleasi procese ca si la interactiunea cu materia fara viata, diferenta fiind aceea ca efectele finale la care conduc aceste interactiuni sunt efecte biologice.Radiatiile ionizante pot actiona asupra organismului in trei moduri: i)actiune directa; ii) actiune indirecta; iii) actiune de la distanta.In urma actiunii directe a radiatiilor asupra organismului sunt lezate macromoleculele de importanta vitala (proteine, acizi nucleici) care sufera transformari datorita ionizarii sau excitarii directe.

Mediul principal in care se desfasoara procesele biologice fiind apa, efectele apar ca rezultat al ionizarii acesteia prin aparitia produsilor de descompunere (ioni sau radicali) care actioneaza ca oxidanti si reducatori asupra unor componente celulare esentiale. Astfel este perturbata buna desfasurare a proceselor biologice din aceste celule.

Marimi si unitati folosite pentru evaluarea efectelor biologice

Diversitatea tipurilor de radiatii a impus definirea unui sistem de masurare a efectelor biologice ale radiatiilor cu marimi corespunzatoare intregului domeniu si unitati de masurare adecvate. A fost introdusa notiunea de factor de calitate (FC) pentru a putea explica faptul ca unele radiatii produc efecte mai daunatoare decat altele.

Doza absorbita D reprezinta raportul dintre ΔW si Δm, in care ΔW este energia medie comunicata de radiatiile nucleare unei cantitati de materie de masa Δm.Unitatea SI pentru doza absorbita este gray (Gy), care corespunde unei energii cedate de un joule pe Kg (1Gy=1J·Kg-1). Se mai mentine temporar si unitatea speciala denumita rad (rontgen absorbed dose)1rad=10-2Gy=10-2 J·Kg-1 sau 1rad=100 er·g-1

Relatia pentru doza absorbita este de forma:D= ΔW/ Δm [Gy] sau [rad]

Debitul dozei absorbite, reprezinta variatia dozei absorbite, ΔD, in intervalul de timp Δt si are ca unitate de masura Gy sau rad cu multiplii sau submultiplii lor, raportata la unitatea de timp: Gy/s; Gy/min; Gy/h sau rad/s; rad/min; rad/h etc.=ΔD/Δt [Gy·h-1] sau [rad·h-1]Echivalentul dozei, H este produsul a trei termeni: D, FC si N, intr-un punct considerat al tesutului unde D este doza absorbita, FC este factorul de calitate si N este produsul tuturor celorlalti factori modificatori intre care si factorul de distributie; N se considera 1. Pentru factorul de calitate, orientativ se dau urmatoarele valori:1-pentru radiatii rontgen, radiatiile gama si electroni;10-pentru neutroni, protoni si particulele cu sarcina unica care au o masa de repaus superioara unitatii de masa atomica;20-pentru particulele alfa si particulele cu sarcini multiple.Unitatea SI a echivalentului dozei, H, se numeste Sievert (Sv), 1Sv=1J·Kg-1.1rem=10-2 Sv; 1Sv=10-2J·Kg-1 sau 1 rem=100 erg/gtesut

Relatia pentru echivalentul dozei este:H=D·(FC) ·N [Sv] sau [rem]

Debitul echivalentului dozei, , reprezinta variatia echivalentului dozei, ΔH, in intervalul de timp, Δt si se masoara in unitati adecvate, derivate din Sv sau rem (multipli si submultipli) raportati la unitatea de timp considerate: Sv/s; Sv/min; Sv/h; rem/s; rem/min; rem/h etc.= [Sv·h-1] [rem·h-1]Din punct de vedere al energiei cedate, se poate considera ca exprimarea iradierii in unitatile rontgen, rad sau rem sunt aproximativ echivalente.

Efectele biologice sunt efecte somatice si efecte genetice.Efecte somatice (asupra organismului)- se manifesta dupa un interval mai scurt: i) efecte imediate cum sunt efecte localizate-eritem, epilare, arsuri ale pielii etc.; ii) efecte care apar dupa intervale mai lungi (ani sau zeci de ani), efecte tardive.

Efecte genetice (asupra urmasilor)- cele care reclama o diminuare a calitatilor urmasilor si apar pana la 3 sau 4 generatii. Numarul mutatiilor genetice depinde numai de doza totala absorbita la nivelul gonadelor si nu de debitul dozei.In functie de modul de expunere, respective de repartizare a iradierilor, distingem: i) iradiere profesionala; ii) iradierea populatiei.

Iradierile profesionale sunt atunci cand rezulta din activitati legate direct de lucrul cu surse de radiatii nucleare (ex.-personalul unitatilor nucleare este expus profesional actiunii radiatiilor ionizante). Limita anuala a echivalentului dozei efectiv pentru lucratori este de 20mSv (2 rem).

Iradierile populatiei sunt atunci cand nu rezulta direct din efectuarea unor lucrari cu substante radioactive sau surse de radiatii ionizante. Limita anuala a echivalentului dozei efectiv pentru persoanele din public este de 1mSv (0.1 rem).

Limita dozei admisa pentru organism, organ sau tesut oarecare trebuie inteleasa ca doza primita atat prin iradierile externe pe durata orelor de lucru cat si prin iradierile interne. In Normele Republicane de Securitate Radiologica (NSR) sunt date atat activitatile maxim admise in organul critic cat si concentratiile maxim admise (CMA) in aer si apa pentru fiecare radionuclid in parte.

Detectarea si masurarea radiatiilorDetectorii de radiatii se bazeaza in special pe efectele produse de radiatii la interactiunea lor cu substanta.

Detectori de radiatii

Detectorii utilizati in mod curent in activitatile nucleare:-detectori bazati pe ionizarea gazelor si conectarea ionilor (camerele de ionizare, contorii proportionali, contorii Geiger-Muler);-detectori bazati pe ionizarea cristalelor (contori cu cristal);-detectori la baza carora sta fenomenul de impresionare a emulsiilor fotografice (emulsiile nucleare, filmele dozimetrice etc.);-detectori bazati pe fenomenul de luminiscenta (contorii cu scintilatie);-detectori bazati pe fenomenele de termoluminiscenta si fotoluminiscenta (detectori termoluminiscenti).

Dozimetrul Rontgen-gama VA-J-15A este un aparat portabil destinat masuratorilor de doze si debite de doze date de radiatiile Rontgen si gama cu un spectru larg de energie 20 KeV…1,2 MeV ca si a detectarii calitative a radiatiilor beta emise de diferiti radionuclizi. Aparatul are ca detector o camera de ionizare cu aer (mediul in care se face masuratoarea). Detectorul este cuplat la partea principala a aparatului. Domeniul de masurare este larg, impartit in doua domenii. Fiecare domeniu are cate 6 scale atat pentru debitul dozei cat si pentru doza integrata. Eroarea de masurare este ±15%.

Dozimetrul Rontgen-gama VA-J-15Acuplarea sondeiprin cablu de legatura

1-stecherul de legatura cu aparatul principal2-instrumentul de masurat3-surubul pentru pozitionarea punctului zero mecanic4-fixatorul punctului zero electric5-salterul, domeniul de masurare fin6-cureaua de purtare7-cuplajul pentru inscriptor8- lacasul bateriilor9- calul de legatura10-stecherul capului de masurare11- salterul “domeniul de masurare grosier”12-butonul pentru sursa de radiatii de control13- camera14-capacul de protectie al camerei

Gamarad DL-7- este destinat masuratorilor de debite ale expunerii externe in campuri de radiatii rontgen si gama

in scopul radioprotectiei personalului cu expunere profesionala la radiatii din unitatile radiologice fig 4.7,

pg 71

El este utilizat in situatii specific ca: i) supraveghere a zonelor de radiatii (monitor);ii) masuratori de rutina ale debitelor expunerii;

iii)localizarea surselor de radiatii;iv)sesizarea cazurilor de contaminare.

Gamarad DL-7

Aparatura pentru supravegherea dozimetrica individuala Printre sistemele de dozimetrie individuala se practica supravegherea cu:

1. camere de ionizare (stilodozimetre);2. fotodozimetre;3. dozimetre TL

Camera de ionizare (stilodozimetru)- este destinat controlului dozimetric individual la iradierea cu radiatii Rontgen, gama si beta dure (beta moi fiind oprite de peretii stiloului). Exista si

stilodozimetre cu electroscop (cu fir), acestea fiind prevazute cu sistem optic ce permite citirea direct, in milirontgen, expunerea in intervalul de timp cat a fost expus radiatiei.

Pentru stilodozimetre este necesara si instalatia de incarcare, utilizata inainte de a fi purtat. Periodic este necesara etalonarea si cu aceasta ocazie se verifica si autodescarcarea stilodozimetrelor. La

terminarea lucrului, stilodozimetrele sunt depozitate in loc ferit de actiunea radiatiilor.

Dozimetrul individual cu film (fotodozimetru)Acesta se compune din film dozimetric si caseta cu filtre metalice in care se inchide filmul. Este destinat masurarii dozelor de radiatii gama, Rontgen, beta si neutroni termici incasate pe timpul lucrului de persoana purtatoare.Filmul dozimetric are doua pelicule de sensibilitati diferite pentru acoperirea unui interval de masurare.Parametrii dozimetrici ce caracterizeaza filmul:

- este un dozimetru integrator;- are interval larg de masurare (0.17 mSv-1Sv);- raspunsul dozimetric functie de energia radiatiilor ionizante este mare,

ceea ce impune utilizarea de filtre atenuatoare si discriminatoare de energie;-precizia de masurare este ≤ 30%;-imaginea fotografica se pastreaza timp indelungat

Dozimetrul individual cu detectoare termoluminiscente (dozimetru TL)Acesta se compune din caseta cu filtre metalice asezate in lacasuri amenajate in care se pun detectoare termoluminiscente. Este destinat masurarii dozelor de radiatii gama, Rontgen, beta si neutroni termici incasate pe timpul lucrului de persoana purtatoare.Detectoarele termoluminiscente sunt de diferite forme (pudra, cips, tablete etc.) cu sensibilitate mare pentru acoperirea unui interval larg de masurare.Parametrii dozimetrici ce caracterizeaza dozimetrul TL:

- este un dozimetru integrator;- are interval larg de masurare (0.10 mSv-1Sv);- raspunsul dozimetric functie de energia radiatiilor ionizante este neglijabila.- eroarea de masurare este ≤ 10%;- citirea detectoarelor termoluminiscente este rapida;- se pot refolosi dupa regenerare pana la 80 ori.

DOSIMETRY

1. Types of nuclear radiation and characteristics2. Interaction between radiation and substance3. Biological effect of radiaton4. Radiation Detectors

Types of nuclear radiation and characteristics

Nuclear radiation dosimetry and dosimetry metrology is a chapter dealing with nuclear radiation nuclear radiation dose measurement.

Dosimetry or radiation dosimetry is a part of nuclear radiation wich treats the measurement of radiation doses.

Radionuclide applications are mainly based on the use of radiation emitted in radioactive decay process.

The term of “particle” is used here in the most general sense, including both photons (particles with zero rest mass) and corpuscles (particles with nonzero rest mass).

Photon beams is electromagnetic radiation and beams corpuscles (alpha, deuterons, electrons, protons etc..) are corpuscular radiations.

Electromagnetic radiation is made up of photon beams and beams corpuscles (alpha, deuteronis, electrons, protons, etc..) constitutes corpuscular radiations.

Nuclear radiation includes some of the electromagnetic radiations, namely the penetrating electromagnetic radiations are: braking radiation, Rontgen rays (or X) annihilation and gamma radiations.

Corpuscular radiations are made up of elementary particles (electrons, mesons, protons, neutrons, etc.) and the moving nuclei of atoms (deuteron, helion etc.).

The main feature of nuclear radiation is that, by interacting with a substance they produce, directly or indirectly, its ionization.

Alpha radiation

Alpha radiation, being helium nuclei, have a big rest mass,( the helium nuclei have large rest mass,) allowing them to move straight and on a short distance (go small). Depending on the energy of alpha radiation, the distance traveled in during air is between 2 and 10 cm, and the energy emitted is between 2 and 9 MeV.In a magnetic field, alpha radiation emitted by a radioactive substance are diverted as a narrow beam, which means that they basically are emitted with the same energy.This type of radiation causes a strong specific ionization. On average, a particle with the energy of 2 MeV produces, in air, 60.000 pairs of ions per cm.

Beta radiation

Beta rays are composed of particles with negative or positive electrical charge.In literature they are known as beta minus radiation (negatrons) and beta plus radiation (positrons). These particles have a rest mass over 7300 times smaller than alpha particles.

In a magnetic field, beta radiation emitted by a radioactive substance are deflected in the form of a broad beam and energy distribution is continuous from very small values (close to zero) to a maximum value well determined. This is called the upper limit of the continuous beta spectrum and average energy of a beta radiation spectrum emitted by a radioactive substance, represents approximately 40% of its value. Continuous energy spectrum of beta radiation causes a period which in large limits. The distance traveled in air is between a few millimeters to several meters and even tens of meters.

Gamma and Rontgen radiation

Rontgen rays and gamma rays are electromagnetic in nature and are characterized by a great distance traveled and a small specific ionization. Depending on their energy can cross, in air, a distance of meters and hundreds of meters.The two types of electromagnetic radiation are not different in nature. Gamma rays generally have a higher energy implying a lower wavelength than the Rontgen rays and more power to penetrate through a material medium.

Neutrons

Neutrons are particles without an electrical charge, with a rest mass equal to that of protons (hydrogen nuclei), four times smaller than that of alpha radiation and almost 2000 times larger than beta radiation. Their course through a material medium is shaped in a zigzag because of their interaction with nuclei and electrons of atoms. Lightweight materials (wax, graphite, water, heavy water, beryllium metal, beryllium oxide etc.) are materials which slow down the neutrons and are called moderators.Boron and cadmium absorbs neutrons and heavy materials slow them down less.

Depending on their energy neutrons are classified as:

- Thermal neutrons with energy <1eV- Resonance neutrons with energy 100 eV to 1000 eV- Intermediate neutrons (epithermal) with energy 103 eV ÷ 106 eV- Fast neutrons with energy> 106 eV

RADIATION INTERACTION WITH SUBSTANCE

The interaction of radiation with substance is important in knowing their effect on detectors and on the body .Processes of interaction of gamma radiation

For the energies of gamma rays emitted by common radioactive sources, the main interaction processes are:a) the photoelectric effectb) Compton effectc) formation of electron pairsPhotoelectric effect – when a gamma photon meets and electron of an atom with which it interacts, the photon gives up all its energy and the electron is plucked away from the atom.Compton effect - happens when a gamma photon with hν energy meets a free electron or a slightly connected one.Formation of pairs (electron-positron materialization effect) – apears from the interaction of gamma rays or Rontgen with an energy greater than 1.022 MeV and core field??? .Rontgen rays and gamma attenuationWhen passing through a substance, the gamma rays are gradually weakened as a result of the interaction processes. The attenuation is exponential as for beta radiation and, for thick layers the intensity drops a lot. But in this case we can speak of a determined travel distance ; attenuation increases proportional to the density of materials.

Interaction processes of neutronsNeutrons interact only with nuclei. They have no electrical charge and do not require a high kinetic energy to cross the coulombian field and to reach the nucleus . Low energy neutrons can also interact with the nuclei . The probability of neutrons penetrating in the nucleus is high, particularly those with low kinetic energy (thermal neutrons).When passing through matter, neutrons have three possible interactions: elastic scattering, inelastic scattering and absorption (neutron capture).

Alpha radiation interaction with substanceWhen passing through a substance, alpha radiation can suffer three types of interaction: collision, slowing down in an electric field and capture by the nucleus. The clash is the interaction with the highest probability . Alpha particles in a collision with a moving atom can produce its excitation due by lifting an electron to a higher energy level. Excitation occurs as an effect of the electric field produced by the alpha particle on the orbital electrons . When the electrons return to fundamental levels, the atoms emit electromagnetic radiation, some in the visible spectrum. Through successive interactions, alpha radiation loses its energy and slows down until its energy falls below a certain limit after which it can not longer produce ionization. At this stage, alpha particles capture two electrons from atoms and turn into helium atoms.

Radiation beam mitigation

After the interaction process, alpha particles gradually lose energy and only at the end, they are scattered. The number of particles in an incident beam does not decrease as they travel through a certain substance. After a certain travel distance through the material they are totally absorbed, the process being called the mitigation of course.

The interaction of Beta radiation with substanceThe interaction mechanism is similar to that of alpha radiation. The difference is that, for beta minus radiation, the forces of atoms acting on electrons are of repulsion. The energy spectrum of the braking radiation is continuous; Rontgen radiation’s braking specter has a maximum energy, considered the upper limit, equal to the maximum energy of the incident beta radiation.Radiation beam attenuationBeta radiation beam loses energy by interacting with electrons and also nuclei of the atoms from the environment it corsses. After each collision, beta radiation has a great static fluctuation regarding energy loss.

Generally, the maximum mass travel, Rmax = Rm · ρ (ρ- material density, [g/cm3]), is used. To calculate the maximum travel distance, for a given substance, the maximum mass travel is divided to the absorbant’s denisity ρ [g/cm3]. In reality, the maximum travel distance of beta radiation in aluminum depends on the radiation energy and is recommended to be read from the diagram.

Biological action of radiation

Just as with matter, radiation interacts with living substance in the same way, only that the effects are biological.Ionizing radiation can affect the body in three ways: i) direct action; ii) indirect action; iii) action from a distance.Following the direct action of radiation on the body macromolecules of vital importance are damaged (proteins, nucleic acids) which are processed due to direct ionization or excitation.The main environment in which biological process occur is water. The effects appear as a result of ionization and that leads to the formation of ions and radicals, which act as ?redox? On essential cellullar components.

Quantities and units used to evaluate the biological effects

The diversity of radiation led to the creation of a system used to measure the biological effects of radiation corresponding to all scales and units. The notion of quality factor (QF) was introduced in order to explain that some radiation are more harmful than others.

Absorbed dose D is the ratio between ΔW and Δm, where ΔW is the mean energy of nuclear radiation transmitted to a mass, Δm.The SI unit for the absorbed dose is the gray (Gy), which corresponds to an energy of one joule per kilogram transferred (1Gy = 1J · kg-1). Another unit sometimes used is the rad (Rontgen absorbed dose).

1rad = 10-2GY = 2.10 J · kg-1 or 1rad = 100 erg ·g-1 The relative absorbed dose is:D = ΔW / Δm [Gy] or [rad]

The rate of dose absorbtion, 𝐷 ̇�, is defined as the variation of the absorbed dose, ΔD, in the time interval Δt and it’s measured in Gy, multiples or submultiples divided by unit of time: Gy /s, Gy /min, Gy /h or rad /s, rad /min, rad / h etc.

= Dd / Dt [Gy · h𝐷 ̇� -1] or [rad · h-1]The dose equivalent, H, is the product of three terms: D, QF and N, in a point considered of tissue volume, where D is the absorbed dose, QF is the quality factor and N is the product of all other modifying factors among which if distribution factor;N is considered 1. Concerning the quality factor, the following guidance values are given:1-for Rontgen rays, gamma rays and electrons;10-for neutrons, protons and particles with a single charge rest mass atomic mass unit higher;20-for alpha particles and particles with multiple tasks.The SI unit of dose equivalent, H, is called the Sievert (Sv), 1Sv = 1J · kg-1.1rem = 2 ·10-1 Sv; 1Sv = 10-2J · kg-1 or 1 rem = 100 erg / g tissueThe formula of dose equivalent is: H = D · (CF) · N [Sv] or [rem]

The equivalent dose rate , , represents the equivalent dose variation, ΔH, in time, Δt 𝑯 ̇�and is measured in appropriate units derived from Sv or rem (multiples and submultiples) per unit of time: Sv / s; Sv / min; Sv / h; rem /s; rem / min; rem / h etc.

= Δ / Δ [Sv · h𝑯 ̇� 𝐻 𝑡 -1] [rem · h-1]In terms of energy transferred, the irradiation expressed in Rontgen, rad or rem are approximately equivalent.

Biological effects are somatic and genetic.Somatic effects (on the body) - occur after a shorter interval: i) immediate effects such as localized effects-erythema, epilation, skin burns and so on, ii) effects that occur after longer intervals (years or decades) , late effects.

Genetic effects (on the descendants) – are those that reduce reduction in offspring qualities and remain up to 3 or 4 generations. The number of gene mutations depends only on the total dose absorbed by the gonads and not the dose rate.Depending on the exposure, respectively on the distribution of irradiation, we can distinguish:i)Professional irradiation;ii) Population irradiation.

Professional irradiation is when it results from activities directly related to manipulation of nuclear radiation sources (eg: the personnel of nuclear units is occupationally exposed to the action of ionizing radiation.The annual limit of effective dose equivalent for workers is 20mSv (2 rem).

Population irradiation is when people are not directly irradiated as a result from carrying out work with radioactive substances or ionizing radiation sources. The annual limit of effective dose equivalent for members of the public is 1mSv (0.1 rem).

The allowable dose limit for the body, any organ or tissue is to be understood as both the external radiation dose received during office hours and as internal irradiation.In the Republican Radiological Safety Norms (NSR) are given both the maximum permissible activities in the critical organ and maximum permissible concentrations (MPC) in air and water for each radionuclide separately.

Radiation detection and measurement

Radiation detectors are based mainly on the effects of radiation in their interaction with substance.

Radiation Detectors

Detectors currently used in nuclear activities:- detectors based on the ionization gas the connection of ions (ionization chambers, proportional counters, Geiger counters, Muler);- detectors based on the ionization of crystals (crystal counters);- detectors based on the phenomenon of photographic emulsions (nuclear emulsions, dosimetic films etc.).- detectors based on the phenomenon of luminescence (scintillation counters);- detectors based on the phenomenon of thermoluminescence and photoluminescence (thermoluminescent detectors)

Rontgen-gamma dosimeter , VA-J-15A, is a portable device used to measure doses and dose flow data given by Rontgen and gamma rays with a wide spectrum of energy: 20 keV ... 1.2 MeV and also the qualitative detection of beta radiation emitted by different radionuclides ???? Ce vrei sa zici????.The device uses as a detector an air ionization chamber (the environment in which the measurements are made). The detector is connected to the main unit. Measuring spectrum is wide, divided into two areas. Each domain has 6 scales for both dose rate and the integrated dose. The measurement error is ± 15%.

1-stecherul de legatura cu aparatul principal2-instrumentul de masurat3-surubul pentru pozitionarea punctului zero mecanic4-fixatorul punctului zero electric5-salterul, domeniul de masurare fin6-cureaua de purtare7-cuplajul pentru inscriptor8- lacasul bateriilor9- calul de legatura10-stecherul capului de masurare11- salterul “domeniul de masurare grosier”12-butonul pentru sursa de radiatii de control13- camera14-capacul de protectie al camerei

DL-7-Gamarad, is used to measure the flow of external exposure in fields of gamma and Rontgenn radiation of the staff with occupational exposure

for radiation protection purposes Figure 4.7, pg 71It is used in specific situations such as:

i) monitoring radiation areas (monitor);ii) Routine measurements of flow exposure;

iii) location of radiation sources;iv) finding cases of contamination.

Equipment for monitoring individual dosimetry

Among individual dosimetry systems practiced with supervision:1. ionisation chambers (pen dosimeters);

2. fotodosimeters;

3. TL dosimeters

Ionization chamber (pen dosimeter) - is designed to control individual dosimetric Rontgen ray irradiation, gamma and hard beta (beta soft walls being stopped by pen). There pen dosimeters the electroscope (wired), which is equipped with an optical system that allows direct reading in milirontgens, exposure time was exposed as radiation.For pen dosimeters is required and loading gear, used before being worn. Periodic calibration is necessary, on this occasion to check and tipping of the pen dosimeters. After work, the pen dosimeters are stored in a place inaccessible to the action of radiation.

Individual dosimetrical film (fotodosimeter)

It consists of dosimetrical film and metal filter box that closes the film. Is intended to measure radiation dose range, Rontgen, beta and thermal neutrons collected in person whilst carrying.Dosimetrical film has two different sensitivities to cover a measuring range.

Dosimetrical parameters characterizing the film:

- Is an integrator dosimeter;- Has a wide measurement range (0.17 mSv-1Sv);- Dosimetrical response by ionizing radiation energy is high, requiring the use of filters and attenuators discriminatory power;-measurement precision is ≤ 30%;-photographic image is preserved for a long time

Individual dosimeter wich thermoluminescent detectors (TL dosimeter)

It consists of metal filter box placed in places arranged to be placed thermoluminescent detectors. Is intended to measure radiation dose range, Rontgen, beta and thermal neutrons collected in person whilst carrying.Thermoluminescent detectors come in different forms (powder, potato chips, tablets etc.) High-sensitivity covering a wide range of measurement.

TL dosimeter dosimetry parameters characterizing:- Is an integrator dosimeter;- Has a wide measurement range (0.10 mSv-1Sv);- Dosimetric response by ionizing radiation energy is negligible.- Measurement error is ≤ 10%;- Reading thermoluminescent detectors is fast;- Can be reused after regeneration up to 80 times.