1. Basic Nuclear Processes in Radioactive Sources

1.1 α decay• α particles: 4He nuclei• Generally emitted by very heavy nuclei, containing to

many nucleons to remain stable• Emission of 4He rather than of a single nucleon is

energetically more advantageous (particularly high binding energy of the α)

• (Z,A) → (Z-2, A-4) + α• First theoretical explanation by Gamow and Condon,

by Gurney: tunneling of α through potential barrier of nucleus

→ α show monoenergetic energy spectrum→ since barrier transmission is dependent on

energy: all α sources generally limited to the range of approx. 4-6 MeV

→ higher energy sources have higher transmission probability and thus shorter half-life

1.1 α decay (cont.)

• Also: most α-decays are directly to ground state of the daughter nucleus since this involves the highest energy change

• Nevertheless: decays to excited states possible → energy spectrum shows several monoenergetic lines corresponding to a decay to one of these states (see table 1.2, p.4 Leo, e.g. 241AM ( Half-life: 433; Energies: a) 5.486 MeV, b) 5.443 MeV; Branching: a) 85%, b)12.8%

1.1 α decay (cont.)

• Because of its double charge, 2e+:α have very rate of energy loss in matter

Range of 5 MeV α’s in air: few cmα sources are designed as thin as possibleMost α sources are made by depositing the isotope on the surface of a suitable backing material and protecting it with an extremely thin layer of metal foil

1.2 β decay• β: fast electron (e-) or positron (e+), resulting from the

weak-interaction decay of a neutron (n) or a proton (p) in a nuclei containing an excess of the respective nucleon

• n rich: n → p + e- + anti-ν• p rich: p → n + e+ + ν• Continuous energy spectrum, because available

energy for the decay (Q) shared between β and ν(usually undetected)

• Typical beta energy spectrum: fig. 1.2, p.5, or slide 8• See there: if small recoil energy of the daughter

nucleus is ignored: Emax=Q• Most β sources: Emax > few tens of keV, < few MeV

β-Decay:

N/Z Ratios in Nuclei:Z

N=Zβ+

emission

β-

emission

Blue line:- valley of stability- extra neutrons

A=N+ZTable of Nuclides: http://www2.bnl.gov/ton/

Τritium: H He + e + ν3 3

1.2 β decay (cont.)• Very many β sources: daughter nucleus in

excited state, decaying immediately with the emission of one or more γ’s (see fig. 1.3, p.5 in Leo for nuclear level diagrams of a few gamma sources)

• Most β sources are also gamma sources• Pure β-emitters exist but list is short (see

table 1.3, p.5 in Leo)• Some β sources have more than one

decay branch:• The total β spectrum from such a source is

a superposition of all branches weighted by their respective decay probabilities

1.2 β decay (cont.)• e- lose their energy relatively easy in

matter → β sources need to be thin• e+ above is particularly important

Why?annihilation of e+ with e- in the source material or in the surrounding conatinerif source too thick: distorted βspectrum, enormous background of 511 keV annihilation photons

1.3 Electron capture (EC)• Alternative to β+ emission of proton-rich nuclei• Capture of an electron from the atomic orbitals: e- + p → n + ν

• Nuclear level diagram identical to β+ emission • However, only a ν is emitted → hard to detect EC?• no, EC leaves hole in atomic shell → emission of

characteristic x-ray or Auger electrons• Most likely K electron is captured, although L-capture

is possible (much lower probability).

1.4 γ Emission• Nucleus is characterized by discrete energy levels →

transitions between these levels by emission (or absorption) of EM radiation of the correct energy (= energy diff. between the levels)

• Energies of these photons: few 100 keV – few MeV(high binding energies of the nuclei)

• High-energy photons historically named γ-rays• Most γ-rays sources result from β-disintegration,

another possible source: excited nuclear states created in nuclear reactions

• Design of γ-ray sources: “Filtering”: e+ and e- are more easily absorbed in matter than γ-rays →enveloping sources with sufficient absorbing material

1.4 γ Emission (cont.)Isometric States

Some nuclear states are long-lived“forbidden” transition: de-excitation hindered by a

large spin difference

Nuclide can be “trapped” in one of these metastablestates → ISOMERS

Examples: 60mCo or 69mZn

1.5 Annihilation Radiation

• Source of high-energy photon radiation• e+ source (e.g. 22Na) enclosed or

allowed to irradiate absorber, e+ will annihilated with the absorber e-→ 2 γ , Eγ = Me = 511 keV

(22Na γ ray spectrum: see page 7, Fig. 1.4 of Leo)

1.6 Internal Conversion• Nuclear excitation energy is directly transferred to an

atomic electron rather than emitted as a photon• e- is ejected with kinetic energy

Ekin = Eex – Eb→ monoenergetic (unlike beta), with energies similar to

competing γ’s (few 100 keV- few MeV• Mostly K-shell• Internal conversion sources thus useful for calibration

purposes(see table 1.4, p. 7 of Leo for examples: 137Cs (624 keV)

1.7 Auger Electrons

• Excitation which arises in the electron shell can also be transferred to an atomic electron rather than to a characteristic x-ray

• Can occur after EC for example • Monoenergetic• like IC lines they can occur in groups, but:

energies are more typical of atomic processes → < few keV → susceptible to self-absorption and difficult to detect

1.8 Neutron Sources

While it is possible to produce isotopes which emit neutrons, natural neutron emitters which could be used in the lab do not exist

Lab neutron sources are based on either spontaneous fission or nuclear reactions

Spontaneous Fission

• Occurs in many transuranium elements → neutrons + fission fragments

• Fission fragments often promptly decay emitting γ and β radiation

• Sufficiently thick container: only neutrons penetrate

• Most common neutron source of this type: 252Cf (half life of 265 years)

Nuclear Reaction• More convenient method of producing

neutrons with reactions (α,n) or (γ,n)• Made by mixing the target material with a

suitable strong α or γ emitter• Most common target material: Berylliumα + 9Be → 13C* → 12C(*) + n (dominant)

8Be + α + n3 α + n

(modes depend on exciting energy)

Nuclear Reaction (cont.)• For fixed alpha energies

energy spectrum of n should show theoretically monoenergetic lines (corresponding to transitions)

• However: smearing of the alpha particle spectrum due to energy loss

• Considerable broadening of as much as 2 MeV

Nuclear Reaction (cont.)

• In case of photo-reaction: only 2 target materials are suitable9Be + γ → 8Be + n2H + γ → 1H + n

• neutrons are more or less monoenergetic (smaller spread)

• Yield 1-2 orders of magnitude lower• Background gamma radiation

Ionizing Radiation:Physics: Carrier: Mechanism:

Electromagnetic interaction: Electrons, He chargeGammas photoeffect

Nuclear (strong) interaction: Neutrons (capture, fission)

charged fragments, gammas

Both: (high enough energy)

4

Parlance: Mediator: Assessment:

α He least penetrating, strong local effectβ e , e moderately penetratingγ photon most penetrating; localized

4

+ -

1.9 Activity

• Activity is the number of transformations (or disintegrations) per time occurring in a radioactive material

• Conventional unit: Curie (Ci)– 1 Ci = 3.7 x1010 disintegrations per second

(1 g of pure radium-226, very large!)• International System (SI): Becquerel (Bq)

– 1 Bq = 1 disintegration per second• 1 Ci = 3.7x1010 Bq

1.9 Radioactive Decay Law• Beginning of the century by Rutherford and Soddy:

activity of radioactive sample decays exponentially in time

• QM: λ is transition probability per unit time, characteristic of the nuclear species, if nuclide has more than one decay mode: λ= λ1 + λ2 + …

→ in sample of N nuclei mean number of nuclei decaying in time dt : dN = - λ N dt, λ: decay constant (N large, continuous)

→ Integration: N(t) = N(0) exp (- λ t)mean life time: τ = 1/ λ (decay to 1/e of initial activity)half-life: T1/2+=1/ λ ln 2 (from ½=exp(- λ T1/2+)

Biological Effects:

Ionizing Radiationcauses:

- one or both DNAstrands to break

- the formation offree radicals

Radiation Units:For radioactive Sources:

Activity: number of disintegrations per time; unit: Becquerel [Bq]or Curie [Ci]

Half-life: time it takes for half the radioactive nuclei to disintegrate

General:

Absorbed dose: amount of energy absorbed per unit massunit: Gray [Gy] or [rad]

Dose equivalent: absorbed dose times a quality factor Q:X,γ,β => Q=1; α => Q=20; n,p => Q=10unit: Sievert [Sv] or [rem]

Exposure and Exposure Rate

• describes amount of ionization produced in air by X-rays or gammas

• technically out-of-date, but still used on many radiation survey instruments

• easily measured by relatively simple instruments

(not in previous list; unit: C/kg or Roentgen)

Absorbed Dose and Absorbed Dose Rate

• measures the amount of energy absorbed per unit mass

• applies to all types of ionizing radiation and all types of materials

• one roentgen of gamma or X-ray radiation is approximately equal to one rad in human tissue

Radiation in Daily Live:

Environmental Dose Rates:

ALARA Principle:As Low As Reasonably Achievable—means making every reasonable effort to maintain exposures to radiation as far below the dose limits as is practicable consistent with the purpose for which the licensed activity is undertaken, taking into account the state of technology, the economics of improvements in relation to the state of technology, the economics of improvements in relation to benefits to the public health and safety, and other societal and socioeconomic considerations, and in relation to utilization of nuclear energy and licensed materials in the public interest.

Radiation Protection:

• Decrease Time• Increase Distance• Increase Shielding

Annual Regulatory Limits

Members of the public 100 mremOccupational Limits:

Total Effective Dose Equivalent 5 remLens of the eye 15 rem“Organ Dose” 50 remSkin Dose 50 remExtremity Dose 50 rem

Acute Effects of Whole Body Exposure on Man

Absorbeddose (Rads) Effect

10,0001,200

600

1005025

5

Death in a few hoursDeath within daysDeath within weeks

Probable recoveryNo observable effectBlood changes definite1st blood change obs