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Chapter 3: Radioactivity

Radioactivity from Direct Excitation X-ray Emission

163 NPRE 441, Principles of Protection An Overview of to US Population

164 NPRE 441, Principles of Chapter 3: Radioactivity X-ray Imaging Examples

165 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – X-ray Tube

Andrew Webb, Introduction to Biomedical Imaging, 2003, Wiley- Interscience.

Motor, Why? Filament Rotating target

Electron beam? How are generated?

166 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – • Target nucleus positive charge (Z·p+) attracts incident e- • Deceleration of an incident e- occurs in the proximity of the target nucleus • lost by e- is gained by the EM (x-ray) generated • The impact parameter distance, the closest approach to the nucleus by the e- determines the amount of E loss • The Coulomb of attraction varies strongly with distance (µ 1/r2); ↓ distance → ↑ deceleration and E loss → ↑ photon E

• Direct impact on the nucleus determines the maximum x-ray E (Emax)

167 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – Bremsstrahlung

Interestingly, this process creates a relatively uniform spectrum.

Intensity = nhu

e0 Photon energy spectrum

168 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity The Unfiltered Bremsstrahlung Spectrum Thick Target X-ray Formation

We can model target as a series of thin targets. Electrons successively loses energy as they moves deeper into the target.

Electrons

X-rays Relative Intensity e0

Each layer produces a flat energy spectrum with decreasing peak .

169 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – Characteristic X-rays

• e- of the target atom have a (BE)that depends on atomic Z 2 (rem: BEKµ Z ) and the shell (BEK> BEL> BEM> ... ) • When e-(KE) incident on the target exceeds the target atom e-(BE), it’s energetically possible for a collisional interaction to eject the bound and ionize the atom. • What would happen then?

170 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – X-ray Tube

Andrew Webb, Introduction to Biomedical Imaging, 2003, Wiley- Interscience.

Motor, Why? Filament Rotating target

Electron beam? How are electrons generated?

171 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – Characteristic X-rays

• Within each shell (other than K) there are discrete E orbitals (ℓ = 0, 1, ... , n-1) → characteristic x-ray fine E splitting • Characteristic x-rays other than those generated through K-shell transitions are unimportant

An experimentally measured X- ray energy spectrum from a micro-focus X-ray source

172 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – Characteristic X-rays

Superimposed multiple flat spectrum with decreasing cutoff energy

Low energy X-rays suffer attenuation inside the anode Further attenuation by the source package.

External filtering to reduce low E à lower does

Beam hardening

173 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – X-ray Tube

174 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity X-ray Generation – Bremmstrahlung • Target nucleus positive charge (Z·p+) attracts incident e- • Deceleration of an incident e- occurs in the proximity of the target atom nucleus • Energy lost by e- is gained by the EM photon (x-ray) generated • The impact parameter distance, the closest approach to the nucleus by the e- determines the amount of E loss • The Coulomb force of attraction varies strongly with distance (µ 1/r2); ↓ distance → ↑ deceleration and E loss → ↑ photon E

• Direct impact on the nucleus determines the maximum x-ray E (Emax)

175 NPRE 441, Principles of Radiation Protection Sources of Electromagnetic Radiation –

Lorentz force: Fm = q (v x B)

176 NPRE 441, Principles of Radiation Protection Sources of Electromagnetic Radiation – Synchrotron Radiation

177 NPRE 441, Principles of Radiation Protection Sources of Electromagnetic Radiation – Synchrotron Radiation

178 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity

X-ray emission is typically associated with other types of , which should be considered for shielding design or dose assessment.

179 NPRE 441, Principles of Radiation Protection Typical Decay Products from Unstable Radioisotopes

Beta decay Beta-plus decay Orbital (E.C.) Alpha particles Daughter nuclei Beta particles Positrons Daughter nuclei Bremsstrahlung gamma rays X-rays Excited Daughter Nuclei

Gamma ray emission (I.C.)

IC electrons Excited Gamma rays Auger electrons Characteristic Bremsstrahlung X-rays X-rays 180 Staining of Tissue sections… Bremsstrahlung X- rays also serve as source of radiation dose beta particles getting out and irridiating the hand

181 Sources

182 NPRE 441, Principles of Radiation Protection Typical Scenarios Involving • Portable neutron sources • Nuclear reactors • Medical applications

183 NPRE 441, Principles of Radiation Protection Neutron Sources –

Cf-252 can be made An engineer tests the prototype Timed Neutron Detector, a device that detects landmines. The neutron source of the landmine extremely compact detector holds a tiny amount of -252. (Photo credit: Pacific Northwest National Lab) 184 NPRE 441, Principles of Radiation Protection Active Interrogation for Finding Special Nuclear Materials

Mobile Dual Neutron/Gamma Interrogation System

185 Inside a

186 Nuclear Fission from Slow and Moderator

187 Chapter 4: Interaction of Radiation with Matter – Interaction of Neutrons with Matter Neutron Induced Reactions

1 10 7 4 0 n+ 5B®3 Li+2 He

F Cross section for thermal neutron is 3840 barns. F Q=2.31MeV when the daughter nucleus is in an excited state (93%) and 2.79MeV when the Li nucleus is in ground state (7%). F Widely used for thermal neutron detection.

BF3 is a gas that can be used directly in a neutron counter. is also used as a liner inside the tubes of proportional counters for neutron detection.

188 NPRE 441, Principles of Radiation Protection, Spring 2020 Therapy

http://www.floridaproton.org/what-is-proton-therapy/benefits

The downside of this strategy is that proton interactions with materials in the beamline will create high-energy secondary neutrons. The high linear energy transfer (LET) of neutrons makes them extremely efficient at ionization, and far more likely to cause cell death than low-LET particles, such as X-rays or . 189 Thermal

Fig.1 boron neutron capture therapy (BNCT) can be performed at a facility with a nuclear reactor or at hospitals that have developed alternative neutron sources. A beam of epithermal neutrons penetrates the brain tissue, reaching the malignancy. Once there the epithermal neutrons slow down and these low-energy neutrons combine with boron-10 (delivered beforehand to the cancer cells by drugs or antibodies) to form boron-11, releasing lethal radiation (alpha particles and ions) that can kill the tumor.[1]

Boron neutron capture therapy

Barth, Rolf F.; Soloway, Albert H.; Fairchild, Ralph G. (1990). "Boron Neutron Capture Therapy for Cancer". Scientific American 263 (4): Figure from: http://en.wikipedia.org/wiki/File:NeutronCaptureTherapyImage.jpg 100–3, 106–7. Bibcode:1990SciAm.263d.100B. doi:10.1038/scientificamerican1090-100. PMID 2173134.190 Neutrons Sources

191 NPRE 441, Principles of Radiation Protection Neutron Sources – Spontaneous Fission

Cf-252 neutron source can be made An engineer tests the prototype Timed Neutron Detector, a device that detects landmines. The neutron source of the landmine extremely compact detector holds a tiny amount of californium-252. (Photo credit: Pacific Northwest National Lab) 192 NPRE 441, Principles of Radiation Protection 193 NPRE 441, Principles of Radiation Protection Neutron Sources – Spontaneous Fission

Spontaneous fission of tarnsuranic heavy , such as 252Cf, produces several fast neutrons, in addition to heavy fission products, prompt fission gamma rays and beta and activities.

• Half-life: 2.65 years • Neutron yield: 0.116n/s per Bq, or 2.3×106 n/s per mg

• Neutron energy peaking at 0.5MeV and extends beyond 10MeV.

Measured neutron energy spectrum from spontaneous fission of 252Cf Page 19-27, Radiation Detection and Measurements, Third Edition, G. F. Knoll, John Wiley & Sons, 1999. 194 NPRE 441, Principles of Radiation Protection Neutron Sources – Radioisotope (a,n) Sources

Energetic alpha particles can induce (a,n) reaction in certain target materials.

4 9 12 1 2a +4 BeÞ 6 C+0 n Q - value : 5.71MeV

The source is normally prepared in the form of alloy (MBe13), where M is alpha-emitting radioisotopes

A practical neutron source

Page 19-27, Radiation Detection and Measurements, Third Edition, G. F. Knoll, John Wiley & Sons, 1999. 195 NPRE 441, Principles of Radiation Protection Neutron Sources – Radioisotope (a,n) Sources

James Tuner, Atoms, radiation and Radiation Protection, p210-p211. 196 NPRE 441, Principles of Radiation Protection Neutron Sources – Radioisotope (a,n) Sources

A typical neutron energy spectrum from an 239Pu/Be source.

• The various peak and valley are due to the distinct excited states of the 12C product nucleus.

• The continuum is the result of variable energy possessed by the alpha particles before reaction.

Page 19-27, Radiation Detection and Measurements, Third Edition, G. F. Knoll, John Wiley & Sons, 1999. 197 NPRE 441, Principles of Radiation Protection Neutron Sources – Radioisotope (a,n) Sources

Practical considerations for choosing appropriate a emitter.

Radium-226 gamma ray spectrum from high purity germanium (HPGe) detector 198 NPRE 441, Principles of Radiation Protection Neutron Sources – Radioisotope (a,n) Sources

Practical considerations for choosing appropriate a emitter.

• Radioisotope (a,n) sources are normally associated with other significant background radiations, especially when 226Ra and 227Ac are used.

• Choice has to be made between specific activity of the alpha emitter (and therefore neutron yield), source life-time and the availability of the .

Page 19-27, Radiation Detection and Measurements, Third Edition, G. F. Knoll, John Wiley & Sons, 1999. 199 NPRE 441, Principles of Radiation Protection Neutron Sources – Photon-Neutron Sources

• Some radioisotope gamma ray emitters can also be used to produce neutrons when combined with an appropriate target material.

9 8 1 4 Be + hvÞ4 Be+0n, Q - value : -1.666MeV 2 1 1 1 H + hvÞ1H +0n, Q - value : -2.226MeV

• A gamma ray photon with an energy greater than the negative of the Q -value is required.

• Some practical gamma ray emitter include: 226Ra, 124Sb, 72Ga, 140La and 24Na.

Page 19-27, Radiation Detection and Measurements, Third Edition, G. F. Knoll, John Wiley & Sons, 1999. 200 NPRE 441, Principles of Radiation Protection Neutron Sources – Photoneutron Sources

If the gamma rays are monoenergetic, the neutrons are also nearly monoenergetic!

M (E + Q) E [(2mM )(m + M )(E + Q)]1/ 2 E (q ) @ g + g g cos(q ) n m + M (m + M )2 where q = angle between gamma photon and neutron direction

Eg = gamma energy M = of recoil nucleus ´ c2 m = mass of neutron ´ c2 Typical structure of photon neutron sources The neutron energy is blurred by • The slight angular dependency. • Neutron scattering inside the source.

Page 19-27, Radiation Detection and Measurements, Third Edition, G. F. Knoll, John Wiley & Sons, 1999. 201 NPRE 441, Principles of Radiation Protection Neutron Sources – Photoneutron Sources

Calculated neutron energy spectra

202 NPRE 441, Principles of Radiation Protection 203 Average Binding Energy Per Comparing Fusion and Fission Reactions

http://230nsc1.phy-astr.gsu.edu/hbase/hframe.html 204 NPRE 441, Principles of Radiation Protection Neutrons Generated by Accelerated Charged Particles • Neutrons can be produced by between accelerated charged particles.

2 2 3 1 The D-D reaction: 1 H + 1 H ⇒ 2 He + 0 n, Q-value: 3.26MeV, En=2.5MeV 2 3 4 1 The D-T reaction: 1 H + 1 H ⇒ 2 He + 0 n, Q-value: 17.6MeV, En=14.1MeV Why accelerated? • Due to the coulomb barrier between the incident deuteron and the target nucleus, only a relatively small accelerating potential is required (about 100 to 300kV) Why to induce the reaction. 14.1MeV? • The neutrons produced by a given nuclear reaction (D-D or D-T) have roughly the same .

Page 19-27, Radiation Detection and Measurements, Third Edition, G. F. Knoll, John Wiley & Sons, 1999. 205 NPRE 441, Principles of Radiation Protection Neutrons Generated by Accelerated Charged Particles

A schematic of a . On the left hand side is an from which a Hydrogen isotope ion beam is extracted. This beam is accelerated with a high voltage towards a target on the right hand side, loaded with and , to produce neutrons in a process. http://ibt.lbl.gov/neutrongamma.html 206 NPRE 441, Principles of Radiation Protection http://ibt.lbl.gov/neutrongamma.html 207 NPRE 441, Principles of Radiation Protection A Typical D-T Neutron Generator

This compact and simple device can generate 5e11 neutrons per second by accelerating deuterium or tritium (depending on the desired neutron spectrum) into a deuterated or tritiated neutron production target.

http://ibt.lbl.gov/neutrongamma.html 208 NPRE 441, Principles of Radiation Protection Coulomb Barrier

• are bounded together in nucleus by the strong force, which has a short range of ~10-15m. • The strong force is powerful enough to overcome the Coulomb repulsion between the positively charged protons.

209 NPRE 441, Principles of Radiation Protection Neutrons Generated by Accelerated Charged Particles

James Tuner, Atoms, radiation and Radiation Protection, p210-p211. 210 NPRE 441, Principles of Radiation Protection Neutrons Generated by Accelerated Charged Particles • Neutrons can be produced by nuclear reaction between accelerated charged particles.

2 2 3 1 The D-D reaction: 1 H + 1 H ⇒ 2 He + 0 n, Q-value: 3.26MeV, En=2.5MeV 2 3 4 1 The D-T reaction: 1 H + 1 H ⇒ 2 He + 0 n, Q-value: 17.6MeV, En=14.1MeV Why accelerated? • Due to the coulomb barrier between the incident deuteron and the light target nucleus, only a relatively small accelerating potential is required (about 100 to 300kV) Why to induce the reaction. 14.1MeV? • The neutrons produced by a given nuclear reaction (D-D or D-T) have roughly the same energies.

Page 19-27, Radiation Detection and Measurements, Third Edition, G. F. Knoll, John Wiley & Sons, 1999. 211 NPRE 441, Principles of Radiation Protection Neutrons Generated by Accelerated Charged Particles – An Interesting Application

Tagged Neutron Method

212 NPRE 441, Principles of Radiation Protection NDA of Potential Misloading with Time-Tagged Neutron Interrogation Techniques (Technical Approach) NDE of Potential Misloading with Time-Tagged Neutron Interrogation Techniques (Technical Approach)

(a) (b)

(a) The commercial associate particle neutron generator (DT108API) from Adelphi . (b) The internal assembly of the anode and the alpha-detector inside the APNG. Both images from [23]. Gamma Rays Generated by Accelerated Charged Particles

Conceptually similar to the neutron tubes, but instead of using a deuterium beam and a deuterium target to generate neutrons through fusion, it uses a proton beam and a boron, fluorine, or lithium target (depending on the desired gamma-ray energy) to generate monoenergetic gammas through a nuclear process.

http://ibt.lbl.gov/neutrongamma.html 215 NPRE 441, Principles of Radiation Protection Large Sized Neutron Sources

Nuclear fission reactors Nuclear fission which takes place within in a reactor produces very large quantities of neutrons and can be used for a variety of purposes including generation and experiments.

Nuclear fusion systems Nuclear fusion, the combining of the heavy of hydrogen, also has the potential to produces large quantities of neutrons. Small scale fusion systems exist for research purposes at many universities and laboratories around the world. A small number of large scale nuclear fusion systems also exist including the National Ignition Facility in the USA, JET in the UK, and soon the recently started ITER experiment in France.

High energy particle accelerators A source is a high-flux source in which protons that have been accelerated to high energies hit a target material, prompting the emission of neutrons.

James Tuner, Atoms, radiation and Radiation Protection, p210-p211. 216 NPRE 441, Principles of Radiation Protection Inside a Nuclear Reactor

217 Nuclear Fission from Slow Neutrons and Water Moderator

218 Nuclear Fission from Slow Neutrons and Water Moderator

219 Spallation Neutron Sources

A spallation neutron source consists mainly of an ion source, a linear accelerator, a synchrotron or storage ring, a target area, and neutron beam lines. A continuous beam of negative ions of hydrogen is obtained from an ion source and accelerated by the linear accelerator. At the Los Alamos spallation source, the ion beam passes through the linear accelerator's radiofrequency quadrupole (RFQ), where it is bunched into pulses and accelerated by RF waves; then it is focused by quadrupole magnets into the storage ring. There the negative ions pass through a foil, which strips all electrons from the negative hydrogen ions, converting them into positive ions, or protons.

http://web.ornl.gov/info/ornlreview/rev28-1/text/rnd.htm 220 NPRE 441, Principles of Radiation Protection Spallation Neutron Sources

A schematic diagram of a pulsed source (based on ISIS, UK)

221 NPRE 441, Principles of Radiation Protection