Secondary Radiation Production and Shielding for Proton Therapy Facilities Nisy Elizabeth Ipe, Ph.D., C.H.P. Consultant, Shielding Design, Dosimetry & Radiation Protection San Carlos, CA, U.S.A. Email: nisy@comcast.net Doing the right thing, doing it right June 15, 2015 AAPMSS 2015 1
Secondary radiation production Intra-nuclear cascade Neutron yield Neutron interactions Unshielded and shielded spectra Attenuation length Dose quantities Angular dose profile Components of dose Calculation Methods Monte Carlo codes, analytical and computational models Outline Source terms and attenuation length Dose in forward and transverse directions Shielding design considerations Treatment parameters Facility/site-specific parameters Examples Mazes Penetrations Skyshine & groundshine Activation (brief) June 15, 2015 AAPMSS 2015 2
Secondary Radiation Produced by interaction of protons with any material in its path Produced during beam losses a term which loosely refers to Unintentional loss of primary proton beam Full beam interception by intentional targets (degrader, patient, etc.) Partial beam interception by devices (collimators, apertures, slits, etc.) Produced at various locations In synchrotron and cyclotron during injection, acceleration and extraction During energy degradation in cyclotron During beam transport to treatment room In beam shaping and bean modifying devices In patient, dosimetric phantom and beam stop Consists of Prompt radiation which is on only when the machine is on Residual radiation from activation which remains even after the machine is turned on June 15, 2015 AAPMSS 2015 3
Consequences of Secondary Radiation Prompt radiation Requires large structural shielding thicknesses (~ 0.5 m 4 m of concrete) in order to protect workers and public Results in secondary dose to patient Residual radiation May result in exposure to individuals working with or on activated components May require local shielding, increased ventilation, and administrative procedures Effective shielding design requires understanding of physics behind secondary radiation production June 15, 2015 AAPMSS 2015 4
Physics: Intra-Nuclear Cascade (INC) p p π 0 n π ± http://s0.geograph.org.uk/geophotos/ 01/30/30/1303004_e8f9a13b.jpg Intra- Nuclear Cascade Interaction of protons with matter results in production of an intra-nuclear cascade Important in shielding design for protons in the therapeutic energy range of 67 MeV -330 MeV Similar to a water cascade, there is a succession of stages in an intra-nuclear cascade June 15, 2015 AAPMSS 2015 5
Stages: Intra-Nuclear Cascade π ± µ ± Muon Production e + π 0 γ γ n e - Electromagnetic Cascade Incoming hadron p n π ± π 0 Intra-nuclear cascade n p α n Evaporation of Nucleons and Fragments α β γ Activation June 15, 2015 PTCOG51 AAPMSS EW N. Ipe 2015 6
Incoming hadron (p, n) interacts with individual nucleons in nucleus, producing a spray of particles Pions are produced above ~ 290 MeV Scattered and recoiling nucleons proceed through nucleus Each nucleon may interact with other nucleons leading to development of cascade Some nucleons may escape nucleus p, n Intra-Nuclear Cascade p Large fraction of energy is transferred to single nucleon This nucleon with E > 150 MeV is forward peaked and propagates the cascade Nucleons with energies between 20 and 150 MeV transfer energy by nuclear interactions to several nucleons (< 10 MeV/nucleon) Charged particles are quickly stopped by ionization Neutrons predominate at low energies June 15, 2015 AAPMSS 2015 7 n π 0 π± Intra-Nuclear Cascade
Energy of nucleons that do not escape nucleus is distributed among remaining nucleons Some nucleons with considerable energy ( pre-compound ) may be ejected Original nucleus is left in an excited state and de-excites by emitting evaporation nucleons, alphas and fragments Low-energy charged particles deposit energy locally Evaporation and Activation α β n α Evaporation nucleons are emitted isotropically Energy of evaporation neutrons extends to 8 MeV Neutrons travel long distances depositing energy continuously Remaining excitation energy emitted as gammas De-excited nucleus may be radioactive (activation) June 15, 2015 AAPMSS 2015 8 n γ p
π 0 Muons and Electromagnetic Cascade π ± µ ± γ γ e + e- n Charged pions decay to muons Muons are penetrating particles and deposit energy by ionization; photonuclear reactions also possible Neutral pions decay to gammas which initiate electromagnetic (EM) cascades EM cascades do not contribute significantly to energy transport Protons and pions (E < 450 MeV) have a high rate of energy loss Neutrons are principal propagators of cascade with increasing depth in shielding June 15, 2015 AAPMSS 2015 9
SPALLATION Spallation is the emission of a large number of nucleons and fragments when a particle (proton, neutron) with kinetic energy > ~ 100 MeV interacts with a heavy target nucleus. Includes intra-nuclear cascade and de-excitation processes Emitted nucleons are referred to as spallation nucleons Spallation plays an important role in activation June 15, 2015 AAPMSS 2015 10
Thick targets have thicknesses proton range Protons are completely stopped Referred to as stopping targets Thick and Thin Targets Thin targets have thicknesses that are significantly < than proton range Protons lose an insignificant amount of energy in the target Softer neutron spectrum in forward direction Kinetic energy available compared to thin targets for neutron production in Use of thick targets in target is the full incident shielding calculations is not proton energy conservative in the forward Harder neutron spectrum direction in forward direction June 15, 2015 AAPMSS 2015 11
Neutron yield is number of neutrons produced per proton Depends upon proton energy, target material and dimensions Thick target yield is proportional to E p 2 between 50 and 500 MeV Thin target yield is proportional to target thickness For shielding calculations, neutron energy and angular distribution are more important! Neutron Yields from Thick Targets* *Except at highest energies Includes measurements and calculations Adapted from K.Tesch, Radiat Prot Dosimetry 11(3), 163, 1985 June 15, 2015 AAPMSS 2015 12
Proton Energy, E p (MeV) Neutron Yields for Protons Incident on a Thick Iron Target (FLUKA) Range (mm) Iron Target Radius (mm) Thickness (mm) Neutron Yield (n/p) E n <19.6 MeV E n > 19.6 MeV Total 100 14.45 10 20 0.118 0.017 0.135 150 29.17 15 30 0.233 0.051 0.284 200 47.65 25 50 0.381 0.096 0.477 250 69.30 58 75 0.586 0.140 0.726 June 15, 2015 AAPMSS 2015 13 Agosteo et al., NIM Phys. Res. B265, 581-589. 2007
Angular Dose Profiles from Unshielded Thick Tissue Targets For Various Proton Energies Total Dose (psv-m 2 /p) 1.E-02 1.E-03 1.E-04 1.E-05 0 50 100 150 200 More shielding is required in the forward direction Angle (degrees) A 250 MeV 230 MeV 220 MeV 160 MeV 120 MeV Ipe Data with FLUKA June 15, 2015 AAPMSS 2015 14 1 psv = 10-12 Sv
Thick Target Tissue (ICRU) Dimensions as a Function of Proton Energy (Ipe Data) Energy (MeV) 250 230 220 160 120 Target Length (cm) 38 34 31 18 11 Target Radius (cm) 20 17 16.5 10 6.5 June 15, 2015 AAPMSS 2015 15
Neutron Energy Classification and Interactions Thermal: Ē n = 0.025 ev at 20ºC Typically E n 0.5 ev Intermediate : 0.5 ev <E n 10 kev Fast: 10 kev< E n 20 MeV Include evaporation neutrons Relativistic E n > 20 MeV Include cascade neutrons For E n < 20 MeV, nearly all interactions are elastic or inelastic scatters Absorption is important at thermal energies and at a few resonances in kev region Resonances are peaks in the cross section which occur at energies where reactions with nuclei are enhanced June 15, 2015 AAPMSS 2015 16
Absorption Cross Section for 56 Fe(n,γ) 57 Fe Thermal Resonances Capture cross section for E n < 1 kev decreases with increasing neutron energy 1 barn = 10-24 cm 2 June 15, 2015 AAPMSS 2015 17
Thermal neutrons gain and lose only small amounts of energy through elastic scatter They diffuse about and are captured by the nucleus Excited nucleus emits capture gamma rays Energy of capture gamma from hydrogen is 2.22 MeV (polyethylene, concrete) Energy of capture gamma from boron is 0.478 MeV (borated polyethylene) Boron capture crosssection is ~10,000 x higher than hydrogen crosssection Thermal Neutron Capture Borated polyethylene is used instead of polyethylene for shielding maze doors Thermal neutron Excited Nucleus Capture Gamma Ray June 15, 2015 AAPMSS 2015 18 http://www.glossary.oilfield.slb.com/display.cfm?term=inelastic%20neutron%20scattering
Kinetic energy and momentum are conserved Fast neutrons lose energy by elastic scatter and become thermal neutrons Interaction with hydrogen is like a billiard ball collision Elastic Scatter Primary process of energy loss below 1 MeV in hydrogenous materials (concrete, polyethylene, etc.) Dominant interaction below 10 MeV for all materials Hydrogenous materials are most effective for fast neutron shielding Water content of concrete (shielding) should be at least 5.5% by weight n Nucleus http://www.glossary.oilfield.slb.com/display.cfm?term=elastic%20neutron%20scattering June 15, 2015 AAPMSS 2015 19
Inelastic Scatter (n, n / ) Kinetic energy is not conserved Occurs only above lowest excited state in material (847 kev in 56 Fe) Nucleus absorbs energy and is left in an excited state De-excites emitting gamma rays Is dominant process above 10 MeV in all materials In high-z materials, inelastic scattering reduces neutron energy, thus making hydrogenous material that follows more effective Lead or steel must always be followed by a hydrogenous material because high-z materials are transparent to lower energy neutrons Inelastic gamma rays Nucleus Nucleus June 15, 2015 AAPMSS 2015 20 http://www.glossary.oilfield.slb.com/display.cfm?term=inelastic%20neutron%20scattering
Fast Neutron Interactions Neutrons can also be absorbed or captured: (n, 2n), (n, p), (n, α) or (n, γ) Non-elastic cross-section is sum of inelastic (n, n') and (n, 2n) cross-sections for E n < 20 MeV Inelastic reactions dominate at lower energies (n, 2n) reactions dominate at higher energies (n, 2n) reaction produces large number of lower-energy neutrons NCRP 79 Cross-Sections in Lead 1 barn = 10-24 cm 2 June 15, 2015 AAPMSS 2015 21
Neutron Inelastic Cross Sections for Various Materials Inelastic cross sections increase with increasing mass number, A Cross sections decrease with increasing energy to a constant value above ~ 150 MeV Neutrons with E n > 150 MeV will control radiation environment for E p > 150 MeV ICRU Report 28 June 15, 2015 AAPMSS 2015 22
Σ in σ N in = Α Α Inelastic Cross-Section Measurements of inelastic nucleon-nucleus crosssection for either protons or neutrons can be represented by σ cm 1 0. 70 in = 43.1Α N mb for mass number A N 3; and E 150 MeV Macroscopic inelastic cross-section is: σ N in Α 1 where N A is Avogadro number and A is Σ = cm in Α molar mass of medium Absorption length for neutrons is: λ = 1 in Σ in cm June 15, 2015 AAPMSS 2015 23
Relativistic Neutron Interactions Relativistic neutrons arise from cascade processes in proton accelerators Important in propagating the radiation field Neutrons with E n > 150 MeV Propagate cascade through shielding Continuously regenerate lower-energy neutrons and charged particles at all depths via inelastic reactions Low-energy neutrons undergo capture reactions resulting in production of capture gamma rays June 15, 2015 AAPMSS 2015 24
Relativistic Neutron Interactions Neutrons (50 MeV < E n <150 MeV) Intra-nuclear cascade Evaporation nucleons Activation Neutrons (20 MeV < E n 50 MeV) Evaporation nucleons Activation June 15, 2015 AAPMSS 2015 25
Unshielded Neutron Spectra at Various Production Angles for 250 MeV Protons Incident on Thick Iron Target (FLUKA) Lethargy = u = ln(e 0 /E) du = de/e E x dn/de = dn/du Evaporation Neutron Peak Radius =5.8cm Thickness=7.5 cm High-Energy Peak June 15, 2015 PTCOG51 AAPMSS EW 2015 N. Ipe 26 Agosteo et al., NIM Phys. Res. B265, 581-589. 2007
Characteristics of Shielded Neutron Field for Protons High-energy cascade neutrons propagate cascade in shield Continuously regenerate lower-energy neutrons and charged particles at all depths in the shield via inelastic reactions Yield of low-energy and high-energy neutrons increases as proton energy increases Greater yield of lower-energy neutrons is more than compensated for by greater attenuation in shield, because of higher cross-sections at lower neutron energies Eventually, radiation field reaches equilibrium condition beyond a few mean free paths (MFP) within shield Deep within shield, high-energy neutrons (E n > 150 MeV) regenerate cascade; they are few in number, but, are accompanied by numerous low-energy neutrons Typical neutron spectrum observed outside a thick shield consists of peaks at ~ 2 MeV and at ~ 100 MeV MFP = average distance between two consecutive interactions June 15, 2015 AAPMSS 2015 27
Normalized Neutron Spectra in Transverse Direction at Surface of Concrete and at Various Depths, for 250 MeV Protons Incident on Thick Iron Target Evaporation Neutron Peak 100 MeV Peak June 15, 2015 AAPMSS 2015 28 Agosteo et al., NIM Phys. Res. B265, 581-589. 2007
Attenuation Length Radiation transmission can be approximated by an exponential function over a limited range of shielding thickness Attenuation length is distance travelled through which radiation dose is reduced to 37% (1/e) of its original value. Neutron attenuation length, λ, is given by: cm where Σ is the total neutron macroscopic cross section It changes with depth as spectrum changes and eventually reaches an equilibrium value known as effective attenuation length Depends upon neutron energy, production angle, composition and density of shielding material Measured in cm or g /cm 2 ( when multiplied by density) Σ = 1 λ June 15, 2015 AAPMSS 2015 29
Calculated Effective Attenuation Lengths for Monoenergetic Neutrons Normally Incident on Concrete (NCRP 144) June 15, 2015 AAPMSS 2015 30
Measured Neutron Attenuation Lengths in Concrete for Various Sources (Forward and Transverse Directions) Attenuation Length (g cm -2 ) 150 100 50 SLAC,NE213 HIMAC,TEPC HIMAC,Cal. ISIS,Rem C. ISIS,Cal. Loma Linda,TEPC Orsay,TEPC TIARA,NE213 CYRIC,NE213 Max ~130 g-cm -2 0 10 1 10 2 10 3 Maximum Source Neutron Energy (MeV) June 15, 2015 AAPMSS 2015 31 Nakamura, T. in Proc. SATIF-7, 2004, NEA, OECD, Paris
H T w R DT, R R, = Radiation Protection Dose Quantities (ICRP, ICRU) Effective Dose, E (Sv) Radiation Energy W R (ICRU 103) E = w H T T T Photons, electrons, muons 1 H T = equivalent dose in tissue T or organ w T = tissue weighting factor Equivalent Dose, H T (Sv) H T w R DT, R R = D T,R = mean absorbed dose in tissue T or organ due to radiation R w R = radiation weighting factor Neutrons, Protons, charged pions Alphas, fission fragments, heavy ions < 1 MeV 2.5 + 18.2 e -[ln(e)]²/6 1 MeV - 50 MeV > 50 MeV 5.0 + 17.0 e - [ln(2 E)]²/6 2.5 + 3.25 e - [ln(0.04 E)]²/6 June 15, 2015 AAPMSS 2015 32 2 20
U.S. Dose Limits (NRC 10CFR20*) Occupational dose limits f or adults: Controlled Areas, Occupancy Factor, T = 1 Annual total effective dose equivalent = 0.05 Sv In interests of ALARA use 5 msv/y (NCRP 151) Dose limits for individual members of the public: Unrestricted Areas Annual total effective dose equivalent to individual = 1 msv (can use appropriate T) Annual dose in any unrestricted area = 20 µsv in 1 h (T=1) *Proposed changes in the works June 15, 2015 AAPMSS 2015 33
ICRU 103 Radiation weighting factors for neutrons have been revised over time. US NRC values are lower than ICRU values. NRC is about to adapt IRCU values. "Neutron radiation weighting factor as a function of kinetic energy" by Ytrottier at English Wikipedia. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/file:neutron_radiation_weighting_factor_as_a_function_of_kinetic_energy.gif#/media/ File:Neutron_radiation_weighting_factor_as_a_function_of_kinetic_energy.gif June 15, 2015 AAPMSS 2015 34
Operational Dose Quantity, H*(d) Effective dose is not directly measurable, so the operational dose quantity can be used The ambient dose equivalent, H*(10), at a point in a radiation field, is the dose equivalent that would be produced by the corresponding expanded and aligned field, in the ICRU sphere (diameter = 30 cm, composition: 76.2 % O, 10.1 % H, 11.1 % C and 2.6 % N) at a depth, 10 mm, on the radius opposing the direction of the aligned field. In the expanded and aligned field, the fluence and its energy distribution have the same values throughout the volume of interest as in the actual field at the point of reference, but the fluence is unidirectional June 15, 2015 AAPMSS 2015 35 https://www.euronuclear.org/info/encyclopedia/ambientdose.htm
Calculation Methods 1. Monte Carlo (MC) Codes Various MC codes are available for shielding calculations FLUKA and MCNP are well benchmarked and widely used Full computer simulation modeling accelerator, beam line and room geometry can be performed Requires use of site-specific composition and density of shielding materials Total dose from all particles should be calculated Should be used for maze and penetration-scatter calculations Be careful in using codes that only transport neutral particles; or that allow user to arbitrarily choose physics models Full MC simulations for specific room design are time consuming and not very cost effective during schematic design phase for determining structural shielding June 15, 2015 AAPMSS 2015 36
2. Analytical Methods - Most models are lineof-sight and assume point source - Limited to transverse shielding and simple geometries - Do not account for changes in production angle, target material/ dimensions, shielding material, density and composition, etc. Calculation Methods PTCOG Report 1 H & d # = 0 exp 2 $! r % λ " June 15, 2015 AAPMSS 2015 37 H H = dose at point of interest H 0 = dose at 1 m from source d = slant thickness r = slant distance to shield λ = attenuation length
Calculation Methods 3. Computational Models Hybrid approach Monte Carlo and Analytical Methods Site-specific source terms and attenuation lengths that are independent of geometry are derived using Monte Carlo Various site-specific parameters are considered Particle energy, production angle, target material/ dimensions, shielding material, composition and density They are particularly useful during schematic design phase Facility layout undergoes several iterations They are faster than Monte Carlo calculations Published source terms/attenuation lengths should not be used because they typically assume thick targets; and theoretical concrete composition/density June 15, 2015 AAPMSS 2015 38
H ( E 0 r 2 p, θ) d exp λθ g( θ) Computational Models H(E p, θ, d/λg(θ)) = H ( E, θ ) exp & d $ % λ g θ ( θ ) Where: H is the dose equivalent at the outside the shield, H 0 is source term at an angle θ with respect to the incident beam, and is assumed to be geometry independent r is the distance between the target and the point at which the dose equivalent is scored, d is the thickness of the shield d/g(θ) is the slant thickness of the shield at an angle θ λ θ is the attenuation length at an angle θ g(θ) is cosθ for forward shielding g(θ) is sinθ for lateral shielding g(θ) = 1 for spherical geometry June 15, 2015 AAPMSS 2015 39 0 r 2 p #! "
Dose in Forward Direction as a Function of Concrete Thickness for Protons incident on Tissue (Ipe Data: FLUKA) H 0 = 0.0166 psv-m2/p λ = 1/(0.019) = 52 cm June 15, 2015 AAPMSS 2015 40
Components of Dose in Forward Direction for Protons incident on Tissue (Ipe Data- FLUKA) June 15, 2015 AAPMSS 2015 41
Dose in Transverse Direction as a Function of Concrete Thickness for Protons incident on Tissue (Ipe Data: FLUKA) June 15, 2015 AAPMSS 2015 42
Shielding Design Considerations Treatment and Beam Parameters Beam shaping and delivery (scanning vs. scattering, etc.) Energy per fraction Dose delivered per fraction Current at each energy to deliver a certain dose rate No. of patients/year No. of fractions/patient at each energy Beam-on time Beam/field size Beam losses and locations Target materials and dimensions Parameters vary from facility to facility June 15, 2015 AAPMSS 2015 43
Typical Beam Currents Required for Dose Rate of 2 Gy/min in Water Target (1 liter) for Various Beam Delivery Techniques (With permission from IBA) Passive Scattering (PS) Uniform Scanning (US) Pencil Beam Scanning (PBS) Distal Icyclo Inozzle Loss Icyclo Inozzle Loss Icyclo Inozzle Loss Energy (na) (na) (na) (na) (na) (na) (na) (na) (na) (MeV) 160 130 11 119 22.5 2.6 19.6 17.36 0.493 16.87 Currents and current loss higher for PS compared to US and PBS More secondary radiation production for PS compared to US and PBS June 15, 2015 AAPMSS 2015 44
Effect of Target Thickness on Dose Attenuation in Forward Direction (Ipe Data: FLUKA) Use of thick targets is not conservative in the forward direction Actual target thickness should be used June 15, 2015 AAPMSS 2015 45
Shielding Design Considerations Accelerator Type Synchrotron Cyclotron Shielding Material Composition Density Water content Facility Layout Adjacent occupancies Type of Area (Controlled, Public, etc.) Above ground, underground.. Country/State Specific Regulatory Dose Limits Shielding design is facility dependent! June 15, 2015 AAPMSS 2015 46
Dose Attenuation of Concretes with Different Compositions but Same Density Dose (psv-m2/p) Dose 0-10º: 250 MeV Protons on Tissue 1.0E-02 1.0E-03 1.0E-04 1.0E-05 1.0E-06 Site-specific composition and density should be used! Concrete A Concrete B Concrete C 448 cm of Concrete A provides same dose attenuation as 405 cm of Concrete C Difference in thickness = 43 cm! 1.0E-07 0 100 200 300 400 500 Concrete Thickness (cm) (Ipe Data: FLUKA) June 15, 2015 AAPMSS 2015 47
Variation in Shielding Thicknesses For Different Proton Facilities (PTCOG Report 1) Shielding thicknesses vary for proton facilities with different machines, usage assumptions, regulatory limits and shielding materials NCC, Korea: IBA 230 MeV Cyclotron Loma Linda, CA: Optivus 250 MeV Synchrotron Rinecker, Germany: Varian 250 MeV Cyclotron June 15, 2015 AAPMSS 2015 48
Mazes Radiation at maze entrance consists of neutrons that scatter through the maze; and neutron capture gamma rays Forward-directed radiation from target should never be aimed toward the maze opening Dose at maze entrance can be reduced by: Reducing maze cross-sectional area Increasing maze length Increasing number of legs Maze legs should be perpendicular to each other At least two full scatters to the entrance are desirable PTCOG Report 1 June 15, 2015 AAPMSS 2015 49
Example of an Pseudo Maze Maze appears to have two legs Legs are not at 90 degrees to each other Single scatter from source reaches maze entrance with very little attenuation Ineffective design June 15, 2015 AAPMSS 2015 50
Design of Penetrations Ensure that no direct radiation from source points in direction of penetration or portion thereof June 15, 2015 AAPMSS 2015 51
Skyshine and Groundshine PTCOG Report 1 June 15, 2015 PTCOG51 AAPMSS EW 2015 N. Ipe 52
Activation: Graph of Nuclides by Type of Decay Stable nuclides lie along black curve Nuclides on either side are unstable Solid black line represents N = Z Both protons and neutrons can cause activation For activation to occur, incident radiation should overcome Coulomb barrier of positive nucleus Requirement favors neutrons which are electrically neutral June 15, 2015 AAPMSS 2015 53 "Table isotopes en" by Table_isotopes.svg: Napy1kenobiderivative work: Sjlegg (talk) - Table_isotopes.svg. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/file:table_isotopes_en.svg#/media/file:table_isotopes_en.svg
Activation in Proton Accelerators Activation in proton therapy facilities is significantly lower than at high-energy accelerator facilities because of the much lower beam intensities Activation is produced wherever beam losses occur Beam-line components, shielding, air, earth, cooling and ground water can be activated Cooling water should be confined to self-contained loop Ventilation is required to reduce air activation Local shielding can be used to reduce external exposure from activated components Delay of entry after irradiation reduces exposure to short-lived radionuclides (RNs) June 15, 2015 AAPMSS 2015 54
Nuclear Reactions Leading to Activation Thermal and intermediate-energy neutron capture (n, γ) Inelastic scatter of neutrons with energy > excited level of target nucleus (n,n ) Neutron threshold reactions (n, p), (n, pn), (n, 2p), (n, n2p), (n, α), etc. for neutron energies between few MeV and 50 MeV Proton threshold reactions (p, n), (p, pn), (p, 2n) (p, p2n), (p, α), etc. proton neutron energies between few MeV and 50 MeV As particle energy increases, number of reaction channels and number of radionuclides increase Spallation neutrons Cause activation at threshold energies > 20 MeV and wide variety of new atoms are formed Excited residual nucleus can de-excite by emitting evaporation nucleons or whole groups of nucleons, or by fission De-excited nucleus is radioactive. June 15, 2015 AAPMSS 2015 55
Neutron and Proton Cross Sections in Iron 56 Courtesy of C. Sunil Thermal neutron capture has highest cross-section Threshold for 56 Fe (p,*) ~ 2.5 MeV Threshold for 56 Fe(n,p) 56 Mn is 5 MeV Cross-sections for threshold reactions rapidly increase and have a peak Iron can be activated by thermal neutrons, fast neutrons and protons June 15, 2015 AAPMSS 2015 56
[Activity]/[Saturation Activity] Change in Activity during Irradiation 1.2 λdt A = A (1 e ) 1 0.8 0.6 0.4 0.2 0 t Saturation Activity T Continuous Irradiation R >>T 1/2 A 0 A T T R D Irradiation Decay T R,T <<T D 1/2 s λ t June 15, 2015 Time AAPMSS 2015 PTCOG Report 1 t = 0 A s A e D λ D = ln(2) T 1/ 2 For continuous ventilation during irradiation, λ D can be replaced by an effective removal constant, λ E λ = λ + E D λ where λ v is the ventilation constant v.
Monte Carlo Calculations A = N φ( E) σ ( E) de E N = number of target nuclei σ(e) = energydependent cross section φ(e) = incident particle fluence Activation Calculations Analytical calculations assume average cross section and flat neutron spectrum Not accurate Cross-section and fluence are energy-dependent Capture cross-sections have several resonances in intermediate energy range In shielded room there is both direct and reflected spectra Reflected spectra also have resonances June 15, 2015 AAPMSS 2015 58
Surface dose rates can be used to determine locations of beam loss Activation of Cyclotron Cannot quantify amount of radioactivity due to geometric effects Measured dose rates do not show uniform beam loss distribution around cyclotron RNs in Copper: 64 Cu (12.8 h) and 61 Cu, (3.32 h) RNs in Iron: 52 Mn (5.6 d), 54 Mn (303 d), and 56 Mn (2.57 h) Measured dose rates in msv/h from activation in PSI 250 MeV cyclotron, 10 cm from the surface measured 50 min after beam termination, for an extracted charge of 805 µa-h at 230 MeV. Courtesy of M. Schippers (Steen et al, Proc. of CYCLOTRONS 2010. September 6 10, 2010) June 15, 2015 AAPMSS 2015 59
Study of the Induced Radioactivity by Therapeutic Proton Beam in National Cancer Center, Korea, Lee et al., Poster Presented at PTCOG 50, 2011. PMMA Phantom Dominant Radionuclides 10 C (19.25 s) 11 C (20.39 min) 13 N (9.96 min), 14 O (1.18 min) 15 O (2.04 min) Rapid decrease in dose rate due to short-lived RNs Brass Collimator Dominant Radionuclides 62 Cu (9.76 min) 63 Zn (38.33 min) Higher dose rate than PMMA June 15, 2015 AAPMSS 2015 60
Concrete Activation Amount of induced radioactivity in concrete is smaller than that in accelerator components directly irradiated by primary proton beam 23 Na (n th, γ) 24 Na (15 h) ) E = 1.369 MeV (99.935%), 2.754 (99.862%) Significant exposure from gamma rays during first few hours after beam termination 59 Co (n th, γ) 60 Co and 151 Eu (n th, γ) from Co and Eu impurities Low amounts of impurities Large cross-sections Long-lived RNs of concern in decommissioning, 22 Na (2.62 y) E max = 1.275 MeV (22%) (spallation) 60 Co (5.26 y) E = 1.17 MeV (100%), 1.33 MeV (100%) 152 Eu (13.52 y) ) E max = 1.408 MeV (22%) Depending upon concrete composition other RNs may need to be considered June 15, 2015 AAPMSS 2015 61
Water Activation Cooling water is activated by spallation reactions of high-energy neutrons with oxygen 11 C is dominant short-lived RN after 1-5 h of irradiation Annihilation photons and gammas increase dose rates around cooling water pipes and heat exchangers Protons may also directly strike cyclotron slits and deflector and activate cooling water High-energy secondary neutrons may penetrate shielding and activate ground water 22 Na (leaches from earth) and 3 H produced directly in ground water are of concern Spallation Radionuclides Nuclide Half-life 3 H 12.3 year β - Decay Mode, γ-ray Energy and Emission Probability 7 Be 53.3 day EC, 0.478 MeV γ 10.5% 11 C 20.4 min β + 13 N 9.97 min β + 14 O 1.18 min β +, 2.3 MeV γ 99.4% 15 O 2.04 min β + June 15, 2015 AAPMSS 2015 62
Radionuclides from Neutron Activation of Air Radionuclide Half Life Emission Parent Element % Parent Element in Air 3 H 12.2 y β - 12 C, 14 N, 16 O 0.012, 75.5, 23 7 Be 53 d γ, EC 12 C, 14 N, 16 O 0.012, 75.5, 23 11 C* 20.5 min β + 12 C, 14 N, 16 O 0.012, 75.5, 23 13 N* 10 min β + 14 N, 16 O 75.5, 23 15 O* 2.1 min β + 16 O 23 41 Ar* 1.8 h β -, γ 40 Ar 0.013 June 15, 2015 AAPMSS 2015 63 *Only RNs that need to be considered based on activities and allowable concentrations
The astute physicist rides the shielding wave undaunted! Hydrogenous Material Who is this? Astute Physicist June 15, 2015 AAPMSS 2015 64 Photograph provided by Paul Pater San Diego