Neutron Interactions Part II

1
Neutron InteractionsPart II

• Rebecca M. Howell, Ph.D.
• Radiation Physics
• rhowell_at_mdanderson.org
• B1.4580

2
Why do we as Medical Physicists care about
neutrons?

• Neutrons in Radiation Therapy
• Neutron Therapy
• Fast Neutron Therapy
• Boron Neutron Capture Therapy
• Contamination neutrons in high energy x-ray
  therapy and proton therapy.
• Patient and personnel dose
• Important component in shielding design

3
Neutron Radiotherapy

• Fast Neutron Therapy Beams
• Boron Neutron Capture Therapy
• History and Current Facilities
• Treatment sites

4
Fast Neutrons Methods of Production

• Neutrons can be produced in a cyclotron by
  accelerating deuterons or protons and impinging
  them on a beryllium target.
• Protons or deuterons must be accelerated to 50
  MeV to produce neutron beams with penetration
  comparable to megavoltage x-rays.

5
Fast Neutrons Methods of Production

• Accelerating deuterons to 50MeV
• Requires very large cyclotron, too large for
  hospital.

• Accelerating protons to 50MeV
• Much smaller cyclotron b/c proton has ½ the mass
  of deuteron.

6
Fast Neutrons from Deuteron Bombardment of Be

• Stripping Process
• Proton is stripped from the deuteron.
• Recoil neutron retains some of the incident
  kinetic energy of the accelerated deuteron.
• For each neutron produced, one atom of Be is
  converted to B.

P
n
n
g
7
Fast Neutron Spectra from Deuteron Bombardment
of Be

• Neutron spectra consists of a single peak, with a
  modal value of about 40 of the energy of the
  incident deuterons.

Fig 24.2a
Hall fig 24.2a
8
Fast Neutrons from Proton Bombardment of Be

• Knock-out Process
• Protons impinge target of beryllium, where they
  knock-out neutrons.
• For each neutron knocked-out, one atom of Be is
  converted to B.

P
n
g
9
Fast Neutron Spectra from Proton Bombardment of
Be

• The neutron spectra spans a wide range of
  energies.
• Necessary to filter out the low energy neutrons
  to achieve acceptable depth dose distribution.

• According to Hall text book a polyethylene
  filter was used to harden the beam.
• Based on our knowledge of neutron interactions
  why would polyethylene be a good choice for
  removing the low energy component from the
  neutron beam?
• Would polyethylene alone solve the problem or
  make it better?

Hall fig 24.2b
Fig 24.2b
10
Isodose Distribution
Bewley, Fig 4.3

• Neutron beam (produced from 50-MeV protons or
  deuterons) has comparable depth dose
  distribution/isodose to 6MV photon beam.

Note differences incearse for low isodose
lines
11
Long Treatment Distances

• Neutron beam treatment distances are 100 to 140
  cm due to large collimator
• Collimator materials
• Hydrogenous material to slow the neutrons
• Absorber material to remove thermal neutrons
• Pb or other high Z material to absorb g-ray
  component (remember that activation follows
  absorption, g-photon is often the result)

12
Clinical Experience with Fast Neutrons

• First experience at Lawrence Berkeley Laboratory
• Hammersmith Hospital in London
• 3 Neutron Therapy Facilities in the US
• Northern Illinois University Institute for
  Neutron Therapy at Fermilab
• University of Washington Medical Center
• Gershenson Radiation Oncology Center at Harper
  University Hospital, Detroit

13
Modern Neutron Therapy Facilities

• University of Washington Medical Center
• Cyclotron accelerates protons (50.5MeV)
• Rotating gantry
• MLC equipped

• Gershenson Radiation Oncology Center Karmanos
  Cancer Center/Wayne State University (KCC/WSU)
• Gantry mounted superconducting cyclotron
  accelerates deuterons (48.5 MeV)
• Rotating Gantry
• MLC equipped

14
University of Washington NeutronClinical Neutron
Therapy System (CNTS)

• University of Washington CNTS Lower Floor
  Schematic

Rotating gantry
Fixed beam line?
15
University of Washington NeutronClinical Neutron
Therapy System (CNTS)

• University of Washington CNTS MLC

16
Neutron therapy facility at the Gershenson
Radiation Oncology Center KCC/WSU

• Schematic gantry mounted superconducting
  cyclotron GMSCC

17
Neutron therapy facility at the Gershenson
Radiation Oncology Center KCC/WSU

• MLC Schematic

• MLC Photo

18
Fast Neutron Therapy

• Considerations
• Who should be treated with neutrons?
• Subgroups of patients that may benefit from
  neutrons.
• Slower growing tumors.
• Cancers w/ good response to neutron Therapy
• adenoidcystic carcinoma (cancer of parotid
  glands)
• locally advanced prostate cancer
• locally advanced head and neck tumors
• inoperable sarcomas
• cancer of the salivary glands

19
Neutrons for Radiation Therapy

• A few references
• Fast neutron radiotherapy for locally advanced
  prostate cancer. Final report of Radiation
  Therapy Oncology Group randomized clinical trial.
  (American Journal of Clinical Oncology. 1993 Apr
  16(2)164-7)
• Fast neutron irradiation of metastatic cervical
  adenopathy The results of a randomized RTOG
  study. (International Journal of Radiation
  Oncology Biology Physics, Vol. 9, pp. 1267-1270)
• Neutron versus photon irradiation for
  unresectable salivary gland tumors Final report
  of an RTOG-MRC randomized clinical trial.
  (International Journal of Radiation Oncology
  Biology Physics, Vol. 27, pp. 235-240)
• Fast neutron radiotherapy for soft tissue and
  cartilaginous sarcomas at high risk for local
  recurrence. (International Journal of Radiation
  Oncology Biology Physics, Vol. 50, No. 2, pp.
  449456)
• Photon versus fast neutron external beam
  radiotherapy in the treatment of locally advanced
  prostate cancer results of a randomized
  prospective trial. (International Journal of
  Radiation Oncology Biology Physics, Vol. 28, pp.
  47-54)

20
Boron-Neutron Capture Therapy

• The idea
• Preferentially deliver Boron containing drug to
  the tumor.
• Then deliver thermal (0.025eV) neutrons, which
  interact with the boron to produce alpha
  particles.
• Recall 10B has large thermal cross section s
  3837 barns
• The 10B absorbs the thermal energy neutron and
  ejects an energetic short-range alpha particle
  (1.47MeV) and lithium ion (0.84MeV) which deposit
  most of their energy within the cell containing
  the original 10B atom.

21
Why Boron???

• Several nuclides have high thermal neutron s, but
  10B is the best choice for several reasons
• it is non-radioactive and readily available,
  comprising approximately 20 of naturally
  occurring boron
• Emitted particles (a and 7Li) have high LET
• Combined path lengths are approximately one cell
  diameter i.e., about 12 microns, theoretically
  limiting the radiation effect to those tumor
  cells that have taken up a sufficient amount of
  10B, and simultaneously sparing normal cells
• Chemistry of boron is well understood and allows
  it to be readily incorporated into a multitude of
  different chemical structures.

22
Boron-Neutron Capture Therapy

• Beam Energy Selection
• Limited penetration of thermal neutrons.
• Thermal neutrons rapidly attenuated by tissue.
• HVL only about 1.5cm.
• Not possible to treat depths greater than a few
  cm.
• Can use epithermal neutrons (1eV-10keV), which
  are theramlized by tissue (via collisions w/ H).
• Peak in dose occurs at 2 to 3cm
• Avoid high surface doses, but still poorly
  penetrating!

23
Boron-10 Neutron Interaction

• An epithermal beam rapidly loses energy by
  elastic scattering in tissue.
• The thermal neutrons are captured by the 10B
  atoms which become 11B atoms in the excited state
  for a very short time ( 10-12 s).
• The 11B atoms then splits into alpha particles,
  7Li recoil nuclei and in 94 of the reactions,
  gamma rays.

http//web.mit.edu/nrl/www/bnct/info/description/d
escription.html
24
BNCT Neutron Source at MIT/Harvard

• The MIT/Harvard group makes use of a fission
  converter based epithermal neutron beam at the
  MITR-II Research Reactor.
• filtered by aluminum, Teflon, cadmium,  and Lead.
  
• provides a broad spectrum epithermal beam with
  low incident gamma and fast neutron contamination
  while maintaining an incident neutron flux of 5
  x 109  neutroncm-2sec-1. 
• permits irradiations for clinical trials to be
  conducted in 1 - 4 fractions in 10 minutes or
  less

25
Treatment Sites for BNCT

• Clinical Trials for
• Glioblastoma Multiform (GBM)
• Sweet and colleagues first demonstrated that
  certain boron compounds would concentrate in
  human brain tumor relative to normal brain
  tissue.1
• Melanoma

1(Sweet, W.H., Javid, M., "The possible Use of
Neutron-capturing Isotopes such as boron-10 in
the treatment of neoplasms," I. Intracranial
Tumors, J. Neurosurg., 9200-209, (1952) )
26
Production of Secondary Neutrons
27
Secondary Neutrons Radiation Therapy

• X-Ray Therapy
• Neutrons can be produced via (g-n) reactions
  primarily with high atomic number materials
  within the treatment head.
• Proton Therapy
• Neutrons can be produced via (p,n) reactions, not
  limited to high Z materials.
• At the high energies other reactions are also
  possible..

28
Production of Neutrons

• (g,n) Reactions

29
Bremsstrahlung Spectrum
30
Particle Production Cross SectionsENDF/B-VII
Incident-Gamma Data
(g,n) cross section
High energy 18MV x-ray treatment beam has max
energy of 18MeV and an average energy of 6MeV.
Secondary neutron Production is possible in high
Z components of linac head. No neutrons for 6MV
x-ray beam all photons below threshold
Threshold energy for (g,n)
Note Magnitude of the cross sections ? 0.1- 0.6
barns
31

• 18 MV beam has more photons above the (g,n)
  threshold and most are in the region where cross
  section dramatically increases.

32
(g-n) Reaction Cross-Sections Elements in Tissue
(C,O,N)

• Note
• Threshold energies are much higher compared to
  high Z
• The magnitude of the reaction x-sections are an
  order of magnitude lower than for high Z (0.005
  0.02).

• These data are from the T-2 Nuclear Information
  Service.

33
Secondary Neutron Spectra from Clinical Photon
beams

• The initial distribution of secondary neutrons
  generated in the linac head from (g,n) reactions
  is approximately isotropic and resembles a
  fission spectrum.
• Then,
• The neutron energy decreases as a consequence of
  their transport through the components of the
  treatment head (primary collimators, flattening
  filter, secondary jaws, MLC, etc).
• The primary mechanisms of energy loss in high Z
  materials in the linac head are inelastic
  scattering and (n,2n) reactions.

34
Photoneutron SpectraEffect of Collimators and
room shielding

• Photoneutron spectrum for 15MeV electrons
  striking W target (designated 15MeV W PN bare)
• Spectrum with 10 cm of W shielding surrounding W
  target.
• Spectrum with 10 cm of W shielding surrounding W
  target inside a concrete room
• A 252Cf fission spectrum shown for comparison

NCRP-79 Fig 25
35
Secondary Neutron SpectraMeasured for Varian
18MV Linac

• Howell et al. Medical Physics, Vol. 36, No. 9,
  4027-4038 (2009)

36
Production of Neutrons

• (p,n) Reactions

37
Proton Spectra

• Clinical proton beams have a much smaller energy
  spread compared to photon beams (Gaussian
  distribution).
• Also, the maximum energies are considerably
  higher and clinical beam energies may include,
  100 MeV, 160 MeV, 200 MeV, and 250 MeV beams.

38
Particle Production Cross Sections ENDF/B-VII
Incident-Proton Data
Note High Z material (e.g. Pb-207) Magnitude
of the (p,n) cross sections are higher than (g,n)
and continue to increase with increasing
energy. The proton beam energies can be as high
as 250MeV, well above the threshold.
39
Particle Production Cross Sections ENDF/B-VII
Incident-Proton Data

• Note low Z material (e.g. C-12)
• Magnitude of the (p,n) cross sections are much
  lower than in high Z materials
  
• energy thresholds for (p,n) in low Z are higher
  compared to high Z
• Energy threshold similar to (g,n)

November 2007 ? Rebecca M. Howell, Ph.D.
40
Particle Production Cross Sections ENDF/B-VII
Incident-Proton Data

• Note low Z material (e.g. C-12)
• Magnitude of the (p,n) cross sections are much
  lower than in high Z materials
  
• energy thresholds for (p,n) in low Z are higher
  compared to high Z
• Energy threshold similar to (g,n)

November 2007 ? Rebecca M. Howell, Ph.D.
41
Secondary Neutron Spectra for Clinical Proton
Beams

• Zhang et al. Phys. Med. Biol. 53 (2008) 187201

42
References

• Eric J. Hall. Radiobiology for the Radiologist
  5th Ed. (2000)
• Frank H. Attix. Introduction to Radiological
  Physics and Radiation Dosimetry. (1986)
• Patton H. McGinley. Shielding Techniques for
  Radiation Oncology Facilities 2nd ed.
• D.K. Bewley. The Physics and Radiobiology of Fast
  Neutron Beams (1989)
• AAPM Report 7 Protocol for Neutron Beam Dosimetry
• ICRU 45 Clinical Neutron Dosimetry
• NCRP 79 Neutron Contamination from Medical
  Electron Accelerators
• NCRP 151 Structural Shielding Design and
  Evaluation for Megavoltage X- and Gamma-Ray
  Radiotherapy Facilities (2005)
• ICRP74/ICRU57 (Jointly published by both ICRU and
  ICRP) Conversion Coefficients for use in
  Radiological Protection against External
  Radiation
• ICRP 60 Recommendations of the International
  Commission on Radiological Protection
• ICRU 66 Determination of Operational Dose
  Equivalent Quantities for Neutrons
• http//web.mit.edu/nrl/www/bnct/info/description/d
  escription.html
• T2.lanl.gov
• http//www.nndc.bnl.gov/nudat2

43
End

• This concludes the material from the neutron
  interactions lectures that will be covered on the
  exam in this course.

44
Extra Information

• The material in the remaining slides is covered
  in Radiation Protection and is beyond the scope
  of todays lecture. In previous years, I have
  covered this material in this course. This year,
  I have decided to cut this material to minimize
  overlap with other courses.
• However, I decided to make the slides available
  to you should you be interested, but will not be
  testing you on this material.

45
Shielding Considerations for Secondary Neutrons

• for
• High Energy X-Ray Beams

46
Shielding for Photoneutrons

• High beams (gt10MV) are contaminated with
  neutrons.
• Produced by high energy x-rays and e-s incident
  on various materials (target, flattening filter,
  collimators, etc.).
• Many more neutrons in x-ray beam than in e- beam.
• X-section for (e,n) reactions smaller than
  x-section for (g,n) by factor of 10.
• In electron mode beam current is about 1000X less
  than in x-ray mode due to inefficiency of Brems.
  Production.

47
Shielding for Photoneutrons

• Neutron contamination increases rapidly with
  energy from 10 to 20 MV, then remains approx.
  constant above 20MV (recall that there are very
  few linacs with max energies greater than 25MeV).
• Neutron contamination at cax for 16-25MV x-ray
  beam is approx. 0.5 of x-ray dose, and falls off
  to about 0.1 outside the field.
• Higher for IMRT? more beam on time to achieve
  same photon dose.

48
Particle Production Cross SectionsENDF/B-VII
Incident-Gamma Data
(g,n) cross section
Neutron contamination increases rapidly with
energy from 10 to 20 MV, then remains approx.
constant above 20MV (recall that there are very
few linacs with max energies greater than 25MeV).
Threshold energy for (g,n)
Note Magnitude of the cross sections ? 0.1- 0.6
barns
November 2007 ? Rebecca M. Howell, Ph.D.
49
Shielding for Photoneutrons

• Concrete barriers designed for x-ray shielding
  are sufficient for photoneutrons.
• Door must be protected against neutrons that
  diffuse into the maze and reach the door.
• Required door shielding can be minimized with a
  good maze.
• Maze gt 5m desirable.
• This length is chosen because the TVL for the
  photoneutrons entering the maze is approximately
  5m (MKcGinley, pg 71)

50
Example of Door in High Energy Vault shielded for
Neutrons
Note Figure (5.5) is taken from Shielding
Techniques 2nd ed. by Patton McGinley
Note the average energy of the neutrons at the
Maze entrance is approximately 100keV (NCRP, 1984)
Note there are many ways to design a door, in
some cases, the lead is placed before the
polyethylene, and in some cases it is sandwiched
between two layers of lead, this is just one
example of a door design.
51
Door Design
Photoneutron Shielding

• After multiple scattering interactions in the
  polyethylene (high H content) the neutrons are
  thermalized.
• Thermal neutrons can undergo neutron capture
  releasing high energy g-rays (n,g) ? g-rays
  energies can exceed 8MeV, and have an average
  energy of 3.6MeV.
• How do we eliminate these high energy g-rays? Add
  5 Boron to the polyethylene.
• Boron absorbs (high thermal absorption cross
  section) the low energy neutrons before they have
  a chance to undergo (n,g) reactions.
• But the reaction also results in a 0.48MeV g.
• Lead is placed after the boronated polyethylene,
  where it attenuates the photons produced in the
  boron (0.48MeV) and any capture gamma rays
  generated in the maze by neutron capture in the
  concrete walls, ceiling, and

McGinley, 2002
52
Boron Interaction with Thermal Neutrons

• Materials with B are effective absorbers of
  thermal neutrons because of the high thermal
  neutron cross section
• The thermal neutrons are captured by the 10B
  atoms which become 11B atoms in the excited state
  for a very short time ( 10-12 s).
• The 11B atoms then fissions producing
  a-particles, 7Li recoil nuclei, and in 94 of the
  reactions, gamma rays (0.48MeV).

53
Door Design for Neutron Shielding Details

• Boronated polyethylene
• The polyethylene (high H content) slows
  (moderates) the fast and intermediate energy
  neutrons to thermal energies.
• The 5 Boron absorbs the low energy neutrons
  (high cross section for thermal neutron
  absorption).
• Lead absorbs the 0.48 MeV photon that results
  from the (n,a) and capture gammas ( from maze
  ceiling, and floor).

Lead
Steel Casing
Maze
Polyethylene 5 Boron
54
Activation of Materials in
Linac components
Reproduced from Table 4.4, McGinley
55
Activation Material in Air

• Air is made radioactive by medical accelerators
  operated above 10 MeV primarily by
• Each reaction produces a positron emitter with a
  relatively short half-life.
• Patients/personnel can be exposed to 0.511 MeV
  annihilation photons.

56
Activation Material in Air

• Maximum permissible concentration in air (MPCa)
  based on a 40-hr work week and typical treatment
  vault (air volume).

Reproduced from Table 7-5, McGinley
57
Activation Material in Air

• McGinley at al (1984) calculated annual total
  dose equivalent to radiation therapists skin
• Conservative Calculation Assumptions
• 40 patients per day
• 5 days per week
• 4Gy/fx
• Daily treatment time 120-s
• Therapist stay time of 600s per patient
• Far below the Maximum Permissible Skin dose
  (0.5Sv).
• Air activation presents minimal hazard.

58
Neutron Dose

• Two main Categories of Neutron Dosimetry
• Clinical Neutron Beam Dosimetry
• Dose from neutron beams used for patient
  treatment.
• Protection Dosimetry for unwanted neutron dose
  (low neutron doses)

59
Quantities used in Radiation Protection

• Quantities used in Radiological Protection
• There are two types of quantities used in
  radiological protection
• Protection Quantities - defined by the
  International Commission on Radiation Protection
  (ICRP).
• Operational Quantities - defined International
  Commission on Radiation Units and Measurements
  (ICRU).

60
ICRP Protection Quantities

• The (ICRP) defines limiting or protection
  quantities as dosimetric quantities specified in
  the human body.
• Recommended dose limits are expressed in terms of
  protection quantities.
• These quantities are not directly measurable but
  may be related by calculation to the radiation
  field in which the exposure occurs.

61
ICRU Operational Quantities

• Operational quantities were designed by the ICRU
  in response to ICRP recommendations on
  radiological protection.
• Used to demonstrate compliance with dose limits.
• Operational quantities provide a reasonable
  estimate of the protection quantities and serve
  as calibration quantities for dosimeters used in
  monitoring.

62
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63
Protection Quantities
64
ICRP Protection Quantities

• Absorbed Dose, DT, is the mean absorbed dose in a
  specified tissue or organ of the human body, T,
• Equivalent Dose, HT, is the absorbed dose
  averaged over the tissue or organ, T, irradiated
  in a radiation field consisting of several
  different radiations with different values of WR,
  
• Effective Dose, E, is the sum of the weighted
  equivalent doses in all the tissues and organs of
  the body,

• where, mT is the tissue or organ mass, DT is the
  absorbed dose in the mass element dm.
• where, DT,R is the average absorbed dose from
  radiation, R, in tissue T.
• where, HT is the equivalent dose in the tissue or
  organ T and WT is the weighting factor for the
  tissue.

65
ICRP Protection Quantities

• ICRP-60 Tissue Weighting Factors

• Note In the case in which a single one of the
  remainder organs receives an equivalent dose in
  excess of the highest dose in any of the 12
  organs for which a weighting factor is specified
• a weighting factor of 0.025 should be applied to
  that tissue and
• a weighting factor of 0.025 applied to the
  average dose in the rest of the remainder organs.

66
Radiation Weighting Factors
67
Radiation Weighting Factors
68
Operational Quantities
69
ICRU Operational QuantitiesDose Equivalent

• The Dose Equivalent, H as defined by the ICRU is
  the product of Q and D at a point in tissue
• where D is the absorbed dose
• Q is the quality factor at this point
• The SI unit for H is the Sievert (Sv).

70
ICRU Operational QuantitiesDose Equivalent

• The quality factor depends on the unrestricted
  linear energy transfer, L, for charged particles
  in water, specified in ICRP publication 60.
• For photons and electrons the quality factor is
  unity. For neutrons, the quality factor is
  strongly energy dependent (ICRU Report 66).

71
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72
Determining Dose Equivalent

• Dose equivalent is essentially unmeasurable.
• But, as shown in the previous diagram, it can be
  determined by calculation or by a combination of
  calculations and measurements.
• Quantities needed to determine dose equivalent
  are
• absorbed dose and the quality factor or
• fluence and fluence-to-dose equivalent conversion
  coefficients.

73
Additional Information on Neutron
InteractionCross-sections

• This information will not be on your exam.

74
Transport of Secondary Neutrons

• Strongly dependent on absorbing material and
  incident neutron energy.
• Example 207Pb
• The next 15 slides of the presentation are
  provided as an example of the data that are
  available for a given isotope, given time
  constraints, these can not be covered in detail.

75
Great Reference for Nuclear DataT2.lanl.gov
76
T2.lanl.gov

• What Data are provided in this file?
• Lets consider an example
• For 207Pb, the PDF file has 179 pages of data.

View PDF files for various elements/ isotopes
77
Principal Cross SectionsLow Energy (Log-Log Plot)
78
Principal Cross SectionsHigh Energy (Linear Plot)
79
Inelastic Cross Sections are provided on Separate
Plots
80
Inelastic Cross Sections

• What is (n,n1), (n,n2), etc.
• There is a discrete energy band gap between
  energy levels in the nucleus.
• 1 refers to promoting a neutron into the first
  excited state.
• 2 refers to promoting a neutron into the second
  excited state.
• (n,x) Anything above 20th excited level (not
  n,g)
• How much energy are we talking?
• http//www.nndc.bnl.gov/nudat2/

81
NuDat 2.4
Select Levels and Gamma Search
http//www.nndc.bnl.gov/nudat2/
82
Enter Isotope of Interest
Search
83
Nuclear Level and Gamma Search
1st excited State
2nd excited State
3rd excited State
84
Threshold Reactions
85
Angular Distributions(provided for all elastic
and inelastic Scattering)

• Angular Distribution of Elastic Scatter
  (0-30MeV)

• Angular Distribution of inelastic (n,n1)Scatter
  (0-30MeV)

86
Definitions from ICRP Report 63

• Elastic denotes a reaction in which incident
  projectile scatters off target nucleus with total
  KE being conserved (final nucleus is the same as
  bombarded nucleus).
• Non-elastic is a general term referring to
  nuclear reaction that are not elastic (i.e. KE is
  not conserved).
• Inelastic refers to specific type of non-elastic
  reaction in which KE not conserved, but final
  nucleus is the same as bombarded nucleus.