1. Session I.4.13 Part I Review of Fundamentals
Module 4 Sources of Radiation
Session 13 Neutron Production
IAEA Post Graduate Educational Course
Radiation Protection and Safe Use of Radiation Sources

2. Overview In this Session we will discuss how neutrons are produced and
We will also discuss how neutrons are used for medical neutron therapy

3. Neutron Production
Neutrons carry no electrical charge and are thus "neutral"
Now let’s discuss various methods of producing neutrons to be used for various purposes.

4. Fission
Neutrons are produced in great numbers by the fission process in reactors.
The theoretical basis for fission is the massive energy release which occurs when a heavy nucleus divides into two smaller ones. Only a few very heavy nuclei undergo fission spontaneously, while others can be encouraged to undergo fission by the addition of energy when a neutron is absorbed. Such fissile materials (as they are known) include 235U and 239Pu.
During the fission process, a number of neutrons are released, and if these go on to induce new fission events, a chain reaction results. The use of a controlled chain reaction is the basis for all nuclear power stations.
The process of nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann and was explained in early 1939 by Lise Meitner and Otto Frisch. The fissionable isotope of uranium, U-235, can be split by bombarding it with a slow, or thermal, neutron. (Slow neutrons are called “thermal” because their average kinetic energies are about the same as those of the molecules of air at ordinary temperatures.) The atomic numbers of the nuclei resulting from the fission add up to 92, which is the atomic number of uranium. A number of pairs of product nuclei are possible, with the most frequently produced fragments being krypton and barium.
Since this reaction also releases an average of 2.5 neutrons, a chain reaction is possible, provided at least one neutron per fission is captured by another nucleus and causes a second fission. In an atomic bomb, the number is greater than 1 and the reaction increases rapidly to an explosion. In a nuclear reactor, where the chain reaction is controlled, the number of neutrons producing additional fission must be exactly 1.0 in order to maintain a steady flow of energy.

5. 3He and Neutron Production
from Fusion
Neutrons can be produced by fusion of two deuterons (deuterium ions).
For light nuclei, which are of interest in controlled nuclear fusion research Ro may be taken to be approximately equal to a nuclear diameter (5 x cm), and since 'e' is 4.8 x esu (statcoulomb) it follows from the equation that the energy required to surmount the Coulomb barrier is:
= 4.6 x 10-7 Zl Z2 erg = .28 Zl Z2 MeV (million electron volts)
Where 1 MeV = 1.6 x 10-6 erg.
It can be seen that the energy which must be acquired by the nuclei before they can undergo fusion increases with the atomic numbers Zl and Z2 . For reactions among hydrogen isotope nuclei (deuterium, for example) for which Zl=Z2=l, the minimum energy according to classical theory is about .28 MeV. Larger energies would be required for reactions involving nuclei of higher atomic numbers because of the increased electrostatic repulsion.
During the fusion reaction, a small amount of mass appears to be lost, about 38 parts out of 10,000. But the matter has not been destroyed but instead has been converted to energy following Einstein's famous equation E=mc2. This equation states that the energy (E) is equal to the lost mass (m) times the square of the velocity of light (c). Even a very small mass can yield a considerable amount of energy. For example, if a 1 gram raisin were completely converted to energy it would equal about 10,000 tons of TNT!

6. Nuclear Reactions Fusion
Neutrons can be produced also by fusion of a deuteron (deuterium ion) and a triton (tritium ion).
The first generation of fusion power plants will use the D-T fusion reaction, shown schematically in the above animation. Nuclei of two isotopes of hydrogen, deuterium (D) and tritium (T) react to produce a helium (He) nucleus and a neutron (n).
In each reaction, 17.6 MeV of energy (2.8 pJ) is liberated:
D T  4He (3.5 MeV) n (14.1 MeV)
The first generation fusion reactors will use deuterium and tritium for fuel because they will fuse at lower temperature. Deuterium can be easily extracted from seawater, where 1 in 6500 hydrogen atoms is deuterium. Tritium can be bred from lithium, which is abundant in the earth's crust. In the fusion reaction a deuterium and tritium atom combine together, or fuse, to form an atom of helium and an energetic neutron.
This figure shows a deuterium ion (deuteron) combining with a tritium ion (triton) to form an unstable compound nucleus which relaxes into a helium ion and an energetic neutron. The "D-T" reaction has the highest reaction rate at the plasma temperatures which are currently achievable; it also has a very high energy release. These properties make it the easiest reaction to use in a man-made fusion reactor. As the figure shows, the products of this reaction include an alpha particle (Helium-4 nucleus) with 3.5 MeV energy, and a neutron with 14.1 MeV energy. The neutron escapes from the plasma (it has no charge and is not confined) and can be trapped in a surrounding "blanket" structure, where the n + Li-6 => He-4 + T reaction can be used to "convert" the neutrons back into tritium fuel.
Additional notes:
1 eV = E-19 joules;
Average particle thermal kinetic energy is 1 eV per 11,600 K.

7. Deuterium-Tritium Neutron Generator
A tritium target is bombarded with deuterons to strip neutrons.

8. Neutron Production by Spallation
Neutrons can also be produced by a process called “spallation.”
A more recent development is the accelerator-based pulsed source which produces neutrons in a totally different manner. In this spallation process, neutrons are released by bombarding a heavy metal target with high energy protons from a powerful accelerator.
Spallation is a more efficient way of extracting neutrons than fission, and with recent progress in accelerator technology and computing power it is now clear that the next generation of neutron sources will be based on this concept. The world’s premier spallation neutron source at present is the ISIS facility in the UK.

9. ISIS Neutron Production Assembly
ISIS is the world's most powerful pulsed spallation neutron source. The facility provides beams of neutrons and muons that enable scientists to probe the structure and dynamics of matter in areas encompassing Physics, Chemistry, Earth Science, Materials Science, Engineering and Biology.
ISIS is the major facility at the Central Laboratory of the Research Councils’ (CLRCs’) Rutherford Appleton Laboratory site in Great Britain.The source was approved in 1977 and the first neutrons were produced in late 1984.
It is well known that tungsten is a better target material than tantalum in two main respects. First it produces more neutrons per proton, and secondly the heat produced from the induced activity in tungsten is about one third that of tantalum. However, tungsten is a more difficult metal to machine and is subject to corrosion by water.
A development programme was put in place to establish fabrication methods to construct a target made from tungsten plates clad in tantalum. The cooling system design was optimised to minimise the number of cooling channels in the target and simplify the complex cooling manifold design required for uranium targets and used on the tantalum ones.
A schematic of the target design is shown in the above figure.

10. Continuous Spallation Neutron Source
This slide shows the SINQ Continuous Spallation Neutron Source in Switzerland.
SINQ at the Paul Scherrer Institut (PSI) in Villigen is the first continuous spallation neutron source world wide. The continuous flow of neutrons and the vertical target arrangement is responsible for an instrument suite characteristic of that installed at reactors.
On December 3, 1996, the first protons were guided onto the SINQ target, while scheduled user operation started in summer SINQ is a national neutron source open to the international community.

11. Proposed US Spallation Neutron Source
This slide shows the proposed spallation neutron source to be built in the United States.
The first third generation spallation neutron source to come into operation will be the American source. The construction of the SNS facility started with ground breaking on December 15, 1999 at Oak Ridge in Tennessee. The approved M$ project is for a 2MW short pulse spallation source operating at 60 Hz.
The facility will have one target station at start-up in A proposal for a Long Wavelength Target Station (up to 600 kW) operating at a lower frequency is being made at present. The design study for the second target station is funded by the National Science Foundation. The full facility could operate with both target stations around 2008.

12. (p,n) reactions (d,n) reactions (,n) reactions Neutron Production
Other reactions which produce neutrons include capture of protons, deuterons, and alpha particles in various nuclei, with emission of neutrons of various energies.

13. Charged Particle Bombardment
Nuclear Reactions
Charged Particle Bombardment
p Zn  67Ga + 2n
 O  F p + n
Many radionuclides can be produced through charged particle (protons, deuterons, alpha particles, 3He+2 ) bombardment of nuclei of stable atoms. Two such nuclear reactions are shown in this slide.
The charged particles must have enough kinetic energy to overcome the repulsive effects of a positively charged nucleus (1-100 MeV per nucleon). Such energies are produced by accelerating particles using a linear accelerator or cyclotron.
The desired isotope almost always has a different atomic number (Z) with respect to the target material. Charged particle reactions yield radionuclides that are predominantly neutron deficient and therefore decay via positron emission or EC (electron capture).

14. Neutron Production by Various Reactions
This slide shows a good summary of various neutron production methods using (p,n) and (d,n) reactions. A disadvantage of the (p,n) reaction is that it produces considerable gamma ray contamination, relative to the (d,n) reaction.
Depending on the planned use of the neutron beam being produced, gamma rays can be undesirable from the standpoint of complicating radiation measurement and also producing unwanted dose to people.

15. Neutron Production (cont)
This slide shows neutron production by means of (,n) and (,n) reactions. The (,n) reactions also produce considerable gamma ray contamination.

16. What is Neutron Therapy?
Neutron therapy is a highly effective form of radiation therapy
Long-term experience with treating cancer has shown that certain tumor types are very difficult to kill using conventional radiation therapy
These types are classified as being "radioresistant"
Neutron therapy specializes in treating inoperable, radioresistant tumors occurring anywhere in the body

17. What is Neutron Therapy?
Conventional radiation therapy includes photon (x-ray) and electron radiation, which is available at many clinics and hospitals
These beams are produced by electron accelerators or from radioactive sources such as cobalt
Particle therapy includes protons and neutrons, which are generated using proton accelerators
The basic effect of ionizing radiation is to destroy the ability of cells to divide and grow by damaging their DNA strands

18. What is Neutron Therapy?
For photon, electron and proton radiation the damage is done primarily by activated radicals produced from atomic interactions
These types of radiation are called low linear-energy-transfer (low LET) radiation
With neutron radiation the damage is done primarily by nuclear interactions
Neutrons are high linear-energy-transfer (high LET) radiation

19. What is Neutron Therapy?
If a tumor cell is damaged by low LET radiation it has a good chance to repair itself and continue to grow
With high LET radiation such as neutron radiation, the chance for a damaged tumor cell to repair itself is very small.
Because the biological effectiveness of neutrons is so high, the required tumor dose is about one-third the dose required with photons, electrons or protons
A full course of neutron therapy is delivered in only 10 to 12 treatments, compared to treatments needed for low LET radiation

20. What is Neutron Therapy?
Side effects for fast neutron therapy are similar to those of low LET therapy
Their severity depends on the total dose delivered and the general health of the patient
Effects on normal tissues are minimized by careful computerized treatment planning

21. History of Neutron Therapy
Neutrons were discovered by Sir James Chadwick in 1932
Just six years later, Dr. Robert Stone began clinical trials treating cancer with neutrons produced by E.O. Lawrence's cyclotron in Berkeley, California
These trials were terminated because the cyclotron was needed for the war effort during World War II
Clinical research began again in 1965 when Hammersmith Hospital in London began irradiating patients with neutron beams

22. History of Neutron Therapy
By 1969, it was clear that for certain tumors, local control could be achieved using neutron irradiation
Encouraged by these results, the M.D. Anderson Hospital and Tumor Institute in Houston, the Naval Research Laboratory in Washington, D.C., and the University of Washington in Seattle began neutron therapy research
They started treating patients in the early 1970s.

23. Boron Neutron Capture Therapy
(BNCT)
Boron Neutron Capture Therapy (BNCT) is a radiotherapy modality for treating cancerous tumours (e.g. glioma – a cancer of the brain)
It utilizes the reaction between the 10B-nucleus and slow (thermal) neutrons:
10B + n  7Li(0.84 MeV) + 4He(1.47 MeV) + g(0.48 MeV)
The released 7Li atom and alpha-particle (4He) have the total energy of 2.31 MeV which is deposited within the range of approximately 5-9 mm (i.e. at a distance corresponding to about one cell diameter)

24. Boron Neutron Capture Therapy
(BNCT)
In principle, one boron-neutron interaction liberates enough energy to kill the cell in which the inter-action takes place
The short distance deposition of the released energy spares the surrounding boron-free tissue of the radiation damage
The effect of BNCT is dependent on two preconditions:
the selective accumulation of boron atoms in the target (tumour) and healthy cells
the tumour site is reached by a sufficient number of neutrons

25. Medical Neutron Therapy
In BNCT, cancer cells are not directly destroyed by irradiated neutrons but indirectly by particles generated by nuclear reaction between borons and neutrons. BNCT stands as an effective remedy for malignant brain tumors and melanoma. Boron does not easily enter healthy brain cells owing to the blood‑brain barrier function that prevents the invasion of toxic substances into the brain, but it easily enters cancer cells, enabling their selective destruction by irradiation. In this method, boron compounds are injected into the patient's body in advance. After the boron compound has entered the brain tumor cells sufficiently, the affected part is irradiated by neutrons flux generated by a reactor. Boron absorbs neutrons and induces a nuclear reaction, and generates alpha and lithium particles which selectively destroy cancer cells containing it.

26. Medical Neutron Therapy
The configuration of a medical irradiation facility is shown here. First, a patient needing treatment undergoes surgery to open his/her skull in the medical treatment room, then is moved to the irradiation room installed beside the reactor core, where the affected area is exposed to neutron by the neutron beam facility, while doctors observe the patient via TV monitors and measuring equipments in the observation area.

27. Medical Neutron Therapy
The neutron beam facility is a main part of a medical irradiation facility. This slide gives a crosssection of the neutron beam facility.
Most important in this facility is to generate the suitable neutron beam to effect a cure by controlling the amount of heavy water in the tank.
Since neutrons from the reactor core are "fast neutrons" and have high energy, their speed must be retarded for use as a remedy. The heavy water tank, used to control their energy (speed), consists of aluminum (gray) and varying thicknesses of heavy water (orange), and is surrounded by graphite (black) to preclude neutron leakage from the tank.
A cadmium shutter next to the heavy water tank is used to alter the thermal or epithermal beam mode. Cadmium absorbs thermal neutrons well and enables the extraction of epithermal neutrons by closing this shutter. To use thermal neutron beam, the shutter is opened.
Bismuth (red) is a shield for alpha‑ray as this ray cause damage to normal brain cells.
A collimator is used to irradiate the converged neutron beam to the affected part. Collimators of 10,15 and 20 cm diameter can be used. In the future, other types of collimators will be prepared according the needs of doctors. The boron and lithium polyethylene (yellow) placed near the patient are neutron absorbers to prevent neutron leakage. The part nearest the patient (brown), made of lead, is alpha‑ray shielding. This equipment is designed to eliminate fast neutron and alpha‑ray that are harmful for normal cells and to concentrate the suitable neutron beam on the affected part.

28. Medical Neutron Therapy
This table shows the properties of medical irradiating facilities in various countries.

29. Medical Neutron Therapy
FERMILAB Neutron Therapy
Over 3000 patients have been treated at the Neutron Therapy Facility (NTF) since it opened on September 7, Neutrons are more effective at killing tumors than conventional radiation therapy. Cure rate depends on the type of cells in the tumor (histology), the size of the tumor (stage and/or grade), whether the tumor has spread to other parts of the body (metastasis), and the patient's general health.

30. Medical Neutron Therapy
HEAD AND NECK - Salivary glands, tongue, pharynx, oral cavity, nasopharynx, brain tumors
CHEST - Localized tumors of lung, mediastinum, pleura, pericardium
ABDOMEN - Pancreas, colon, bile duct, gallbladder, ampula of vater, peritoneum
PELVIS - Prostate, bladder, uterus, rectosigmoid
EXTREMITIES AND TRUNK - Soft tissue, bone, cartilage
PALLIATIVE - Large tumors and metastasis from neutron-sensitive tumors
Neutrons may be appropriate for the following sites, depending on stage and histology

31. Medical Neutron Therapy
The first superconducting cyclotron for medicine was designed and constructed at the NSCL and is now in use for cancer treatment at the Gershenson Radiation Oncology Center at Harper University Hospital in Detroit. It was completed at the NSCL in 1990 and was then moved to Detroit to be installed in the hospital. The cyclotron itself accelerates deuterons. Sometimes known as ““heavy hydrogen,”” deuterons differ from normal hydrogen in that they have a neutron as well as a proton in their nucleus. The cyclotron’’s fast-moving deuterons are stopped in a target of beryllium just before their exit from the cyclotron. This produces a beam of high-energy neutrons, which is then directed against the cancer patient’’s tumor. Since the cyclotron is superconducting, its physical size is much smaller than a room-temperature cyclotron would be. This ““miniaturization”” allows the cyclotron to be mounted on gantry rings that rotate around the patient so that the cancer can be irradiated from several angles. The Harper facility has become the most active neutron-therapy center in the world, and a new treatment modality using a combination of neutrons and x-rays has been found to be particularly effective for the treatment of advanced prostate cancer

32. Medical Neutron Therapy

33. Medical Neutron Therapy

34. Medical Neutron Therapy

35. Medical Neutron Therapy
This slide indicates the locations of fast neutron therapy facilities around the world. The next two slides provides more information about these facilities.

36. Fast Neutron Therapy Facilities
(E < 30 MeV)
Place Country Source Mean Energy
(MeV)
Obninsk Russia Reactor
Garching Germany Reactor 1.8
Chelyabinsk Russia d(0.5) + T 14.3
Tomsk* Russia d(14) + Be 5.9
Minsk* Belorus d(14) + Be 5.9
Essen* Germany d(14.3) + Be 6.0
*Cyclotron

37. Fast Neutron Therapy Facilities
(E > 30 MeV)
Place Country Source
Orleans France p(34) + Be
Beijingº China p(35) + Be
Detroit, MI USA d(50) + Be
Seattle, WA USA p(50) + Be
Seoul+ South Korea p(50) + Be
Nice+ France p(60) + Be
Louvain‑la‑Neuve+ Belgium p(65) + Be
Batavia, IL USA p(66) + Be
Faure South Africa p(66) + Be
+Not operational at present
ºLinac (all other accelerators are cyclotrons)

38. Neutrons
Although the previous slides have indicated the usefulness of neutron beams for medical therapy, neutrons can also be a nuisance byproduct of medical therapy such as in high energy Linear Accelerator facilities treating patients with high energy photons.
The remaining slides summarize this issue.

39. Neutrons
Neutrons are produced by (gamma,n) production from high energy linear accelerators (E > 10MV)
Issues are neutron shielding and activation of items in the beam
The picture shows a physicist performing a neutron survey using a proportional counter with ‘Bonner spheres’ to moderate the neutron to thermal energies to be able to detect them.

40. Neutron Shielding Different concept from X-ray shielding
Neutrons scatter more
Attenuation (and scatter) depend VERY strongly on the neutron energy
Best shielding materials contain hydrogen or boron (with high cross section for thermal neutrons)

41. Features of Neutron Shielding
Long maze - many ‘bounces’
Neutron door - typically filled with borated paraffin
Care is required as neutrons generate gammas which may require other materials for shielding

42. Activation Neutrons can activate materials
High energy linacs are designed with materials with low activation cross section
After high energy photon irradiation, beam modifiers such as wedges or compensators may become activated
After prolonged use of high energy photons (e.g. for commissioning) it is advisable to let activation products decay prior to entering the room (>10min)
This is a different problem with neutrons which may affect staff working in a bunker used for irradiations with high (>10MV) energy photons.
It is important from a radiation safety perspective that the radiation generated here can be detected using common personal dosimeters, such as film or TLD badges.

43. More Information on Neutrons

44. Neutron Evaluation
if the facility has an accelerator with an energy >15 MeV, then radiation scans should include a neutron survey, especially near the entrance to the maze
the survey instrument used for neutrons should be a suitable type (see AAPM report 19)
The picture shows a Gas filled (BF3) proportional counter at the centre of a polyethylene block. Polyethylene slows down (thermalizes) the neutrons
The instrument is used to measure neutron levels at linac maze entrance

45. Where to Get More Information
Useful Websites
www-radonc.med.ohio-state.edu/clinbysite/ brain/bnct/radiumsoc/sld001.htm
sist.fnal.gov/archive/2001‑topics/Ramkissoon/Ramkissoon.htm

46. Where to Get More Information
Cember, H., Introduction to Health Physics, 3rd Edition, McGraw-Hill, New York (2000)
Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8th Edition, 1999 update), Wiley, New York (1999)
International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)