Lecture 1 Neutron Fundamentals Microscopic Interactions

22.106 Neutron Interactions and Applications Spring 2010

Objectives

• B asic Neutron Properties

• Concept of Microscopic x.s.

• B rief review of Neutron Interactions

• E nergy dependence of neutrons

– N eutron – nucleus interactions

– N eutron sources

Neutron

• D iscovered by James Chadwick in 1932

– N o electric charge

– M ass of 1.008664915 amu (slightly more than a proton)

– S pin of ½

• Its existence explained the atomic model and the cohesion of the nucleus. The strong nuclear forces are able to keep the protons from repulsion.

• Bound neutrons (i.e. in the nucleus) are stable

• F ree neutrons are unstable

– T hey are produced by:

• F ission

• F u s i o n

• Photoneutrons

• Alpha sources (PuBe, AmBe, AmLi)

– D ecay s by beta emission

– H alf life of 885.7s (roughly 15 min)

• Bounded nucleus may also use beta decay if unstable

– T his is the case for the fissi on products who are neutron rich.

Successive beta d e cay b rings them closer to stability.

Stability of fission products

Image removed due to copyright restrictions.

• N eutron rich fission products beta decay towards stability

• N eutron interacts with materials for different purposes or consequences:

– D etection

• Based on the interaction of neutrons with other materials

– S hielding

• N eutron absorption or reflection prevents neutrons from escaping

– I nducing fission

• Active detection systems

• I onization chambers

– S tructural displacement

• Material damage by creating defects in lattice

– M aterial assay

• N eutron Diffraction

• S mall Angle Neutron Scattering

Microscopic x.s.

• Probability of a particular event occurring between a neutron and a nucleus is expressed through the concept of a cross-section.

• C onsider a beam of mono-energetic neutrons of intensity I (neutrons/cm 2 .s) incident on a very thin sample material such that there are N A atom per cm 2

I n e u t r o n s / c m 2 . s e c

N A n u c l e i / c m 2

x

Image by MIT OpenCourseWare.

Microscopic Cross section

R   I N A

 # 

 cm 2  

#   # 

  cm 2 s  

    cm 2 s  

  cm 2  

I n e u t r o n s / c m 2 . s e c

N A n u c l e i / c m 2

x

Image by MIT OpenCourseWare.

• C ross-sections are strongly dependent of neutron incoming energy. This relation is important in the understanding of x.s. but also to estimate the time a neutron spends in a particular region.

– Classical expression for K.E. is sufficiently accurate because rest mass of a neutron is fairly large (939.55 MeV)

E = ½ m v 2

E = 5.227 x 10 -15 v 2 or v = 1.383 x 10 7 E 1/2

Examples

• 1 MeV neutron has a speed of

– 1 .383 x 10 7 m/s

– W ill cross a 15 cm sample in 11 nanoseconds

• 0.025 eV neutron has a speed of

– 2187 m/s

– W ill cross a 15 cm sample in 70 microseconds

• C onclusion: When studying neutron distributions, the energy levels involved and the size of the systems usually make it possible to neglect the beta decay of the free neutron.

Types of Interactions

• T wo major types

– S cattering: Speed and Direction change but nucleus is left with same number of neutrons and protons

– A bsorption: Original neutron disappears within the nucleus

Elastic Scattering (n,n)

• O ne of the most difficult neutron interactions to measure and the most complicated to analyze theoretically.

– It is also one of the most important reactions in reactors, characterization devices, …

• E lastic Scattering requires to consider the double differential cross section which describes the angular distribution and the energy distribution as well as the total elastic cross- section.

Inelastic Scattering (n,n’)

• P lays a key role in neutron slowing down a high energies

– R equires a threshold energy to excite the nucleus to a quantum state

– T he neutron can thus lose a considerable amount of energy in one collision since most of it is used to bring the nucleus to this quantum state

Radiative Capture (n,  )

1 n 

A X  

A  1 X  * 

A  1 X  

• N eutron combines with target of mass A to produce nucleus of mass A+1 in a quantum state. The excitation of this state is equal to the sum of the neutron binding energy and the kinetic of the neutron.

• E nergy is released in one or more gamma rays

• T his reaction is very important in the production of radioactive medical isotopes.

Cobalt- 60

• P roduction of radioactive medical isotopes

– S terilization

• M edical Equipment

• F ood

• B lood

– C hemical tracer

27 Co

60

0.31 MeV 

5.26 a

1.17 MeV 

1.33 MeV 

28 Ni

60

Image by MIT OpenCourseWare.

Technitium- 99m

• T echnetium-99m is used in 20 million diagnostic nuclear medical procedures every year.

• It is a gamma ray emitting isotope used as a radioactive tracer that medical equipment can detect in the body. It is well suited to the role because it emits readily detectable 140 keV gamma rays (about the same wavelength emitted by conventional X-ray diagnostic equipment), and its half-life for gamma emission is 6.01 hours. The short half life of the isotope allows for scanning procedures which collect data rapidly, but keep total patient radiation exposure low.

• M olybdenum-99 is produced mainly by fission of HEU

– D ecay s to Tc-99m (half-life of 66hrs)

– T c-99m is chemic ally extracted from Mo-99 fairly eas ily

(n,2n), (n,3n), …

• T wo step process

– Incident neutron is inelastically scattered by a target nucleus (threshold reaction)

– I f residual energy is left with excitation energy above binding energy of last neutron, the neutron is free to escape

• (n,2n) threshold usually is 7-10 MeV

– 1 .8 MeV for Be

• (n,3n) threshold usually is 11-30 MeV

(n,2n), (n,3n) for U-238

Image by MIT OpenCourseWare.

Charged particles (n,p), (n,  )

• U sually requires lots of energy to happen with one notable exception

– 10 B (n,  ) 7 Li: very high x.s. at low energy which is why

10 B is used as a thermal absorber

• V ery small x.s. in heavy nucleis, because emitted charge particle must overcome Coulomb barrier in order to escape nucleus

– D elay is so long that compound nucleus (CN) will scatter (elastic or inelastic) instead.

Fission Reaction (n,f)

1 n  A X  A 1 X  A 2 X

 neutrons  200 Me V

0 Z Z 1 Z 2

• Activation energy required to induce fission. Energy not available in ground state of 236 U but it is supplied in the form of the neutron binding energy in ( 236 U)*

• F ission usually provides two fission products (FP)

– Sc ission in 3 or more FPs is possible but very rare

• 1/400 scission in 3 FPs

• 1/3000 scission in 4 FPs

• R eleases roughly 200MeV of energy and ν neutrons (typically 2-3).

Energy Dependence of x.s.

• G eneral Rule: x.s. decreases with increasing energy of incoming neutron

• A t low energies (<1 MeV), (n,n) is nearly constant whereas (n,  ) is proportional to “1/v”

• In the keV range, resonances are superimposed on the “1/v” trend, these resonances are formed when neutron energy + binding energy correspond to an excitation level of the nucleus.

• Heavy Nucleus

– eV range: resonances appear

– k eV range: resonances are too close to be distinguished

– M eV range: resonances are broader and small, x.s. becomes smooth and rolling.

• Light Nucleus

– R esonances appear only in MeV range. They are broad and small

– E xceptions

• 1 H and 2 H have no resonances at all (no excitation level in the nucleus)

• N uclides with “magic number” o f protons and neutrons may behave more like light nuclei

C-12 total x.s.

Image by MIT OpenCourseWare.

• N eutrons are produced with varying energy depending on the source.

– F ission produces 2-3 neutrons/fission

– L arge fraction is prompt (10 -13 seconds)

– Small fraction is delayed (decay of FPs)

Image by MIT OpenCourseWare.

 ( E )

 0.453 e  1.036 E

s inh

• S pontaneous Fission from Cf252

10 2

10 1

10 0

1

2

3

4

5

Neutron ener gy (MeV)

Measured neutron ener gy spectrum from the spontaneous fission of 252 Cf.

Image by MIT OpenCourseWare.

• A lpha Neutron Source

8

6

Stilbene

4

Emulsions

2

0

2

4

6

8

10

Neutron ener gy (MeV)

Measured ener gy spectra for neutrons from a 239 Pu/Be source containing 80g of the isotope.

Image by MIT OpenCourseWare.

• S pallation N eutron Source

– High energy proton bombard a heavy target and creates spallation particles

1) Negative hydrogen ions (1 proton and 2 electrons) are generated in pulses

2) Accelerated to 1GeV (almost 90% of speed of light) by a linear accelerator

3) Electrons are stripped and protons are concentrated in a 2MW beam

4) Directed at liquid Mercury

• Liquid at room temperature

• A bsorbs rapidly changes in temperature and can survive intens e bombardment shock

• E jects 20-30 neutrons per collis ion (avg energy of 20-25 MeV)

M IT OpenCourseWare http://ocw.mit.edu

22.106 Neutron Interactions and Applications

Spring 2010

For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .