Fundamentals of Neutron Physics

M. Scott Dewey

National Institute of Standards and Technology

11/10/2011 Radiation Metrology Workshop, Buenos

Aires, Argentina

Acknowledgements for slides

Geoff Greene, Pieter Mumm, Dan Neuman,…

Short History Lesson

Ernest Rutherford

1920 Noting that atomic number (Z) does not correspond to

atomic weight, Rutherford suggests that, in addition to

“bare” protons, the nucleus contains some tightly bound

“proton- electron pairs” or neutrons.

1930 Bothe and Becker discovered a penetrating, neutral

radiation when alpha particles hit a Be target.

1931 Mme Curie shows that they are not gamma rays and they

have sufficient momentum to eject p’s from paraffin.

?

Irene Curie

Walter Bothe

nCBe 129

1932 Chadwick replaced the paraffin with a variety of other

targets and, by measuring the recoil energies of the

ejected particles, was able to determine the mass of the

neutral particle

M = 1.15 ± ~10%

Chadwick claimed this was Rutherford’s “Neutron”

J. Chadwick, Proc. Roy. Soc., A 136 692 (1932)

1933 Bainbridge makes precision measurements of the atomic masses

of the proton and the deuteron using the mass spectrograph

1934 Chadwick and Goldhaber make the first “precision”

measurement of the neutron mass by looking at the

photo-disassociation of the deuteron

Using 2.62MeV gammas from Thorium and determining the

recoil energy of the protons they were able to determine*:

1. The neutron cannot be a bound “proton-electron pair” 2. It is energetically possible for a neutron to decay to e-+ p+

*Chadwick and Goldhaber, Nature, 134 237 (1934)

h d p n

0005.00080.1nM

KEY OBSERVATION: Mn > Mp + Me

Sources of neutrons

• Neutron generators – D + D n + 3He yields 2.5 MeV neutrons

– D + T n + 4He yields 14.1 MeV neutrons

• Radioactive sources – 252Cf, half-life 2.6 years

– Am-Be, half-life 432.2 years

Spellation Neutron Source

NIST Center for Neutron Research

H and D scatter very differently

Neutrons are very sensitive to hydrogen!

H

D O

Si

C Appropriate wavelength and energy

=> geometry of key motions

Weak neutron – nucleus interaction

=> penetrating

=> easily modeled

Why Neutrons?

Magnetic moment

Source

Thermal neutron instrumentation

Liquid hydrogen cold neutron source

Cold neutron instrumentation

Neutron guides

NIST Develops Neutron Instrumentation & Makes it Available to the Scientific Community

www.ncnr.nist.gov

The NCNR Has 25 Operating Beam Instruments Tailored to Specific Needs …

www.ncnr.nist.gov

NG6 Neutron

Physics NG7 Prompt

NG7 Interferometer

BT2 Neutron

Imaging Facil.

Thermal

Column

Diffraction

Instruments

Spectrometers

Other Neutron Methods

NG3 30 m SANS

NG7 30 m SANS

NG1 Vert. Refl.

NG1 AND/R

NG7 Hor. Refl.

BT8 Resid.

Stress Diff.

BT1

Powd.

Diff. BT5 USANS

NG0 MACS

NG2 Backscattering Spec.

NG5 Spin-Echo Spec. NG-5 SPINS

NG4 Disk Chopper

TOF Spec.

BT7 3-Axis Spec.

BT9 3-Axis Spec.

BT4 FANS

NG2 Backscattering Spec.

NG5 Spin-Echo Spec. NG-5 SPINS

NG4 Disk Chopper

TOF Spec.

NG3 30 m SANS

BT5 USANS

NG1 Depth Pofiling

Emission rate of a neutron source

Concept of the Mn bath

Role of the manganese

Time (hours)

Where do the neutrons go?

NIST Manganese Baths

The neutron exhibits much of the richness of nuclear physics, but is vastly simpler, and thus more interpretable, than nuclei.

The neutron can be used to probe Strong, Weak, EM and Gravitational phenomena as well as serving as probe for new interactions.

Neutron decay is the archetype for all nuclear beta decay and is a key process in astrophysics.

The neutron is well suited as a laboratory for tests of physics beyond the Standard Model.

Why Study Neutrons?

The Neutron is complicated enough to be interesting…

But is simple enough to be understandable.

Some Neutron Properties Mechanical Properties

Mass

Gravitational Mass (equivalence principle test)

Spin

Electromagnetic Properties

Charge (or limit on neutrality)

Internal Charge Distribution

Magnetic Dipole Moment

Electric Dipole Moment

Neutron Decay

Neutron Mean Lifetime

Correlations in Neutron Decay

“Exotic” Decay modes

Miscellaneous Quantum Numbers:

Intrinsic Parity (P), Isospin (I), Baryon Number (B), Strangeness (S), …

Neutron Decay

1930 Pauli proposes the “neutrino” to explain apparent energy

and angular momentum non-conservation in beta decay

1934 Fermi takes the neutrino idea seriously and develops his

theory of beta decay

1935 The β decay of the neutron is predicted by Chadwick and

Goldhaber based on their observation that Mn>Mp+Me .

Based on their ΔM, the neutron lifetime is estimated at ~½

hr.

1948 Snell and Miller observe neutron decay at Oak Ridge

1951 Robson makes the first “measurement” of the neutron

lifetime

Wolfgang Pauli

Enrico Fermi

Modern View of Neutron Decay:

Fermi’s View of Neutron Decay:

Neutron Lifetime (The need for a new measurement)

900

890

880

870

Neu

tro

n L

ifet

ime

(s)

2005200019951990

Year

beam UCN bottle

PDG tau = (885.7 +/- 0.8) sNew result from Serebrov et al.

Neutron lifetime dominates the theoretical uncertainty of 4He abundance.

Thermal Equilibrium (T > 1 MeV)

After Freezeout n/p decreases due to neutron decay

Nucleosynthesis (T~0.1 MeV) Light elements are formed.

almost all neutrons

present are 4He

e-

e-

Big Bang Nucleosynthesis

Measuring the Neutron Lifetime

Step 1. Get One Neutron “Bottle”

Measuring the Neutron Lifetime

Step 1. Fill Neutron “Bottle”

neutrons

Measuring the Neutron Lifetime

Step 3. Let neutron decays for time t~τn

Time

Decay R

ate

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

n

p

e

ep

pp

pn

Measuring the Neutron Lifetime

Step 4. Pour neutrons out and count

0 n

t

N t N e

Mampe et al, PRL 63 (1989) Serebrov et al, Phys Lett B605 (2005)

Magnet form

Racetrack coil

Cupronickel tube

Acrylic lightguide

TPB-coated acrylic tube

Solenoid

Neutron shielding Collimator

Beam stop

Trapping region

Huffman, et al, Nature, 403 (2000)

Some Neutron Bottles

NIST Lifetime Apparatus