an ultra cold neutron source at the ncstate pulstar reactor l.
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An Ultra-Cold Neutron Source at the NCState Pulstar Reactor. R. Young NCState University. The Collaboration. Physics Department: C. Gould, A. R. Young Nuclear Engineering Department: B. W. Wehring, A. Hawari Hahn-Meitner Institute (plan: NCState in Jan, 2004) R. Golub, E. Korobkina.

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The Collaboration

  • Physics Department:
    • C. Gould, A. R. Young
  • Nuclear Engineering Department:
    • B. W. Wehring, A. Hawari
  • Hahn-Meitner Institute (plan: NCState in Jan, 2004)
    • R. Golub, E. Korobkina

Local research groups with overlapping interests:

  • new NCState physics faculty in fundamental neutron physics (offer being made now…)
  • H. Gao & D. Dutta (in the EDM collaboration)
  • H. Karwowski and T. Clegg (weak interactions res.)

All of the collaboration members have experience with neutron-related physics research and/or UCN production

  • R. Golub: co-invented superthermal source technique
  • B. Wehring: constructed a CN source at the Nuclear Engineering Teaching Laboratory TRIGA Mark II reactor at University of Texas, Austin
  • A. Hawari: active research program in neutron moderator modeling

PULSTAR facility is ideal for exploring new ideas for UCN production and experimentation


The PULSTAR UCN Source Project

  • Establish a university-based UCN facility with a strong focus on nuclear physics applications for UCN
  • Integrate the UCN facility into the undergraduate curriculum
  • Involve the local nuclear physics groups (NCState, UNC and Duke, through TUNL) in fundamental physics with cold and ultracold neutrons.


  • Sintered UO2 pellets
  • 4% enriched
  • 1-MW power
  • Light water moderated and cooled
  • Just issued a new license for about 10 years of operation.
  • PULSTAR design has several advantages for a UCN source:
    • high fast flux leakage
    • long core lifetime

28 ft

Source located in thermal column



Conceptual Design I

(top view)

Takes advantage of:

  • large fast flux leakage – channel fast and thermal neutrons into D2O tank
  • very low heating – use solid methane moderator

Details of UCN Source

  • UCN Converter
    • Solid ortho D2
    • 4-cm thick
    • 18-cm diameter
  • CN Source
    • Solid methane
    • 1-cm thick cup around SD2

Parametric design calculations

  • CN fluxes in the UCN converter and heating rates by MCNP simulations
  • UCN production rates by integrating the converter CN energy spectrum with the UCN production cross sections—physics based on LANSCE measurements.
  • UCN intensity at end of an open UCN guide using UCN-transport calculations.

CN Flux (MCNP)

  • Averaged over UCN converter
  • Integrated, 0 to 10 meV CN energies

φ = 0.9 x 1012 CN/cm2-s


Neutron and Gamma Heating Rates



  • UCN converter, 200 g 1.7 W
  • UCN converter chamber, 696 g 3.1 W
  • CN source, 558 g 5.6 W
  • CN source chamber, 1529 g 6.0 W

UCN Production Rate and Limiting Density

Io = 2.7 x 107 UCN/s

For SD2 = 43 ms,  = 1,160 UCN/cm3

Lifetime assumes SD2 at 5K, 1.5% para-deuterium, no H2


Partially Optimized Design

(side view)

  • CN flux averaged over UCN converter
    • 4-cm thick x 18-cm diameter

φ = 1.0 x 1012 CN/cm2-s

  • UCN intensity at end of open Ni-58 guide
    • 50-cm rise, 2-m level

Io = 1.0 x 107 UCN/s

  • UCN limiting density

 = 1,290 cm3


SD2 Source Summary

  • For 1MW reactor operating power:

Io = 3.0  107 UCN/s

 = 1,300 UCN/cm3

  • Very small heat loads (1.7 W total to UCN converter)
    • cryostat designs straightforward (D. G. Haase)
    • lower operating temperatures feasible
  • Accessibility of source is excellent, available year-round, reactor operable by students
  • Upgrade of reactor power to 2MW being planned

Rough Comparison with Other Sources

Facility UCN (1000/cm3) Comments


A Nuclear Physics Science Program

  • Observed baryon-antibaryon asymmetry  physics beyond the standard model
  • How do we explore these issues at a university-based facility?
  • Measure T invariance in neutron decay (D coefficient)
  • Contribute to the UCN EDM project
  • Perform source development work as a part of implementing a UCN neutron-antineutron oscillations experiment (NNbar)

T non-invariance

Baryon number violating interactions


Measurement of T-noninvariance in -decay


Envisioned facility

Experiments go here

(He liquifier not shown)

UCN guide

UCN source

Neutron decay directional angular correlations:




The term proportional to D violates T symmetry: need to observe decay ’s and protons in coincidence use a cell geometry with UCN

a potential d measurement with ucns
A Potential D Measurement with UCNs.

From complete PENELOPE MC:


1109 decays

25 UCN/cc -10 days

Much higher densities ultimately available…up to ~ 1000 with this source

  • Why this experiment is suitable for a small, university facility:
  • Relatively compact (about 3m long)
  • Detectors are inexpensive and relatively straightforward to implement
  • Does not require a large superconducting spectrometer magnet
  • Does not require high precision polarimetry

Possible Contributions to the UCN EDM Project (M. Cooper and S. K. Lamoreaux, PIs)

Local members of the EDM collaboration:

H. Gau, D. Dutta, R. Golub, E. Korobkina

  • Possible measurement programs using the NCState source as a test facility:
      • UCN storage
      • UCN depolarization
      • UCN production of scintillation light
      • Dressed UCN interaction with polarized 3He

NNbar and source development

NNbar workshop at the IUCF/LENS facility, Sept. 2002:

Evaluated idealized geometry & conclusion:

Need more UCN 

Source R&D

(At NCState: 4 years of running produce factor of 7 improvement over ILL results (PSI or US national facility somewhat more effective)


Source Development Projects: Solid Oxygen and Liquid He

Solid oxygen (part of thesis of Chen-Yu Liu):


Freeze out magnons at 2K

UCN lifetime 9 x SD2

Optimal production w/CN at 8-10K ~ 1.8 RSD2

Limiting UCN density SO2~ 16SD2

If UCN elastic scattering length is long in SO2, more gains possible!


Liquid He: R. Golub and E. Korobkina

NCState CN flux well-suited to UCN production in liquid He

Korobkina et al. calculate contribution from single and multiphonon prod for various CN distributions

Large gains possible (need to do pilot experiments)


Source Development in a University


  • A Systematic investigation of source parameters is required to optimize UCN production rates and densities
    • CN moderators  optimize temperature and total flux of CN
    • UCN converters  explore physics of production, lifetimes, cooling, engineering issues

University facilities such as NCState PULSTAR and LENS:

  • Easy access (by students, staff, etc…) excellent for exploring performance of various source geometries
  • Low heating rate makes possible the investigation of more “fragile” moderators and converters
  • Low heating rate also permits straightforward cryostat design

Educational Program
























  • Undergraduate students: already mechanism for integrating research projects at the reactor into the curriculum:
    • Every undergraduate in the NE program must do project at the reactor
    • Physics department’s advanced physics lab (PY 452) involves students doing projects in research labs; only requirement is “measure something with an error bar” (two in my lab this semester)

Nuclear Engineering Enrollment at NCSU


Graduate students: local facilities are a powerful draw for students.

    • Fundamental neutron physics is being established as one of the primary activities at TUNL, providing exposure to a large pool of nuclear physics graduate students
    • Training in neutron science and engineering is being established as a priority in the NE department (a director of reactor research is being created to expand the neutron research capabilities of the PULSTAR facility)
  • Faculty: NCState is committed to expanding its role at the SNS, and both the NE and physics departments are seeking to make joint hires in neutron/nuclear physics related research—this is explicitly stated in the “compact plan” for each of these departments, in which departmental funding priorities are established.

Facilities and Operations Costs

Reactor operations: funded by State of North Carolina

director: A. Hawari

budget: $490,000/y

staff: 7 technical staff, 1 secretary

adequate for daily operations: 1 shift of 8 hr/day

Rennovation costs requested in compact plan


Source Equipment Costs & Operating Grant Costs

  • $1,035,905 over 3 years
    • $392,315 for cryostat & related equipment (year 1)
    • $408,700 for Model 1410 He liquifier (year 2)
    • $234,890 for polarizer/spin-flipper magnet (year 3)
  • increase to operating costs for nuclear physics group ~$80,000/year (materials and supplies, LHe and at least one more student)


  • There is now the nucleus of a strong fundamental neutron physics group at NCState, with more faculty and staff to be joining
  • Two unique local resources: the PULSTAR reactor and TUNL
  • Timing is perfect to begin building a strong user group and training students for the SNS and future experiments
  • We should build this source