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Polyethylene‐Reflected Plutonium Metal Sphere: Subcritical Neutron and Gamma Measurements

 1. Name of Experiment:
    Polyethylene‐Reflected Plutonium Metal Sphere: Subcritical Neutron and Gamma Measurements (~1987)

 2. Purpose and Phenomena Tested:
    These benchmark data were collected by measuring a 4.5 kg 94%‐239Pu plutonium 
    metal spherereflected by spherical polyethylene shells. Six different 
    configurations of the plutonium sphere were measured: bare, and reflected 
    by polyethylene 1.27, 2.54, 3.81, 7.62, and 15.24 cm thick. The neutron 
    and photon emissions from this source were measured using three 
    instruments: a high‐efficiency highpurity germanium (HPGe) gamma spectrometer, 
    a gross neutron counter using moderated helium‐3,and a neutron multiplicity 
    counter using moderated helium‐3.  The gamma spectrometry data consist of 
    gamma spectra with 32,768 channels and a maximum photon energy of 11.8 MeV. 
    The gross neutron counting data consist of neutron count rates measured with 
    two different configurations of detector moderation. The neutron multiplicity 
    data consist of time‐stamped list mode detection events acquired with one 
    microsecond time resolution.

    These data provide observations of the total neutron production rate, the 
    neutron multiplicity distribution, and the gamma spectrum of plutonium metal 
    with neutron multiplication between approximately 4 and 17. 

 3. Description of the Source and Experimental Configuration:
    The plutonium source for this series of experiments was the BeRP Ball, a 
    4.5-kg sphere of α-phase, weapons-grade plutonium fabricated by Los Alamos 
    National Laboratory (LANL) in October 1980.1Figure 1 The sphere is clad in 
    stainless steel.
    The BeRP ball was cast and machined to a mean radius of 3.7938 cm. The 
    theoretical density of α-phase plutonium metal is 19.655 g/cm3. However, the 
    measured mass of the plutonium sphere was 4483.884 g, and the volume of the 
    sphere was 228.72 cm3 (based on the mean diameter). Consequently, the 
    calculated density of the plutonium sphere is 19.604 g/cm3. LANL documentation 
    describing the assembly of the BeRP ball indicates the plutonium sphere was 
    partially immersed in Freon to shrink it just prior to its final assembly in 
    the steel cladding. The dimensions of the plutonium sphere following this 
    Freon bath were not recorded by LANL. As a result, the actual density of the 
    plutonium sphere may be higher than its calculated density if the Freon bath 
    permanently reduced the volume of the sphere by changing the grain structure 
    of the metal.
    The BeRP ball was encased in a cladding of stainless steel 304 that is nominally 
    0.0305 cm thick. The nominal composition of the steel cladding is listed in
    Table 1. Its nominal density is 7.62 g/cm3. As shown in Figure 1, the cladding 
    was constructed from two hemishells, each with a nominal inside radius of 3.8278 
    cm and a nominal outside radius of 3.8583 cm. Each hemishell also had a 4.3764-cm 
    radius, 0.0457-cm thick flange. When the plutonium sphere was assembled in the 
    cladding, the two hemishells were electron-beam-welded together at this flange. 
    Because the outside of the radius of the plutonium sphere is smaller than the inside 
    radius of the cladding by 0.0340 cm, there is a gap between the plutonium sphere and 
    the cladding that is nominally 0.0680 cm at its widest point.

     The polyethylene reflectors were constructed as a series of five nesting spherical 
     shells. Each individual shell was constructed of two mating hemishells. Figure 3 is a 
     photograph of the plutonium source in the nested hemishells.
     The mating surfaces of the hemishells were stepped to eliminate any streaming path. 
     Each shell was supported by its own aluminum support stand. The stands were designed to 
     keep the center of the plutonium source 8.3 inches above the work surface

 4. Measurement System and Uncertainties:

    Gross neutron counting, neutron multiplicity counting, and gamma spectrometry 
    measurements were performed on the plutonium source in six configurations:
    • Bare
    • Reflected by 0.5 inch of HDPE
    • Reflected by 1.0 inch of HDPE
    • Reflected by 1.5 inches of HDPE
    • Reflected by 3.0 inches of HDPE
    • Reflected by 6.0 inches of HDPE
    Gross neutron counting, neutron multiplicity counting, and gamma spectrometry 
    measurements were collected for each of the preceding reflected configurations 
    of the plutonium source. 

    In addition to the calibration and benchmark measurements described in preceding 
    sections, several other auxiliary measurements were performed. These measurements 
    were conducted to characterize
    • the effect of the polyethylene reflectors’ temperature on the response of the 
      neutron multiplicity counter.
    • the effect of the neutron multiplicity counter’s large moderator on the response 
      of the gross neutron counter.
    • the differences between the reflectors constructed by SNL and reflectors 
      constructed at LANL.
    • the response of the instruments to the californium neutron calibration source in 
      the polyethylene reflectors.

    Gamma spectra were acquired using an Ortec DigiDart multichannel analyzer (MCA). The 
    MCA was controlled using custom software developed by LANL. All spectra were saved 
    in Ortec’s floating point “.chn” format. Gamma spectra were collected for 10-minute, 
    20-minute, and 60-minute dwell times. For a given reflected configuration of the 
    plutonium source, typically all gamma spectra were collected either on the same day or 
    on consecutive days.

    Statistical and systematic uncertainties are provided with the measurements.

 5. Description of Results and Analysis:

    Gross neutron counting measurements acquired with the LANL SNAP are listed in Table 22.
    Measurements were performed with the SNAP polyethylene cover both on and off. Table 23 
    lists the SNAP neutron count rate and uncertainty versus the thickness of the 
    polyethylene reflector. The measured count rates are plotted in Figure 47.

    Neutron multiplicity counting measurements acquired with the LANL NPOD are listed in 
    Table 24. The measured count rate and its uncertainty are given in Table 25 versus 
    the thickness of the polyethylene reflector, and the count rate is plotted in Figure 48. 
    Figure 49 shows the entire neutron multiplicity distribution versus coincidence gate width, 
    and Figure 50 shows the measured neutron multiplicity distribution for a coincidence gate 
    width of 1024 μs. Figure 51 and Figure 52 respectively show the measured Feynman-Y and     Rossi-α versus reflector thickness.

    Note that both the gross neutron counting measurements and the neutron multiplicity 
    measurements demonstrate competition 
    • Increasing neutron multiplication with increasing reflector thickness
    • Decreasing neutron leakage with increasing reflector thickness
    As a result, most of the neutron metrics are maximized for the 1.5-inch thick reflector, 
    where the product of multiplication and leakage probability are near their maximum.

    Gamma spectrometry measurements acquired with the HPGe detector are listed in Table 26. 
    Recall that measurements were performed both with and without the NPOD and SNAP present. 
    For each reflected configuration of the plutonium source, gamma spectra were collected for 
    10, 20, and 60 minutes.

 6. Special Features:


 7. Author/Organizer
    Experiment and analysis:
    John Mattingly
    Sandia National Laboratories
    1515 Eubank Boulevard Northeast
    Mail Stop 0782
    Albuquerque, New Mexico 87123 USA

 8. Availability:


 9. References:
    SAND2009-5804-R2.pdf John Mattingly "Polyethylene-Reflected Plutonium Metal Sphere: Subcritical Neutron and Gamma Measurements" 
    Sandia National Laboratories Albuquerque, New Mexico 87185 and Livermore, California 94550 SAND2009-5804 Revision 2 (December 2009)

10. Data and Format:
        Filename     Size[bytes]   Content
    ---------------- ----------- -------------

1 berp_poly_2009-a.htm 31,898 This information file
2 berp_poly_2009-e.htm 37,935 Description of experiment
3 berp_ball_construction.pdf 1,047,389 berp_ball_construction.pdf: Internal memorandum on assembly of Pu-239 ball
4 berp_ball_drawing.pdf 8,386 berp_ball_drawing.pdf: Schematic of berp-ball (high quality)
5 HPGe Measurements.xlsx 20,778 HPGe Measurements.xlsx: Excel Spreadsheet listing HPGe detector measurement file names
6 instrument_configuration.pdf 74,591 instrument_configuration.pdf: Measurement locations (high quality)
7 neutron_counter_stand.pdf 158,813 neutron_counter_stand.pdf: schematic of neutron detector stand (high quality)
8 NPOD Measurements.xlsx 21,695 NPOD Measurements.xlsx: Excel Spreadsheet listing NPOD detector measurements file names
9 room_layout.png 170,660 room_layout.png: Drawing detailing source and detector placement
10 SAND2009-5804-R2.pdf 7,871,598 SAND2009-5804-R2.pdf: Sandia report of Polyethylene-Reflected Plutonium Metal Sphere: Subcritical Neutron and Gamma Measurements
11 SNAP Measurements.xlsx 55,311 SNAP Measurements.xlsx: Excel Spreadsheet listing SNAP detector measurements
12 steel_cart.pdf 60,912 steel_cart.pdf: Schematic steel cart used in experiment (high quality)
13 Synopsis.pdf 78,664 Synopsis.pdf: Brief description of this experiment
14 ba133_certificate.pdf 1,027,580 ba133_certificate.pdf: activity certificate for Ba-133 calibration source
15 calibration_sources.pdf 21,291 calibration_sources.pdf: List of calibration source
16 calibration_sources.xlsx 1,244,548 calibration_sources.xlsx: list of calibration sources
17 cf252_certificate.pdf 1,244,548 cf252_certificate.pdf: certificate for Cf-252 calibration source
18 cf252_details.xlsx 15,522 cf252_details.xlsx: detailed calculations of Cf-252 neutron source strength
19 co60_certificate.pdf 1,005,626 co60_certificate.pdf: activity certificate for Co-60 calibration source
20 cs137_certificate.pdf 980,351 cs137_certificate.pdf: activity certificate for Cs-137 calibration source
21 nist_sources.pdf 115,340 nist_sources.pdf: document describing NIST source construction
22 u232_info.pdf 31,494 u232_info.pdf: information on NIST U-232 calibration source
File berp_poly_2009-e.htm contains the following tables: Table 1: Nominal composition of the steel cladding Table 2: Material specifications Table 3: Isotopic composition of the plutonium Table 4: Estimated composition of the plutonium metal on 6 January 2009 Table 22: Gross neutron counting benchmark measurements Table 23: Gross neutron counter count rate versus polyethylene reflector Table 24: Neutron multiplicity counter benchmark measurements Table 25: Neutron multiplicity counter count rate versus polyethylene Table 26: High-resolution gamma spectrometry benchmark measurements Table 27: Neutron multiplicity counting measurements taken versus the surface temperature Table 28: Gross neutron counter response with neutron multiplicity counter present and absent Table 29: Gross neutron counter response for LANL and SNL reflectors Table 30: Neutron multiplicity measurements comparing LANL and SNL reflectors Table 31: Gamma spectrometry measurements comparing LANL and SNL reflectors Table 32: Gross neutron counter count rate versus polyethylene reflector thickness Table 33: Neutron multiplicity measurements with the californium calibration source Table 34: Gamma spectrometry measurements where the californium calibration source SINBAD Benchmark Generation Date: 05/2010 SINBAD Benchmark Last Update: 12/2012