Measurement of prompt fission neutron spectrum for spontaneous fission of ${}^{252}\mathrm{Cf}$ using $\gamma$ multiplicity tagging

The prompt fission neutron spectrum from spontaneous fission of ${}^{252}\mathrm{Cf}$ is an integral part of several aspects of nuclear data. Not only is the spectrum itself of interest, but neutron detectors often use the spectrum for calibration, and other prompt fission neutron spectra are measured as a ratio to ${}^{252}\mathrm{Cf}$. Therefore, reducing the uncertainties in this spectrum will allow for more accurate nuclear data to be available across a wide range of fields. The prompt fission neutron spectrum for the spontaneous fission of ${}^{252}\mathrm{Cf}$ was measured at Rensselaer Polytechnic Institute using the multiple $\gamma$ tagging method with a 18.4-ng fission sample. An EJ-301 liquid scintillator fast neutron detector was used to measure the high energy portion of the spectrum, 0.5–7 MeV, and a thin EJ-204 plastic scintillator was used to measure the low energy portion of the spectrum, from 50 keV to 2 MeV. These spectra both show good agreement with the current evaluation of ${}^{252}\mathrm{Cf}$ and have low associated uncertainties providing a new high precision measurement that helps reduce the uncertainties in the prompt fission neutron spectrum for the spontaneous fission of ${}^{252}\mathrm{Cf}$.

Figure 1

A comparison of several measurements and uncertainties for the spontaneous fission of ${}^{252}\mathrm{Cf}$ as well as the current ENDF/B-VII.1 [4] evaluation. The data sets are shown as a ratio to a Maxwellian distribution with the kT parameter of 1.42 MeV. This highlights the lack of experimental data below 0.5 MeV and above 7 MeV as well as the high uncertainties in the data sets.

Figure 2

The raw and smoothed flux measured with the ${}^{235}\mathrm{U}$ fission chamber, and the flux found from direct measurement of the EJ-301 detector in beam.

Figure 3

The experimental setup utilized for the measurement highlighting the location of the ${}^{252}\mathrm{Cf}$ fission chamber, the ${\mathrm{BaF}}_2\gamma$ detectors, and the EJ-204 and EJ-301 neutron detectors. The $\gamma$ detectors are located 10 cm away from the sample, the EJ-301 detectors are located 50 cm away from the sample, and the EJ-204 detector is located 48 cm away from the sample.

Figure 4

The constant time-independent room background as seen as the average count rate from 500 to 600 ns after the fission event. The background continues to remain constant at longer time of flights.

Figure 5

A comparison of the measured $\gamma$ signal from an EJ-301 detector using PSA to the simulated signal from mcnp-polimi. The spectra are normalized at the carbon scattering peak at 40 ns. This shows an increase in the simulated counts at longer ToFs.

Figure 6

The background components found using mcnp-polimi simulations for the EJ-204 plastic scintillation detector. The neutron background is found from simulating the neutron scattering off the surrounding materials and the $\gamma$ background is from the decay of isomeric states. The 50-keV line is shown to signify where the experimental data ends for the PFNS measurement.

Figure 7

The effects of neutron scattering in an mcnp-polimi simulation from several background materials. This shows that the floor, experiment table, and central flight tube need to be accurately modeled to predict the time-dependent neutron background for the experiment. The 0.5-MeV line shows the lower limit for the PFNS for the EJ-301 liquid scintillator.

Figure 8

A diagram (not to scale) showing a side view of the geometry used to model the mcnp-polimi simulations. The three primary sources of neutron scattering background: The experiment table, floor, and central flight tube are shown.

Figure 9

A comparison of the experimental ToF PFNS for the EJ-301 liquid scintillator compared to the mcnp-polimi obtained neutron background. Also shown is the time-independent room background. This shows good agreement between the simulation and the experiment, from 100 to 300 ns, a region past the PFNS where only scattered background neutrons are present.

Figure 10

The high energy portion of the ${}^{252}\mathrm{Cf}$ spontaneous fission PFNS taken with an EJ-301 liquid scintillator in the region from 0.5 to 7 MeV. Also shown are the ENDF/B VII.1 evaluation and several prominent data sets showing good agreement between the RPI data and the current evaluations and data sets. The data sets were normalized to the ENDF evaluation at 1 MeV except for the Maerten data which was normalized at 5 MeV.

Figure 11

The low energy portion of the ${}^{252}\mathrm{Cf}$ spontaneous fission PFNS taken with an EJ-204 plastic scintillator in the region from 50 keV to 2 MeV. Also shown are the ENDF/B VII.1 evaluation and several prominent data sets showing good agreement between the RPI data and the current evaluations and data sets. The data sets were normalized to the ENDF evaluation at 1.2 MeV.