Prompt neutron multiplicity in correlation with fragments from spontaneous fission of ${}^{252}\mathrm{Cf}$

The spontaneous fission of ${}^{252}\mathrm{Cf}$ serves as an excellent benchmark of prompt emission in fission since experimental data can be obtained without the need of an incident beam. With the purpose of providing experimental data on the prompt fission neutron properties in correlation with fission-fragment characteristics, an experiment on ${}^{252}\mathrm{Cf}$(SF) has been performed. In addition, the experiment serves as a benchmark of setup and analysis procedures for measurements of fluctuations in the prompt-neutron properties as a function of incident neutron energy in fission of the major actinides ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$. The experiment employs a twin Frisch grid ionization chamber as fission-fragment detector while neutrons were counted by using a liquid scintillator placed along the symmetry axis of the ionization chamber. Average neutron multiplicity has been obtained as a function of fission-fragment mass and total kinetic energy (TKE). The average multiplicity as a function of mass agrees well with available data in the literature in the mass range from 80 to 170 u. The existence of additional sawtooth structures in the far asymmetric mass region could not be confirmed, although the statistical accuracy of the present experiment is as good as the previous study where such structures have been reported [Nucl. Phys. A 490, 307 (1988).]. The available data in the literature on the TKE dependence of the multiplicity show strong deviations. Therefore, effort was focused on investigating experimental factors in low-efficiency neutron-counting experiments that may lead to faulty determination of this dependence. Taking these factors into account, a result that agrees well with data from high-efficiency neutron-counting experiments is obtained. The experimental arrangement allows determination of the angle between the detected neutron and the fission axis, which permits the neutron properties to be transformed into the fission-fragment rest frame. Fission neutron emission spectra in the fragment center-of-mass frame have thereby been obtained as a function of the fission-fragment mass and TKE.

Figure 1

Schematic drawing of the experimental setup (left) with the NE-213-type neutron detector (LS-301) and the twin-Frisch grid ionization chamber, as well as the associated electronic setup (right). The abbreviations are CFD for constant fraction discriminator, and TFA for timing filter amplifier.

Figure 2

Cathode-current signal wave forms for a clean fission event (black line), an event suffering from an $\alpha$-particle pileup (blue line), and an event suffering from a fission pileup (red line), as identified by the pileup identification algorithm.

Figure 3

(a) Anode-pulse-height distribution before (black line) and after (red line) pileup rejection; the latter has been scaled to coincide with the former at the light fragment peak. (b) Time-of-flight spectrum before (black line) and after (red line) pileup rejection; the latter has been scaled to coincide with the former at the prompt-$\gamma$ peak.

Figure 4

(a) Distribution of fission fragments as a function of the detected $cos\theta$ value. (b) Distribution of the difference in $cos\theta$ values detected for fragments on the sample and backing side, together with a Gaussian fit.

Figure 5

(a) Comparison of the figure of merit of $n-\gamma$ separation as a function of the proton recoil energy calibrated pulse height for the two methods of pulse-shape discrimination. (b) Time-of-flight spectrum before and after PSD.

Figure 6

(a) Fission-fragment yield as a function of pre-neutron mass and TKE, (b) pre-neutron-mass distribution, (c) average TKE, and (d) standard deviation of the TKE as a function of the fission-fragment mass number from this work, compared with data from Ref. [15].

Figure 7

Vector diagram of the kinematics of neutron emission from a fully accelerated fragment and the transformation into the fragment rest frame (c.m.), black lines represent the fragment detected in the same hemisphere as the neutron, while red lines represent the complimentary fragment. The label for fragment velocity is ${\vec{v}}_{\mathrm{F}}$, for complimentary fragment velocity it is ${\vec{v}}_{\mathrm{F}}^{\prime }$, for neutron velocity in laboratory frame it is ${\vec{v}}_{\mathrm{L}}$, and for neutron velocity in the center-of-mass frame it is ${\vec{v}}_{\mathrm{c}.\mathrm{m}.}$.

Figure 8

Calculated contribution of neutron coincidences from fragments directed away from the neutron detector (closed points) compared with the recorded number of coincidences (open points) as a function of fragment mass number.

Figure 9

Prompt-fission-neutron angular distribution in the rest frame of the fission fragments. The red line represents the best fit of a second-order Legendre polynomial to the range $cos{\theta }_{\mathrm{c}.\mathrm{m}.}\in [0,0.85]$.

Figure 10

Average prompt-fission neutron multiplicity as a function of fission-fragment mass, in comparison with data from Refs. [11, 22, 23, 24, 25].

Figure 11

Total average prompt-fission neutron multiplicity as a function of TKE, in comparison with experimental data from Refs. [6, 11, 22, 26].

Figure 12

(a) Influence of neutron recoil correction on the average total prompt-fission-neutron multiplicity as a function of TKE in the present experiment. (b) Influence of electronic temperature drift on the average total prompt-fission-neutron multiplicity as a function of TKE.

Figure 13

Dependence of the average number of neutrons emitted per light and heavy fission fragment (closed black and open white circles, respectively) as well as the average total neutron multiplicity of the pair of fragments (red crosses) as a function of TKE.

Figure 14

Dependence of the average number of neutrons emitted per light and heavy fission fragment (closed black and open white circles, respectively) as well as the average total neutron multiplicity of the pair of fragments (red crosses) as a function of TKE.

Figure 15

(a) Slope of the neutron multiplicity per fragment with respect to the TKE as a function of fragment mass number. (b) TKE where the neutron emission stops as a function of fragment mass number. Data from Ref. [11] is included for comparison.

Figure 16

Integral prompt-fission neutron spectrum in the fragment center-of-mass reference frame.

Figure 17

Contour plot of prompt-fission neutron spectra in the fragment center-of-mass reference frame as a function of the fragment mass number. The spectra are normalized to yield the same integral for each mass number.

Figure 18

(a) The average neutron energy in the fission-fragment center-of-mass reference frame, (b) nuclear temperature, and (c) cascade neutron emission coefficient as a function of fragment mass number.