Tagging fast neutrons from a 252Cf fission-fragment source

Coincidence and time-of-flight measurement techniques are employed to tag fission neutrons emitted from a 252Cf source sealed on one side with a very thin layer of Au. The source is positioned within a gaseous 4He scintillator detector. Together with alpha particles, both light and heavy fission fragments pass through the thin layer of Au and are detected. The fragments enable the corresponding fission neutrons, which are detected in a NE-213 liquid-scintillator detector, to be tagged. The resulting continuous polychromatic beam of tagged neutrons has an energy dependence that agrees qualitatively with expectations. We anticipate that this technique will provide a cost-effective means for the characterization of neutron-detector efficiency in the energy range 1 - 6 MeV.


Introduction
We recently reported on our efforts to "tag" fast neutrons from an 241 Am/ 9 Be source [1] as the first step towards the development of a source-based fastneutron irradiation facility. Here, we report on our investigation of a 252 Cf fission-fragment fast-neutron tagging technique very similar to that reported on by Reiter et al. [2]. In contrast to Reiter et al. who employed a thin layer of plastic scintillator to detect the fragments, we use a gaseous 4 He-based scintillator detector. The corresponding fission neutrons are detected in a NE-213 [3] liquid-scintillator detector. This effort represents our first step towards the development of an apparatus for the measurement of absolute neutron-detection efficiency at our facility. 252 Cf is an intense source of fast neutrons. With an overall half life of 2.645 years and a specific activity of 0.536 mCi/µg, it decays by both α-particle emission (96.908%) and spontaneous fission (3.092%) [4]. The weighted average α-particle energy is ∼6111.69 keV. The prompt-neutron yield is ∼3.75 neutrons per fission event [5,6]. The resulting fast-neutron energy spectrum follows the Watt distribution [7] and is very well known, with a most-probable energy of 0.7 MeV and an average energy of 2.1 MeV. Our californium source [8] has an active diameter of 5 mm and is mounted a capsule that has a thick platinum-clad nickel backside and a thin 50 µg/cm 2 sputtered-gold front side which allows both α particles and fission fragments to escape. The (nominal) activity is 3.7 MBq [9]. While trace activity comes from 249 Cf (<0.2%) and 251 Cf (<0.04%), the majority comes from 250 Cf (∼7.5%) and 252 Cf (∼92.3%).

Californium fission-fragment source
We estimate a neutron emission rate of ∼4 × 10 5 neutrons per second.

Gaseous 4 He fission-fragment detector
The noble gas 4 He is a good scintillator with an ultra-violet light yield of about the same magnitude as intrinsic (non Tl-doped) NaI crystals [10][11][12][13]. In this measurement, we employed a gas cell built originally as a prototype active target for recent 4 He photoreaction measurements [14]. The cell was machined from a solid aluminum block and has a cylindrical interior volume measuring 72 mm long × 58 mm , for an inner volume of ∼0.35 liters (see Fig. 1).  with amplitudes of about −2200 mV correspond to light fission fragments. We note that the average α-particle energy is ∼6.1 MeV, while the average heavy fission-fragment energy is 80 MeV and the average light fission-fragment energy is 104 MeV [25]. See also the histogram presented in Fig. 6.

NE-213 fast-neutron and gamma-ray liquid-scintillator detector
NE-213 is an organic liquid scintillator that has been employed for decades as a fast-neutron detector. The NE-213 liquid-scintillator detector used here has been reported upon earlier [1,18,19]. It consisted of a 62 mm long × 94 mm cylindrical aluminum "cup" fitted with a borosilicate glass optical window [20].
The filled cell was dry-fitted against a cylindrical PMMA UVT lightguide [21] and coupled to a µ-metal shielded 7.62 cm ET Enterprises 9821KB PMT and base [22]. Operating voltage was set at about −1900 V, and the energy calibration was determined using standard gamma-ray sources together with a slightly modified version of the method of Knox and Miller [23] as described in Ref. [19].
The detector threshold was set at 150 keV electron equivalent (keV ee ), corresponding to a neutron depositing an energy of about 1 MeV. SIS 1100/3100 PCI-VME bus adapter was used to connect the VMEbus to a LINUX PC-based DAQ system. The signals were recorded and processed using ROOT-based software [24]. This could result from non-uniform scintillation-light collection, different energy losses of different particle types in the source as well as the thin Au source window, non-linearity of the scintillation, or even fission fragments striking the source holder. This will be examined in more detail in a future publication.

Results
Recall that the average α-particle energy is ∼6.1 MeV, while the average heavy fission-fragment energy is 80 MeV and the average light fission-fragment energy is 104 MeV. If we calibrate our QDC based upon the average energy deposition of the two types of fission fragments and then apply this calibration to the α-particle distribution, we reconstruct the α peak at ∼12 MeV. 4 He is often assumed to be a linear scintillator, while this preliminary analysis suggests an apparent non-linearity. However, as outlined above, there are several factors which will affect the apparent scintillation-pulse height. The degree of nonlinearity in the scintillation (if any) requires an in-depth study. Note that for the data presented subsquently in this paper, a software fission-fragment cut located at channel 520 was employed. Figure 7 shows a fission-neutron TOF spectrum obtained using the signal in the 4 He scintillator detector to start a TDC and a signal from the NE-213 detector to stop it. Note that the spectrum shown corresponds to events lying above the software fission-fragment cut at channel 520 shown in Fig. 6. After this cut, interpretation of the resulting TOF spectrum is straightforward.
The sharp peak to the left of the spectrum centered at about 5 ns and labeled "gamma-flash" corresponds to the detection of a fission fragment in the 4 He scintillator detector and a correlated fission-event gamma-ray in the NE-213 detector. The ∼1.8 ns FWHM of the gamma-flash distribution is consistent with the observed timing jitter on our PMT signals and the slight tail in the distribution is possibly due to time walk in the electronics. Note that 4 He scintillator is highly insensitive to gamma-rays [14,18] and any electrons produced via Compton scattering or pair production will result in only a very small scintillation signal. These events will be entirely suppressed by the relatively high software  Also shown are three representations of the neutron-kinetic energy distribution constructed using the information presented by Thomas in Ref. [26]. We note that there are small differences between the representations, so a normalization factor has been applied to each so that they coincide with our data at ±1% level of agreement between all three representations of the 252 Cf fissionneutron energy spectrum over the energy region from 1 to 6 MeV is well within any systematic uncertainty that we are likely to obtain in measurements of neutron-detection efficiency using the tagging technique presented here. Thus, they provide an excellent benchmark from which it will be possible to evaluate the neutron-detection efficiency.

Summary
As a first step towards the development of an apparatus for the measurement of neutron-detection efficiency at our source-based fast-neutron irradiation facility, we have employed coincidence and time-of-flight measurement techniques to "tag" neutrons emitted by a 252 Cf source. The spontaneous-fission fragments are detected in a gaseous 4 He scintillator detector. The neutrons are detected in a NE-213 liquid-scintillator detector. The resulting continuous polychromatic beam of tagged neutrons has a measured energy dependence that agrees qualitatively with expectations. This preliminary study strongly suggests that the method of neutron-energy tagging will work well and future investigations will concentrate on quantifying systematic effects in order to optimize the perfor- 13 mance. We anticipate that the technique will provide a cost effective means for the characterization of neutron-detector efficiency, and note that this technique will work equally well for all spontaneous-fission neutron sources.