A multimodal neutron-based technique for the elemental analysis of materials in bulk

Neutrons and gamma rays are effective probes for the elemental characterisation of bulk samples. Neutrons interactions with matter are characteristic of the nature of the target nucleus, and the incident neutron energy. Most nuclides exhibit distinctive structure in their total and differential cross sections, thus elements can be differentiated via their total, elastic and inelastic scattering cross sections, and the energies of prompt gamma rays produced in inelastic collisions. In this work we report on the use of the fast neutron transmission analysis technique for the analysis of 12C in graphite. We also present results of prompt gamma ray neutron activation analysis of graphite using 14 MeV neutrons, which was explored for the purpose of developing a multimodal neutron-based technique for elemental analysis of materials in bulk.


Introduction
Developing methods to non-destructively determine the elemental composition of bulk materials is important in a broad range of contexts, including food and agriculture, coal and minerals processing, contraband detection, and nuclear regulation [1].Neutron-based techniques are advantageous as neutrons are highly penetrating, sensitive to low mass nuclei and produce characteristic secondary radiation for each nuclide.When a sample of unknown composition is exposed to a field of neutrons, with known intensity, energy and angular distribution, an array of radiation signatures which are characteristic of the sample composition is produced.These signatures may be in the form of prompt and delayed gamma rays, and scattered and transmitted neutrons [2].Well established techniques exist to utilize each of these signatures in isolation as a means of materials analysis, and each technique will be more, or less sensitive to a different subset of elements [2,3].
In this work we present the results of proof-of-principle measurements that exploit transmitted neutrons and prompt (de-excitation) gamma (γ) rays for the analysis of 12 C in graphite.The measurements were taken at the n-lab [4] with a broad aim of developing a multimodal neutron-based technique for elemental analysis of materials in bulk.

Measurements at the n-lab
The graphite samples were analysed using fast neutron transmission analysis (FNTA) and prompt gamma ray neutron activation analysis (PGNAA).The FNTA technique exploits transmitted neutrons as a signature, while in PGNAA de-excitation γ rays are used.The measurements were taken using a D-T MP320 (Thermo Fisher) sealed tube neutron generator (STNG) and a 220 GBq 241 Am- 9 Be radioisotopic source (AmBe) [4,5].During FNTA, a Ø 5 cm x 10 cm graphite sample was irradiated with a Ø 0.8 cm pencil beam of AmBe neutrons.The transmitted neutron fluence was measured using a well-characterized Ø 2" x 2" organic liquid scintillator (EJ-301) capable of pulse shape discrimination (PSD) and operated at a negative bias of 854 V.The fast and slow signals from the anode and dynode, respectively, were acquired with a CAEN DT5370 digitizer coupled to the QtDAQ software [6].To select only events that were induced by neutrons, PSD was implemented through QtDAQ using the anode signal [7].Energy information was obtained from the pulse height of the dynode signal after shaping and amplification with an Ortec 113 pre-amplifier, and then with a 527A amplifier.The PGNAA measurements were taken using the STNG.The de-excitation γ rays from the inelastic interaction between 14 MeV neutrons and 12 C nuclei ( 12 C(n, n′) 12 C * ) were detected with two Ø 3" x 3" NaI(Tl) detectors, each positioned next to the sample and oriented 90 degrees with respect to the beam axis.

Fast neutron transmission analysis
The incident (no sample) and transmitted (10 cm graphite) neutron light output (L) spectra were calibrated using a series of gamma ray sources to produce an electron equivalent energy (MeV ee ) scaling [5].The neutron energy spectra were obtained by performing spectrum unfolding analysis with MAXED software [8] using the measured L spectra, known scintillator detector response functions and the ISO-recommended AmBe spectrum [9] as the default spectrum.The effective removal cross section Σ R was used as a measure of probability of interaction per unit length in a material, and is dependent on neutron energy and material composition.For a sample with thickness x, Σ R can be obtained by relating the transmitted neutron flux ϕ(x) to the incident neutron flux ϕ(0) using Eq.1: ( The Σ R values of the graphite sample for neutrons in the 2.0-10.4MeV energy range in the unfolded spectrum (Fig. 1) were calculated using Eq. 1, with the neutron flux in the unfolded no sample spectrum taken to represent ϕ(0).The Σ R values were used to obtain the microscopic (elemental) removal cross section σ R as where N D is the number density of 12 C in the graphite sample.The calculated σ R values, along with the total cross sections σ T from the ENDF/B-VIII.0data library [10], are plotted as a function of energy and shown also in Fig. 1.

Prompt Gamma Neutron Activation Analysis
During irradiation with the STNG, γ ray spectra were measured with and without the graphite sample in position to measure background (no sample) γ rays and the induced de-excitation γ rays, respectively.The expected de-excitation γ ray energy of 4.4 MeV from 12 C in the graphite sample was measured by both the NaI(Tl) detectors; the γ ray spectra from one of the detectors are shown in Fig. 2.

Discussion and conclusion
The proof-of-principle measurements taken at the n-lab demonstrate the potential to analyse carbon in bulk samples using FNTA and PGNAA.The effective removal cross section of 12 C for neutron energies between 2.0 MeV and 10.4 MeV was measured and good agreement in the overall trend was observed when compared to the total interaction cross section.It was expected that the removal cross section would be approximately 2/3 of the total [11], however, that is not the case for most of the removal cross sections.Possible reasons for this are still under investigation.The 4.4 MeV de-excitation γ rays from the 12 C(n, n′) 12 C * interaction were detected by a pair of NaI(Tl) detectors during the PGNAA measurements, which provides an additional signature that can be utilised to identify carbon in bulk samples.In this work, the FNTA and PGNAA signatures for carbon were measured individually, and will be combined to form a multimodal signature for use in elemental unfolding [12].The responses for other elements of interest are currently being investigated, and the use of combined neutron/γ ray transmission analysis is also being explored.The measurement of several radiation signatures ultimately aims to extend the range of applications of fast neutron-based techniques for bulk materials analysis due to their relative sensitivities to each element.

Figure 1 .
Figure 1.Measured energy spectra for AmBe neutrons transmitted through a graphite sample (left) and the microscopic cross sections for 12 C in the 2.0-10.4MeV energy range (right).

Figure 2 .
Figure 2. The γ ray energy spectra measured with (red) and without (blue) the graphite sample present.The 4.4 MeV de-excitation γ rays from the 12 C(n,n′) 12 C * interaction, along with the single and double escape peaks, are clearly visible.Natural and neutron induced background signatures are indicated.