Sudeep Mitra

Evaluation of three field-based methods for quantifying soil carbon

Three advanced technologies to measure soil carbon (C) density (g C m−2) are deployed in the field and the results compared against those obtained by the dry combustion (DC) method. The advanced methods are: a) Laser Induced Breakdown Spectroscopy (LIBS), b) Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS), and c) Inelastic Neutron Scattering (INS). The measurements and soil samples were acquired at Beltsville, MD, USA and at Centro International para el Mejoramiento del Maíz y el Trigo (CIMMYT) at El Batán, Mexico. At Beltsville, soil samples were extracted at three depth intervals (0–5, 5–15, and 15–30 cm) and processed for analysis in the field with the LIBS and DRIFTS instruments. The INS instrument determined soil C density to a depth of 30 cm via scanning and stationary measurements. Subsequently, soil core samples were analyzed in the laboratory for soil bulk density (kg m−3), C concentration (g kg−1) by DC, and results reported as soil C density (kg m−2). Results from each technique were derived independently and contributed to a blind test against results from the reference (DC) method. A similar procedure was employed at CIMMYT in Mexico employing but only with the LIBS and DRIFTS instruments. Following conversion to common units, we found that the LIBS, DRIFTS, and INS results can be compared directly with those obtained by the DC method. The first two methods and the standard DC require soil sampling and need soil bulk density information to convert soil C concentrations to soil C densities while the INS method does not require soil sampling. We conclude that, in comparison with the DC method, the three instruments (a) showed acceptable performances although further work is needed to improve calibration techniques and (b) demonstrated their portability and their capacity to perform under field conditions.

Optimizing the gate-pulse width for fast neutron induced gamma-ray spectroscopy

We used a pulsed 14 MeV neutron generator (NG) to acquire two concurrent gamma-ray spectra induced by inelastic neutron scattering (INS) and thermal neutron capture (TNC) in Si, O, and C, which are key elements in soil analyses. These separate spectra were acquired by gating the data acquisition system during the neutron pulse, to obtain an INS spectrum, and in between the neutron pulses, to obtain a TNC spectrum. Despite this separation, TNC gamma rays are still counted in the INS window due to the steady state achieved in the former reaction. With the NG operating at 10 kHz and a 25% duty cycle, the magnitude of the single-escape gamma rays from the Si 4.93 MeV gamma-ray peak in the TNC spectrum to the 4.43 MeV carbon region in the INS spectrum is 10.1% of the 4.93 MeV peak intensity. This percentage depends on the neutron repetition rate and duty cycle. It can be reduced to 4.9% by using a narrower gate-pulse that closely fits the neutron burst. We also show that under these conditions the net count rate in the individual peaks of soil elements, Si and O (6.13 MeV) of the TNC spectrum reaches a steady state between the neutron pulses, but the total count rate from the entire spectrum does not.

Development of an instrument for non-destructive identification of Unexploded Ordnance using tagged neutrons - a proof of concept study

This work reports on the efficacy of using 14 MeV neutrons tagged by the associated particle neutron time-of-flight technique (APnTOF) to extract neutron induced characteristic γ-rays from an object-of-interest with high signal-to-noise ratio (SNR) and without interference from nearby clutter. A small portable API120 neutron generator was operated at a continuous output of ∼ 1×107 n/s while the γ-rays were detected using a 12.7×12.7 cm diameter NaI(Tl) detector. The α-particle detection system comprised a 5-cm diameter fast Amperex XP2020 photomultiplier tube which was mated with the 6.5 cm fiber-optic face-plate of the neutron generators ZnO(Ga) α-detector. Standard NIM electronics was used for the α-γ coincidence spectroscopy. A graphite cube of dimension 15.2 cm (6 kg) was used as the object-of-interest while a half-gallon bottle of water and a 5-mm thick iron slab were used as clutter items in non-overlapping and overlapping modes. In all cases, carbon (C) signals from the graphite were successfully extracted without interference from the oxygen (O) signals from water. The SNR at the peak energies of 4.43 (C) and 6.13 MeV (O) were found to be 1.3 when spectra were acquired without the time coincidence mode. The SNRs were vastly improved; 22 and 10 for C and O respectively in the coincidence mode.

FPGA based field deployable instrumentation system for neutron time of flight sub nanosecond coincidence spectroscopy

In this report, we present a novel FPGA based electronics architecture for sub-nanosecond coincidence spectroscopy in a neutron time-of- flight system employing the Associated Particle Technique (APT).. In the APT [S. MITRA, J.E. WOLFF, et.al,,] the alpha particle associated with a 14 MeV neutron produced by the 3H(d,n)4He reaction in the neutron generator is emitted in nearly the opposite direction. When a neutron collides with the soil matrix, one probable interaction is an inelastic scattering by a C atom producing the characteristic gamma-ray emission. The elapsed time between detection of the alpha particle and detection of the gamma-ray reveals the distance traveled by the neutron and provides the depth of neutron interaction with C in the soil as shown in Fig.l. Since 14 MeV neutrons travel at a speed of 5 cm/ns, it is important to achieve sub nanosecond time resolutions to be able to resolve depth profiles of C layers less than 5 cm thick.

Monitoring neutron generator output in a mixed neutron-gamma field using a plastic scintillator

Quantitative neutron-induced gamma-ray spectroscopy employing neutron generators (NGs) entails monitoring them for possible fluctuations in their neutron output. We accomplished this using a plastic scintillator and recording a spectrum from which we selected a neutron region- of-interest (nROI) to discriminate between neutrons and the accompanying high-energy gamma-rays. We show that the selected nROI is insensitive to changes in the gamma-ray background, thus allowing satisfactory normalization of the gamma-ray spectra of an in-situ system for analyzing soil carbon.

Search for elemental and mineral biomarkers using inelastic-neutron-scattering gamma spectroscopy

Life on Earth is characterized by a select group of low Z elements: C, H, N, O, P, K, S, Na, Cl. The presence of these elements and their ratios can provide indications of possible biogenicity and thus they may constitute valuable biomarkers that may help determine the best locations to seek more definitive evidence of life. We discuss the possible applications and significance of the inelastic neutron scattering induced gamma spectroscopy (INSGS) for future Astrobiology Missions to Mars or other solar System bodies. The general requirements and capabilities of the proposed approach are presented.