M.M. Nagadi

Performance comparison of an 241Am-Be neutron source-based PGNAA setup with the KFUPM PGNAA setup

Monte Carlo calculations have been carried out to compare the performance of an 241Am-Be neutron source-based prompt gamma neutron activation analysis (PGNAA) setup with that of the 2.8 MeV neutron-based PGNAA setup at King Fahd University of Petroleum and Minerals (KFUPM) to analyze Portland cement samples. This work is a part of a wide Monte Carlo studies being conducted at KFUPM in search of a more efficient neutron source for its 2.8 MeV neutrons, from the D(d,n) reaction, based PGNAA facility. In this study an 241Am-Be neutron source-based PGNAA setup was simulated. For comparison, the diameter of a cylindrical external moderator of the 241Am-Be neutron source, based PGNAA setup, was assumed to be similar to that used in the KFUPM PGNAA setup. The results of this study revealed that although the optimum geometry of the 241Am-Be neutron source-based setup is similar to that of the KFUPM PGNAA facility, the performance of the 241Am-Be neutron source-based setup is slightly poorer than that of the 2.8 MeV neutron-based setup.

Response function measurements of an NE102A organic scintillator using an 241Am-Be source

The response function of a 125 mm diameter NE102A organic scintillation detector has been measured over the 2.7–14.8 MeV neutron energy range. The detector response function was derived from the light output for monoenergetic neutrons and gamma rays. The light output of the detector for monoenergetic neutrons was measured by selecting narrowgates in the time-of-flight (TOF) spectrum for a 241Am-Be neutron source. In order to provide check points on the data, the detector light output was also measured for monoenergetic neutrons from the D(d, n) and T(d, n) reactions. The response function of the NE102A detector is in good agreement (within 1–5%) with the published data of Cecil et al. [Nucl. Instr. and Meth. 161 (1979) 439].

Prompt gamma analysis of fly ash, silica fume and Superpozz blended cement concrete specimen.

Preventive measures against corrosion of reinforcing steel require making the concrete dense by adding pozzolanic materials, such as fly ash, silica fume, Superpozz, blast furnace slag, etc. to Portland cement. In order to obtain the desired strength and durability of concrete, it is desirable to monitor the concentration of the pozzolan in the blended cement concrete. Addition of pozzolan to blended cement changes the overall concentration of calcium and silicon in the blended cement concrete. The resulting variation in calcium and silicon gamma-ray yield ratio from blended cement concrete has found to have an inverse correlation with concentration of fly ash, silica fume, Superpozz, blast furnace slag in the blended cement concrete. For experimental verification of the correlation, intensities of calcium and silicon prompt gamma-ray due to capture of thermal neutrons in blended cement concrete samples containing 5-80% (by weight of cement) silica fume, fly ash and Superpozz were measured. The gamma-ray intensity ratio was measured from 6.42 MeV gamma-rays from calcium and 4.94 MeV gamma-ray from silicon. The experimentally measured values of calcium to silicon gamma-ray yield ratio in the fly ash, silica fume and Superpozz cement concrete specimens agree very well with the results of the Monte Carlo simulations.

Effect of silica fume addition on the PGNAA measurement of chlorine in concrete

Pozzolanic materials, such as fly ash (FA), silica fume (SF), and blast furnace slag (BFS) are added to Portland cement in concrete to prevent reinforcement steel corrosion in concrete. Further preventive measure against reinforcement steel corrosion require monitoring of chloride salts concentration in concrete using non-destructive techniques, such as the prompt gamma-ray neutron activation analysis (PGNAA) technique. Due to interferences between gamma-rays from chlorine and calcium in PGNAA technique, detection limit of chlorine in concrete strongly depends upon calcium concentration in concrete. SF mainly contains silica and its addition to cement concrete reduces overall concentration of calcium in concrete. This may result in an improvement in detection limit of chlorine in SF-based concrete in PGNAA studies. Particularly for chlorine detection using 6.11 and 6.62 MeV prompt gamma-rays that strongly interfere with 6.42 MeV prompt gamma-rays from calcium. In this study, SF was added to Portland cement to prevent concrete reinforcement steel from corrosion. The chlorine concentration in SF cement concrete specimens containing 0.2-3.0 wt% chlorine was measured through yield of 1.16, 1.95, 6.11, 6.62, 7.41, 7.79, and 8.58 MeV chlorine gamma-rays using PGNAA technique. An excellent agreement was noted between the experimental yield of the prompt gamma-rays and the gamma-ray yield calculated through the Monte Carlo simulations. Further the minimum detectable concentration (MDC) of chlorine in SF cement concrete was calculated and compared with the MDC values of chlorine in plain concrete and concrete mixed with fly ash cement. The MDC of chlorine in SF-based concrete through 6.11 MeV, and 6.62 MeV chlorine gamma-rays was found to be improved as compared to those in plain concrete and concrete mixed with fly ash cement.

Response function measurement of a deuterated scintillator using a 241AmBe source

The response function of 50 mm diameter deuterated (C6D6), plastic (NE102A) and liquid (NE213) scintillators have been measured below 6 MeV deuteron/proton energies using a 241AmBe source. Also the energy resolution of C6D6 scintillator was measured using a gamma-gamma coincident technique. The energy resolution of the C6D6 scintillator varies from 21.7 to 11.9% over 0.34–1.1 MeV electron energy range. The response function of the C6D6 scintillator agrees within 1–6% with that of Bovet et al. For the 50 mm NE102A and NE213 detectors, the response functions data agree with the published result of 125 mm NE102A and NE213 detectors. In the energy range below 5 MeV, the light output of the C6D6 scintillator is 17–24% lower than that of the NE213 detector. As the deuteron energy increases above 5 MeV, the difference between the light outputs of the two detectors starts to decrease up to 9 MeV, where light outputs of both detectors correspond to the same electron energy. This trend in difference of light output of C6D6 and NE213 scintillator is consistent with the published results of light output for C6D6 and C6H6 scintillators.

Response of a PGNAA setup for pozzolan-based cement concrete specimens.

Pozzolanic materials are added to Portland cement concrete to increase its durability, particularly corrosion-resistance. In this study the elemental composition of a pozzolanic cement concrete was measured non-destructively utilizing an accelerator-based Prompt Gamma Ray Neutron Activation Analysis (PGNAA) setup. The optimum size of the pozzolanic cement concrete specimen was obtained through Monte Carlo simulations. The simulation results were experimentally verified through the gamma-ray yield measurement from the pozzolanic cement concrete specimens as a function of their radii. The concentration of the pozzolanic material in the cement concrete specimens was evaluated by measuring gamma-ray yield for calcium and iron from pozzolanic cement concrete specimens containing 5-80 wt% pozzolan. A good agreement was noted between the experimental values and the Monte Carlo simulation results, indicating an excellent response of the KFUPM accelerator-based PGNAA setup for pozzolan based concrete.

Performance comparison of NE213 detectors for their application in moisture measurement

The pulse shape discrimination (PSD) characteristic and neutron detection efficiency of NE213 detectors have been measured for their application in moisture measurements using 252Cf and 241Am-Be sources. In PSD studies, neutron peak to valley (Pn/V) ratio and figure of merit M were measured at four different bias values for cylindrical 50, 125 and 250 mm diameter NE213 detectors. The result of this study has shown that better PSD performance with the NE213 detector can be achieved with a smaller volume detector in conjunction with a neutron source with smaller gamma-ray/neutron ratio. The neutron detection efficiency of the 125 mm diameter NE213 detector for 241Am-Be and 252Cf source spectra was determined at 0.85, 1.25 and 1.75 MeV bias energies using the experimental neutron detection efficiency data of the same detector over 0.1-10 MeV energy range. Due to different energy spectra of the 241Am-Be and 252Cf sources, integrated efficiency of the 125 mm diameter NE213 detector for the two sources shows bias dependence. At smaller bias, 252Cf source has larger efficiency but as the bias is increased, the detector has larger efficiency for 241Am-Be source. This study has revealed that NE213 detector has better performance (such as PSD and neutron detection efficiency) in simultaneous detection of neutron and gamma-rays in moisture measurements, if it is used in conjunction with 241Am-Be source at higher detector bias.

Verification of design calculations of a PGNAA setup using nuclear track detectors

Abstract A rectangular moderator assembly has been designed for the PGNAA setup at King Fahd University of Petroleum and Minerals (KFUPM). The design calculations of the rectangular moderator, which were obtained through Monte Carlo simulation, have been verified experimentally through thermal neutron yield measurement using CR-39 nuclear track detectors (NTDs). These measurements were carried out at the KFUPM 350 keV accelerator using 2.8 MeV pulsed neutron beam from D( d , n ) reaction. The thermal neutron yield was measured inside the sample volume of the rectangular moderator by two NTDs fixed at back and front end of the sample cavity. The good agreement between the experimental results and the results of the calculations shows useful application of NTDs in verification of design calculations of a PGNAA setup.

350 keV accelerator-based neutron transmission setup at KFUPM for hydrogen detection

An experimental setup has been developed to determine hydrogen contents of bulk samples using fast neutron transmission technique. Neutrons with 3 MeV energy were produced via D(d, n) reaction. The neutrons transmitted through the sample were detected by a NE213 scintillation detector. Preliminary tests of the setup were carried out using soil samples with different moisture contents. In addition to experimental study, Monte Carlo simulations were carried out to generate calibration curve of the experimental setup. Finally, experimental tests results were compared with the results of Monte Carlo simulations. A good agreement has been obtained between the simulation results and experimental results.

Light output measurements of C6H6 and C6H12 scintillators for protons

Abstract Light output of C6H6 and C6H12 proton scintillators has been measured for neutrons using a 241Am-Be source. In these measurements the energy of the neutrons (recoil protons) was measured using time-of-flight technique. The light output of the C6H6 scintillator was measured for seven proton energies ranging from 2.185 to 4.236 MeV while that of the C6H12 scintillator was measured for eight neutron energies ranging from 2.201 to 5.147 MeV. The measured light output data of the C6H6 scintillator is in good agreement with the published data. Additionally the proton and deuteron response ratio of a C6D6 scintillator identical in shape and size was derived from these measurements. The response ratio of the C6D6 scintillator agrees with the published data.