M.A. Garwan

AMS of heavy ions with small accelerators

Abstract Recent advances in the detection and the routine measurements of heavy elements by accelerator mass spectrometry (AMS) are reviewed. Particular emphasis will be given to the measurement of low energy (⩽ 15 MeV) and high-Z ions using small (⩽ 3 MV) accelerators.

A negative ion survey; towards the completion of the periodic table of the negative ions

Abstract The negative ion properties of many of the elements of the periodic table have been known for some years, however, there were some 36 elements for which conclusive experimental evidence for the existence of a stable negative ion had not been found. After a brief overview of the experimental method used, we will present the results obtained for the three elemental groups studied (Group II, lanthanides, actinides a total of 24 elements). We will then conclude by stressing the similitudes between different elements and different elemental groups.

Radium, actinides, and their molecular negative ions from a cesium sputter ion source

Abstract The negative ions of the isotopes 226 Ra, 231 Pa and 244 Pu, produced by a Cs sputter ion source, have been observed for the first time using the IsoTrace heavy element AMS system. The properties of these negative ions have been compared with those of Th and U, and the electron affinities of all these elements (Ra, Th, Pa, Th and Pu) have been found to be similar and greater than 50 meV. Some molecular negative ions of these elements, in particular carbides and oxides, have also been measured. It was found that di-carbide is generally a profitable molecule form for measuring Ra, Th and Pa, while mono-oxide is more efficient for measuring U and Pu.

Validity test of design calculations of a PGNAA setup

A rectangular moderator has been designed for the prompt gamma ray neutron activation analysis (PGNAA) setup at King Fahd University of Petroleum and Minerals (KFUPM) to analyze Portland cement samples. The design of the moderator assembly was obtained using Monte Carlo calculations. The design calculations of the new rectangular moderator of the KFUPM PGNAA setup have been verified experimentally through prompt gamma ray yield measurement as a function of the front moderator thickness. In this study the yield of the 3.54 and 4.94 MeV prompt gamma rays from silicon in a soil sample was measured as a function of thickness of the front moderator of the rectangular moderator. The experimental results were compared with the results of the Monte Carlo simulations. A good agreement has been achieved between the experimental results and the results of the calculations. The experimental results have provided useful information about the PGNAA setup performance, neutron moderation, and gamma ray attenuation in the PGNAA sample.

Heavy element analysis by low energy accelerator mass spectrometry

Improvements have been made to the mass and energy analysis system at the IsoTrace Laboratory that have resulted in a factor of 30 increase in detection efficiency for some heavy ions and have reduced interference from the fragmentation of hydrides and other molecules. The implications of these improvements for the detection of rare atomic negative ions and radioisotopes such as 129I will be discussed. Evidence is also presented for the existence of a barium negative ion that is completely resolved from barium mono-, di- and trihydride interferences and which is clearly distinguished from any interference due to the fragmentation of BaH−2 to BaH− before the first analyzing magnet.

Electric dissociation of negative ions — II

Abstract As an alternative to naturally occurring negative ion discrimination, we consider the possibility of electric dissociation to discriminate between isobars, by destroying the weaker negative ion of a pair. The theoretical basis of negative ion dissociation using high electric gradients and an appropriate experimental setup are discussed. The possibility of negative ion destruction by electric dissociation during the first stage of tandem acceleration and at the ion source is also considered. In consequence, it is possible to place a limit on the electron affinity of some elements considering their survival in the AMS system. Mn−, Ca− and C−∗ among others are considered.

Low energy prompt gamma-ray tests of a large volume BGO detector

Tests of a large volume Bismuth Germinate (BGO) detector were carried out to detect low energy prompt gamma-rays from boron and cadmium-contaminated water samples using a portable neutron generator-based Prompt Gamma Neutron Activation Analysis (PGNAA) setup. Inspite of strong interference between the sample- and the detector-associated prompt gamma-rays, an excellent agreement has been observed between the experimental and calculated yields of the prompt gamma-rays, indicating successful application of the large volume BGO detector in the PGNAA analysis of bulk samples using low energy prompt gamma-rays.

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.

Lanthanide negative ion detection using accelerator mass spectrometry

Abstract Accelerator mass spectrometric methods have been used in the detection of the negative ions of the lanthanides. All of the lanthanide negative ions (La−−Lu−) have been observed except Pm−, Ho−, and Er−. The heavy element analysis line at the IsoTrace Laboratory was used to count the positive ions resulting from the atomic negative ions produced in a Cs sputter ion source and passed through the tandem accelerator. Because of its very low electron affinity, Dy− had a terminal voltage dependent yield, and its study required the reduction of the electric field gradients used to accelerate the ions. The relative negative ion yields among the lanthanides will be discussed. Both Yb− and Lu− have recently been theoretically predicted to exist in negative parity ground states.

The negative ions of strontium and barium

Abstract Recent theoretical calculations have predicted a tendency toward higher electron affinities for heavier alkaline elements. Experimental evidence has been obtained for the existence of strontium and barium negative ions created from pure elements in a caesium sputter ion source. Accelerator mass spectrometric techniques were employed to resolve the above elemental negative ions from the interfering molecular species.