K. Ishii

PIXE spectrum analysis taking into account bremsstrahlung spectra

Abstract Continuous background appearing over the wide region of X-ray energy in the PIXE spectrum is very troublesome presence in the peak fitting. In the usual manner, the spectrum of continuous background is predicted as a function of polynomial and is subtracted from the X-ray spectrum. However, the parameters of the polynomial are determined with difficulty in the case that the continuous background exists under many peaks of characteristic X-rays. We calculated the production cross sections of continuous X-rays for several elements on the basis of the theories of quasi-free electron bremsstrahlung (QFEB), secondary electron bremsstrahlung (SEB) and atomic bremsstrahlung (AB), and obtained the continuous X-ray spectrum as a function of atomic number and X-ray energy. X-ray spectra of a standard sample and of a bovine liver sample were analyzed by a pattern analysis method assuming the reference spectra consisting of characteristic X-rays and continuous X-rays for each element. The results of analysis are quite satisfactory. By the present method, the PIXE spectra can be analyzed under little influence of the background subtraction, and it enables us a full auto-analysis of PIXE spectrum.

Development of a Submilli-PIXE Camera

We developed a PIXE analysis system which provides spatial distribution images of elements in a region of several cm2 with a spatial resolution of < 0.5 mm. We call this system a submilli-PIXE camera. This system consists of a submilli-beam line, beam scanners and a data acquisition system in which the X-ray energy and the beam position are simultaneously measured. We demonstrate the usefulness of the submilli-PIXE camera by analyzing the surface of a shell and of granite.

Development of a Micro-PIXE Camera

We developed a system of μ-PIXE analysis at the division of Takasaki ion accelerator for advanced radiation application (TIARA) in Japan Atomic Energy Research Institute (JAERI), which consists of a microbeam apparatus, a multi-parameter data acquisition system and a personal computer. Elemental analysis in the region of 500 μm × 500 μm can be performed with a spatial resolution of < 0.3 μm and multi-elemental distributions are presented as images on a computer display even during measurement. We call this system a micro-PIXE camera.

Preparation of Thin Polycarbonate Film and Its Application to PIXE Analysis of Anionic Elements in Water Samples

A polycarbonate film of thin and uniform thickness was prepared. A casting solution for film-formation was made up by diluting a solution of poly (bisphenol A carbonate) in chloroform by a factor of two to three with benzene. A uniform film was created by dropping 0.2-0.3 cm3 of the casting solution slowly on a water surface within an aperture (20 mm diameter) of Mylar target frame floating on 50 wt% sucrose aqueous solution. Films of 0.14-0.27 mg/cm2 thickness thus prepared offer a good combination of mechanical strength and low continuum backgrounds. To test PIXE analysis of anionic species in water, targets containing SO42-, Cr2O72-, AsO43-, and Ga3+-internal standard were prepared by depositing 100 μl of the test solutions onto the polycarbonate film, and irradiated in vacuum by 3 MeV proton beams. The difference between the nominal and the analyzed concentrations seldom exceeded ± 15 % in the range from 10 to 2000 ppb.

Determination of Heavy-Metal Concentrations in Water by PIXE Analysis Using Zr as an Internal Standard

A metal-preconcentration procedure, which consists of a combination of chelation by dibenzyldithiocarbamate ions with subsequent condensation into dibenzylidene-D-sorbitol gels, has been reformed by using Zr as an internal standard. The colloidal gels formed in solutions were collected on a nuclepore filter for direct irradiation by 3 MeV proton beams in a system of vertical beam type in-air PIXE. The methodology was tested for simultaneous determination of the concentrations of seven metals (Fe, Co, Ni, Cu, Cd, Hg, Pb) in 25 ml of an aqueous sample. A good linear relationship was obtained between the nominal concentration and the elemental X-ray yields normalized with Zr-Kα ones. Absolute values of concentration in the sample solutions can be determined with an experimental error less than ±14 % in the 5-100 ppb range by employing published data pertinent to the inner shell ionization and subsequent X-ray emission of the heavy metals.

An endurance test of Kapton foil in in-air PIXE system

We have tested endurance of Kapton foil for beam irradiation, which is used as an exit window of in-air PIXE system. Kapton foil is strong enough to withstand the pressure differential, thin enough to transmit the proton beam with minimal energy loss and does not contain heavier elements. However, little is known about the relation between its lifetime until breakdown and beam parameters such as beam current, beam diameter and energy loss. Endurance tests were made by an in-air PIXE system at Tohoku university. Irradiation was performed by proton beam with 2 or 3 MeV until the foil was broken for various beam parameters. The total charge until breakdown of the Kapton foil was analyzed in consideration of beam current, diameter and energy loss. Total charge until breakdown decrease with increase in the beam current density in case of the beam current density higher than 100 nA/mm2. On the other hand, total charge until breakdown does not decrease so much in the beam current density up to 100 nA/mm2. The li...

RADIATION DAMAGE OF PAPER SAMPLES IN IN-AIR PIXE ANALYSIS

Degradation of paper caused by beam irradiation was investigated from a viewpoint of discoloration in PIXE analysis and its application to the paper samples of archaeology. Two types of paper (Japanese paper and fine quality paper) were tested in in-air PIXE analysis with 3 MeV protons. The degree of discoloration was quantitatively measured by the use of a calorimeter. The degree of discoloration was different for each tested paper and corresponded to the radiation dose of ions. It is resulted that even the in-air PIXE analysis should be carefully applied to archaeological treasures. Because discoloration of all tested paper decreased gradually at first but then increased after a few weeks. However, this phenomenon can be used to develop a technique of funny coloration.

OBSERVATION OF INTERFERENCE BETWEEN ATOMIC BREMSSTRAHLUNG AND NUCLEAR BREMSSTRAHLUNG

An aluminum target was bombarded with 1.5 MeV protons and continuous x-rays were measured at the angles of 45°, 90° and 135° with respect to the beam direction. By investigating the shape of the x-ray energy spectrum, it was recognized that, the continuous x-rays below 12 keV are atomic bremsstrahlung (AB) and those of above 12 keV are nuclear bremsstrahlung (NB), and AB and NB are mingled in the energy region of around 12 keV The x-ray energy dependence of angular distributions presented well a change from the process of AB to that of NB in the continuous x-ray spectrum. Interference between AB and NB were discussed on the basis of PWBA theory. Continuous x-ray production cross sections were calculated on the basis of PWBA BEA and a semi-classical theory and compared with the experimental results. The theoretical prediction reproduced well the experimental cross sections over the wide range of 6 orders in magnitude and of 2 keV – 35 keV in the energy except for the energy region mingled with AB and NB. The ratio of the theoretical cross sections to the experimental ones showed an interference effect between AB and NB in their mingled region.

APPLICATION OF VERTICAL-BEAM IN-AIR PIXE TO SURFACE ANALYSIS OF PLANT ROOT EXPOSED TO ALUMINUM STRESS

Elemental composition of living cells and tissues reflects their physiological function and status. However, it has been difficult to know in-situ elemental distribution by conventional analytical methods. In-air PIXE seems suitable for surface analysis of living cells and tissues because any treatment (e.g. freeze drying, digestion) is not required before and during measurement. We applied Via (vertical-beam in-air) PIXE to surface analysis of plant roots exposed to aluminum (Al). Aluminum stress is a major factor that limits elongation of plant roots in acid soils. We previously reported decrease in atomic ratio of potassium to phosphorus (K/P ratio) of dried root-tip of alfalfa (Medicago sativa L.) under Al stress using in-vacuum PIXE. In Via PIXE, 5 to 7-minute irradiation by 3 MeV proton beams of 200 pA was sufficient to obtain X-ray spectra without drying root samples. Decrease in K/P ratio in surface cells of root-tips was observed by short-term (6-8 h) exposure of root to Al. Via PIXE is recognized as a powerful tool for in-situ surface analysis of plant material.

AN ATTEMPT TO SUPPRESS THE COMPTON SCATTERING BACKGROUND APPEARING IN THE PIXE SPECTRUM

The background due to the Compton scattering of γ-rays in a Si(Li) X-ray detector was measured with the coincidence system which consists of a Si(Li) X-ray detector and NaI(Tl) scintillation detector. The Si(Li) X-ray detector is a through type to detect γ-rays of the Compton scattering into the forward direction. The energy spectra of the Si(Li) detector were measured for γ-rays of 662 keV from a 137Cs source. The contribution of recoiled electrons to the PIXE spectrum in the energy region of 0~100 keV was ~20 % of its background spectrum. It was not enough to suppress the Compton scattering background by using the present system.