QUEEN’S UNIVERSITY

Department of Physics, Engineering Physics and Astronomy

PHYSICS 352 – Measurement, Instrumentation and Experiment Design

X-Ray Fluorescence: Purpose: Quantitative analysis of the elemental content of brass using characteristic

X-rays. References: Sayer and Mansingh, Measurement, Instrumentation and

Experimental Design H.C. Evans supplementary notes Introduction: X-ray fluorescence is a useful technique for non-destructive qualitative and quantitative material analysis. The analysis is done by radiating a sample with X-rays of sufficient energy to ionize the atoms, knocking an electron from the inner shell. An electron from a higher energy shell can drop down to fill this vacant spot, emitting an X-ray photon whose energy is characteristic of the transition and of the element involved: Kα radiation for L-shell to K-shell transitions, and Kβ radiation for M-shell to K-shell transitions. The energy of the x-ray is characteristic of the element, and the intensity can be used for quantitative analysis. One complication in the analysis is self-absorption; but substantial simplification of the analysis can be achieved by careful design of the apparatus. See the supplemental notes for these details. Technique: X-rays characteristic of the elements present can be excited by the bombardment of the material by higher energy X-rays. In this experiment the incident X-rays are

K Shell

L-Shell

M-Shell

Incident X-ray

Kα X-ray

Kβ X-ray

Figure 1: Principle of x-ray fluorescence

produced by an X-ray tube that produces a broad spectrum of energies, some of which are high enough to eject K-shell electrons from copper and zinc in brass. A silicon lithium-drifted detector [Si(Li)] is a device capable of resolving X-rays of different energies such that an electrical pulse with a charge proportional to the X-ray energy is formed. This charge is integrated and amplified with a pre-amplifier and pulse shaping amplifier and the pulse heights are analyzed with a multi-channel analyzer. Instructions for operating the apparatus are available in the laboratory. The sample is placed on a beryllium window – THIS WINDOW IS VERY FRAGILE, DO NOT ALLOW ANYTHING TO FALL THROUGH THE CENTRAL HOLE ONTO THE WINDOW. The X-ray tube is aligned such that the incident x-rays hit the sample and only the secondary X-rays are incident on the detector. Safety: The X-ray source has the potential to emit a dangerous amount of radiation. Be sure to use only the very small current needed for the experiment. Keep the source shielded at all times. The detector and X-ray tube both use high voltage – make sure all cables are correctly connected before turning on the equipment. To maximize the lifetime of the X-ray tube and to reduce the risk of accidental exposure to radiation, please keep the ENABLE switch on the front panel of the X-ray generator control box in the off (“0”) position unless you are acquiring a spectrum. Read through the operations manual for the Amptek Eclipse III X-ray generator, available in the lab. Method:

1. Place the Cu and Zn samples on the detector and accumulate a spectrum for each. Determine the areas of the Kα and Κβ peaks and determine their locations.

2. Accumulate a spectrum from the brass sample and obtain a value and uncertainty for the energy of all peaks observed. Identify the peaks in the spectrum and determine the areas of the Kα and Κβ peaks for Cu and Zn.

Figure 2: Apparatus schematic

X-ray tube

Power supply

Detector

Amplifier

MCA

Sample

Pre-Amplifier

3. The samples provided are essentially infinitely thick. Follow the analysis described in the supplementary reference to account for: a) absorption of the incident X-rays b) fluorescence yield c) self-absorption of the characteristic X-rays in the material being tested.