2/12/06 clc

X-ray production, diffraction and absorption

 

            This experiment introduces you to the principles of x-ray production and x-ray diffraction using the compact and safety interlocked TEL 580 x-ray spectrometer.  Be sure you have read the laboratory notes on the scattering of waves by crystals, or the equivalent . You should understand the principles of Bragg diffraction before you start.

 

The apparatus includes:

 

Supplementary documents include:

 

     

 

Even though this apparatus has safety interlocks, care should be taken in using it.  The x-ray source does have sufficient power to produce dangerous radiation.  Use common sense and never attempt to override any safety interlock for any reason.  If at any time you experience any difficulty in operating the system, see your instructor. The Teltron manual describes the operation and use of the apparatus and contains the description of a large number of experiments. You will do several.  The TEL580 has been equipped with a stepping motor and interface for controlling the position of the detector arm and for accumulating the x-ray spectra (x-ray counts vs.) in a PC. The spectrometer is very compact and easy to use.  Use 80 microamps for all experiments. Do not exceed this.

Do the following experiments:

 

0. Familiarize yourself with how the equipment works. Do not fool with the tube or current adjustments: let your instructor do this. However, you might want to verify that the q – 2 q relationship holds before starting a long scan. Be sure you know how to mount and dismount crystals. Do not touch the surfaces of the crystals with your hands- use tweezers. Use the “dull” side of the crystal (you may want to use the shiny side for alignment first, but then reverse it). Before doing a real scan you probably should do a dry run with the computer so you know how it works.

 

 

 

 

 

1.  X-ray Emission.  Experiment D15 p18 (~ 1  hours)

            In this experiment you will study the properties of the continuous radiation from the x-ray tube, and determine the maximum energy/minimum wavelength photons produced, from Planck’s relations

and the energy relationship

(i.e.; e = 1 in this case).  Perform the experiment with the 30 kV setting only.  Be sure to arrange the proportional counter such that you can go in to at least 10 degrees. Use the NaCl and LiF crystals, and a 2 mm slit in front of the PC. You need scan only to about 40 degrees at most.

 

2.  Bragg Scattering – Experiment D14 p17 (~ 1.5 hours)

            In this experiment a various crystals are used to study the Bragg scattering formula given by

This formula relates the angle  at which constructive interference of photons of wavelength  are scattered from crystal planes separated by a distance d.  n is the number of wavelengths between the reflected waves from adjacent planes (the order of the scattering). Use the 1 mm slit in front of the PC to improve your angular resolution. Scan to the maximun angle the apparatus will allow. Use NaCl, LiF , KCl and RbCl.

 

3.  X-ray Absorption  Experiment D16 p19 (~ 1 hour)

            In this experiment the absorption of the x-rays produced by the Cu anode of the spectrometer is studied by placing intermediate foils of Cu, Ni and Co between the source and the crystal.  The x-rays are attenuated by the relationship

 is the linear absorption coefficient and x is the foil thickness.  The important concept is the absorption edge of the absorber.  This is determined by the energy required to ionize the absorber atoms.  The energy is slightly larger than the  x-ray energy of the absorbing material, and therefore Cu will not effectively reabsorb its own characteristic x-ray lines. Use the LiF crystal.

 

Tips: Be sure the crystal faces the right way. Before starting a long run, scan the apparatus by motor to the location of K alpha, and be sure there is a good peak. Do not play with the processor speed. Be sure you are in the right directory: after booting, type CD NEW, then run X RAY.