PHYSICS 320

X-RADIATION LABORATORY MEASUREMENTS

APPARATUS:

The special X-ray unit TEL-X-OMETER (TEL 580), associated items in the kit (TEL 582), a G-M tube, a ratemeter, a scaler, and a pre-set timer.

SAFETY:  This is a very safe x-ray unit with a very low output intensity.  Please follow the procedures described below to minimize any danger of exposure.  This x-ray unit is registered with the North Carolina Department of Environment and Natural Resources, Division of Radiation Protection.  Your instructor is knowledgeable in the operating and safety procedures for this x-ray unit.  If you have concerns with the operation of this unit or feel that you have been needlessly or accidentally exposed to the x-ray radiation, please talk to your instructor, or to Dr. Dan Boye, the Resident Safety Officer for this unit.

THEORY: Download this document to review the theory underlying this experiment.

EXPERIMENTAL PROCEDURES:

(1)       Preliminary Warm-up:

Before a high voltage is connected to the X-ray tube, the filament current alone should be activated for about five minutes in order that any condensation which might be in the tube can be evaporated by the heat generated by the filament current. Proceed as follows:

(a) Check that the unit's power cord is connected to 120 VAC.

(b) Energize the filament by turning the key in the POWER ON switch to horizontal and rotating the TIME SWITCH clockwise. When this is performed, both the filament in the X-ray tube and the POWER-ON light (WHITE) should be illuminated. Since the Time Switch will automatically cut-off the equipment after the elapse of the pre-set time, it is desirable to keep the time dial rotated upscale throughout the experiment.

(c) Generally, leave the filament on while setting up the accessories for the experiments.

(2)  Spectrometer Mechanism:

Open the TEL-X-OMETER by first displacing the Scatter Shield cover sideways with respect to the hinge at the rear. When the Carriage Arm is positioned so the cursor gives an accurate, no-parallax, zero reading on the (2q) scale, the Slave Plate just under the carriage should have its index lines opposite the zero lines on the scale. If the lines do not make a reasonable match, the Clutch Plate with the knurled edge located just under the central crystal post can be unscrewed to release the drive, and the Slave Plate can be rotated until its index lines do match at q = 0. It may be necessary to "mean out" small differences on each side of the scale. Now, with the Carriage Arm cursor and the Slave Plate index lines on zero, tighten the Clutch Plate again.

(3)  X-Ray Production:

X-rays can never be produced unless the Scatter Shield is closed and properly centered. If it is not exactly centered, the micro-switches at the hinges will not allow the activation of the high voltage. This Shield positioning serves as a safety mechanism. Thus, whenever you wish to cut-off the high voltage and stop X-ray production, simply displace the Scatter Shield sideways from the central position. With the Shield closed and centered, connect a micro-ammeter to the X-ray tube by way of the jack-plug on the right of the control panel. Activate high voltage for the X-ray tube by depressing the X-RAYS ON button. [If on the first attempt the (RED) lamp does not illuminate to indicate X-ray production, check that the Scatter Shield is properly centered. Re-center and depress the X-RAYS ON button again.] The X-ray tube current can be adjusted with a screw driver at the Set Tube Current aperture on the left of the control panel. It is recommended that the current never be allowed to exceed 80 micro-amps.

IN CASE OF EMERGENCY: Either displace the cover or unplug the unit from the AC power outlet.  Notify your instructor immediately.

(4)  Mounting Crystals:

Position one of the short edges of a rectangular crystal onto the central post "step". Ascertain that the face having a "flat matte" appearance is butting against the chamfered protrusion of the post: x-rays reflect better off the shiny surface.  Screw the clamp so the crystal is held securely by the rubber jaw.

(5)  Mounting Detector:

Mount the G-M Tube, TEL 546, in the holder, TEL 547, and position the holder in one of the carriage slots. The orientation should have the co-axial cable leading out from the underside of the holder with the end-window of the G-M Tube facing the crystal post. It is advisable to leave the plastic guard over the end-window. Rotate the Carriage Arm to the left side and close the hinged, radiation Scatter Shield. The co-axial cable can be trailed out to the side of the Carriage Arm from beneath the closed shield. The G-M Tube cable can be connected to a Ratemeter Monitor 807 or a Scaler Monitor 806.

EXPERIMENT I: The X-Ray Spectrum

(1)  Mount the NaCl crystal, TEL 582.004, on the central post.

(2)  Set the High Voltage Switch to 30 kV.

(3)  Attach the 1 mm Collimator, TEL 582.001, onto the Basic Port on the side of the glass dome covering the X-ray tube. Adjust it so the slit is vertical.

(4)  Position the 3 mm Collimator, TEL 562.016, into Carriage Slot #14.

(5)  Position the 1 mm Collimator, TEL 562.015, into Carriage Slot #18.

(6)  Locate the G-M Tube with holder in Carriage Slot #26. Connect the cable to the Ratemeter Monitor and set G-M voltage to 425 volts.

(7)  Click on this link ( xrays.zip ) and save the file to the desktop.  Unzip the file, then double-click on the file's icon or run the DataStudio software and open this experiment.  This program is set to collect data every 1/16 per second with a scan interval of 20 (320 samples).  Connect Channel A of the Pasco Interface to the Power Amp and the Amp to the stepper motor drive.  The specs for the Power Amp should be a 3.5 Volt amplitude, 1Hz positive square wave set to automatically trigger.  Channel C of the Pasco Interface should be connected to the BNC connector on the ratemeter and the voltage probe should be set on low sensitivity in the software.

(8)  Place the arm at just less than (2 q) = 20. Mesh the gear teeth of the motor with those of the carriage arm, and fix the motor position with the tightening plug. The stepper motor switches should be set at Start, Forward, Remote, and Continuous for each run.  Use the Jog switch on the stepper control to locate the arm at exactly (2q) = 20.   This is done by setting the switches to Local and Jog Enable.  The knob controls the Speed at which you can move the stepper control.  A single toggle of the Jog switch moves the detector 1/16.  Make sure to eliminate backlash (the last motor motion should be in the measurement direction) and reset your switches to the original settings before starting your scan.

(9)   * Close and re-center the Scatter Shield. Activate the radiation by depressing the X-rays ON button.
        * Set the X-ray tube current to 80 micro-amps.
        * Manually scan to find a peak and choose the ratemeter scale (250 or 750) that gives a good needle deflection without pegging. 
        * Then click START in the Data window of the computer program to start the recording process. The program will record the count rate every 1/16 degree. The step rate is 1Hz.  Make sure your scan reaches 40 so that you know there was no slipping during the scan.
        * When the program finishes the data taking, displace the shield to turn off the x-rays.

(10)    Now you will repeat this operation to scan a region for the second order peaks.  For this scan, start the arm at (2q)=50 and choose a more sensitive scale on the ratemeter.

(11)  For each set of data, highlight the Table window and under File on the menu bar choose Export Data to save the data.   Choose the RUN number that you want to save and click OK.  Then save the file.  Convert time to angle and plot the signal as a function of 2q.

(12)  Observe that the peaks represent a "pair" of emission lines that reappear in the second order. In each case, the more energetic K-beta peak is less intense than the longer wavelength K-alpha line. Using the Bragg equation, with d = 0.282 nm for NaCl 

n l = 2d sin q

calculate the wavelength corresponding to each of the four peaks.  Compare these wavelength's to the accepted values for Ka and  Kb for copper.

EXPERIMENT II: X-RAY ABSORPTION

(0) Return the ratemeter scale to the setting used in (9) above.

(1) You will be using the data from the 20-40 range that was taken above as a standard to which the absorption of elemental metals is compared.  You may want to check those peaks now. 

(2)  Place the Zinc filter, TEL 564.009, in the Auxiliary Carriage at Slot #13.  Take the x-ray transmission spectrum for the 20-40 range.

(3)  Remove the Zinc filter and replace it with the Copper filter, TEL 564.006, in Carriage Slot #13. Take the x-ray transmission spectrum for the 20-40 range.

(4)  Remove the Copper filter and replace it with the Nickel filter, TEL 564.004, in Carriage Slot #13.  Take the x-ray transmission spectrum for the 20-40 range.

(5)  Remove the Nickel filter and replace it with the Cobalt filter, TEL 564.008,  in Carriage Slot #13. Take the x-ray transmission spectrum for the 20-40 range.

(6)  a)  Explain in general the absorption of x rays and describe what absorption limits are.  How and why do they depend on the size of the atom?  The absorption limits of Zn, Cu, Ni, and Co are 0.128nm, 0.138nm, 0.149nm, and 0.161nm, respectively.  Are your results consistent with these limits?
      b)   Be sure to explain why copper does not absorb it's own x-rays.