X-RAYS AND ORIENTATION OF QUARTZ CRYSTALS 



295 



the target used. In most cases the best filter-metal for the Kj3 radiation 

 is the second element below the target metal in the periodic table. For 

 example, a nickel filter 0.0005 inches thick is used with the copper target 

 which is used for X-ray goniometry of quartz. 



The minimum voltage that will excite the Kai radiation of copper (wave- 

 length X = 1.5374A) is 8.86 kv., but a voltage of 30 or 40 kv. is usually 

 used to obtain adequate intensity. 



1.0 I.I 1.2 13 14 15 16 1.7 

 WAVE LENGTH IN ANGSTROM UNITS 



Fig. 3.2 — Variation of intensity with wave-length of X-rays from a copper target at 



20,000 volts (estimated) 



3.2 Detection of X-Rays 



X-rays may be detected by means of heat effects, fluorescence of appro- 

 priate screens, photographic efifects and by ionization of gases. 



Heat measuring devices are not suited to routine intensity measurements. 

 The other three means of X-ray detection are all being used for various 

 types of quartz work but the ionization of gases is used most widely. 



The ionization method involves a chamber which consists essentially of 

 a gas-filled metallic cylinder containing an electrode. A potential is main- 

 tained between the electrode and the cylinder so that when the gas is 

 ionized by the X-rays the positive and negative ions produced are drawn 

 to the oppositely charged electrodes, thus constituting an electric current, 

 which current is proportional to the intensity of the X-rays entering the 

 chamber. This current is commonly indicated by a current meter pre- 

 ceded by a special vacuum tube amplifier. 



The gas used in the chamber is often air but methyl bromide is about 

 30 times more readily ionized, xenon 155 times. The only advantage of 

 air is that the chamber need not be gas-tight. The entrance to a sealed 

 ionization chamber must be closed by a substance which does not result 

 in undesirable absorption of the X-rays. Since a nickel filter is needed 



