ropics in..
.
Chemical Instrumenfation feature Edited by GALEN W. EWING, Seton Holi University, So. O r a n g e ,
N.J.
07079
I t is a n honor to a s s u m e t h e editorship of this series from Dr. Lewin whose unflagging efforts over t h e past seven years have resulted in such a notable series. 1 shall do my best t o uphold his high standards. The general titleof thecolumn suggests t h a t all phases of instrumentation of chemical significance are suitable for inclusion. Thus I shall attempt to provide articles describing instrumentation of primary application in chemical engineering, and in physical, biochemical, and other fields, a s well a s in t h e area where my own experience lies, analytical chemistry. Also, i trust that readers will not insist on any very precise boundaries t o t h e domain properly referred to a s "instrumentation." Comments from readers, a n d especially suggestions for topics to be covered (preferably with names of appropriate authors) will be most welcome. Galen W. Ewing
XXXII. X-Ray Diffraction Analysis' Part One-Safety
and Generators
Reuben Rudman, Deporfmeni o f Chemisfry, Brookhoven Nofionol Loborofory, Upion, N.Y. 7 1973 This review is limited to a disrussion of (,he principles, features, and availxhilit,y of X-ray dilTractirrtr eqnipmenb. X-ray ditl'mrtiorl theory has been kepi to a minimum. (For more detailed discussions of thermtiral priwiples the reader is advised tu consnll the many texts that, are available (5-11); an extensive list of hooks iu this and closely related fields is f o ~ l u d in thc catalog of l'olyrllystal B w k Service.) I)wing the POIITSB of a n X-ray diffmction amlysir, the investigator often uses appitrntw which is not unique t o lhia field. Examples are: mnterials for sample prepamtion, radinliou detertors, electnmir rompor~enls for detector pa~iel.;, voltage stnidizers, and film eomparalon. 111 such eases representative examples of nvnilnhle equipmeni are given, with emphasis on the equipment designed for specific applications to the field of X-ray diffraoiim. I n l i w wilh the policy of this series, the discus~ion has hem limited, wilh few exceptions, to mn~wfartrwers with ades o~rt,lelsi l l the 1T.S. The scope of this review is further limited by the fact that apparatus used in the determination oi the crystnl atrncture from intellsily data, a- well as those instn~mentsdesigned fur
*Prepared under the nnspieea of the Atomir Xileigy Commiaaiom
st,udying crystal perfertim nud si~rincer, have not been inrh~ded. The preparation of lhis nrlide would not have heen possible withoub the err operation of the representnt,ives of the many companies whore prodwt* are discussed. I would like lo thank them for their assistance in f,mishing upto-date technicnl information and photographs. The scale of price ranges m d n complete listing of "3lanufactwers and Ilistritw tors" are pnhlished as all nppendix t,o this installment (page A42). I!eforences in laler iosl.nllments will be made lo this list.
A erystd, eonsistingof atoms arranged in a pattern which is repeated regularly in three dimensions, arts as n. three-dimem sianal diffrartion grat,ing for X-rays (wil h wavelengths from n h m t 0.&2.5 A). The X-rays interact wilh the ntomv (more precisely, with their electro~ts)present in the cryclal and the waves of scattered Xrays reinforce one auolher in cerlaiin directions. Bragg has shown that reinforeement will orcur when the rays diffracted from pxrxllel planes are in phase with one another, i.e., when the path difference is sn integral mtmher of wavelengths. From reference to Fignre 1 we see lhat this ran be represented by nh = 2rl silt 8,
Reuben Rudrnan obtained his PhD. in X-roy cry$tollogrophy and inorganic chemistry from the Polytechnic lnrtitvte of Brooklyn in 1966. He received his B.A. from Yerhiva University (1957). Dr. Rudmon is in the latter half of a two-year appointment or a Rereorch Asrociote ot Brookhaven National Laboratory, ofter which he intends to pursue on academic eoreer. His principal research interests include low-temperature X-roy diffraction techniques, rolid-solid phore tronsilions, ond materials research. He is presently studying ~ r y r t ostructures l "ring single-crystal X-roy and neutron diffraction techniques. Dr. Rvdmon is a member of the American Crystallographic Arrociotion, the American Chemical Society and the American Association for the Advancement of Science. He has also conrvlted for industry.
where h is the wavelength, d is the distxnre between parallel planes, 8 is the angle of incidence (and reflection) of the X-rays, and n is an integer. This is the well known Bragg Law.
Figure 1. Brogg Lor; Reinforcement of the diffracted rays occurs when the path difference integrol number of wovelengths is equal to - on i.e., AB f BC = nh, where n is on integer.
-
-
Since AB = BC = d sin 8, oh = 2d $in 8.
(Cmlinzred oan page A S ) Volume 44, Number 1, January 1967
/
A7
Chemical Instrumentation X-rays may he produced when highenergy electrons strike matter of any sort. I n practice, X-rays are produced in "Xray tubes" when electrons, emitted from a hot filament (cathode), are accelerated toward and strike a metallic target (anode). The cathode is generally maintained a t a potential of thirty to fifty thousand volts relative to the anode. The rsdidiation emitted when the electrons strike the target can he classified as: (a) the continuous spectrum; ( 6 ) the characteristic or line spectrum. The continuous spectrum is due to Xrays emitted by electrons interacting with the fields of force of the nuclei in atoms of the target material. The minimum wavelength in the continuous spectrum is a function of the accelerating voltage only; it is independent of the target material. An increase in the applied voltage causes the minimum wavelength to shift toward smaller values (shorter wavelengths correspond to higher frequencies or higher energies). When the applied voltage is wfficiently hieh. - , the emitted radiation contains a group of sharp lines (the line spectrum) in addition to the continuous spectrum. These lines occur a t specific wavelengths whose locations are characteristic of the composition of the target. On striking the target, the impinging electrons interact with electrons of the target atoms, raising them to higher energy levels or ejecting them from the atoms. A vacancy then exists in the original electron energy level; it is filled almost instantaneously by an electron from a, higher level. When an electronic transition of this sort occurs, from a higher to a lower energy level, a photon is emitted. The wavelength (or frwuencv) - . of the hot on is rs function of the energy difference between the two levels. I t is therefore characteristic of the target element. All types of X-ray apparatus have eerb i n characteristics in common: an Xray unit consisting essentially of a transformer, which converts line voltage to the high voltwes needed, together with the necessary controls; an X-ray tube; a system of slits (collimators) to define the x-ray beam; mems of holding, orienting, and rotating the specimen in the X-ray beam; and radiation sensitive devices, usually film or counters, to record the scat, tered radiation.
.
RADIATION HAZARDSAND SAFETY PRECAUTIONS The potential danger to the operator of X-rsy diffraction equipment, and to others, makes it mandatory for him to he familiar with basic safety precautions. This discussion of radiation hazards and safety precautions associated with X-ray diffraction equipment has been taken from the following sources: Report of Apparatus and Standards Committee of the American Crystallographic Association (la); Report of the International Union of Crystallography, Commission of Crystallographic Apparatus (9);and Section 6, Volume I11 of the International Tables for X-Ray Crystal-
A8
/
Journal o f Chemical Education
lography (3). Each of these reports contains references to other material descrih ing safety precautions and governmental regnlations. Exposure to ionizing radiation can result in injuries that manifest themselves in the exposed individual and in his descendants; these are called somatic and genetic injuries, respectively. The objectives of radiation protection are t,o prevent or minimize somatic injuries of persons occupationally exposed to ionizing radiation and to minimize deterioration in the genetic constitution of the population. I t should be strensed that reeommendations in respect of the instdation and operation of the equipment are not in themselves sufficient to guarantee adequate protection. Such protection depends largely on the expert knowledge of the staff and on their cooperation in carrying out the instructions prepared in the interests of radiation protection. When X-ray analysis equipment is used for research purposes, the nonroutine nature of the work greatly enhances the possibility of radiation injuries. I t is therefore essential that the personnel should be aware of the hazards and how to guard against them. I n vsrions localities a number of regulations already exist, which usually apply to the use of equipment in factories, educational establishments, etc., and to which individuals are required, or advised, to adhere. One aspect of the nature of radiation leakage as it applies generally to X-ray diffraction equipment must he stressed: I n many cases, this type of leakage is in the formof highly collimated beams of high intensity. Thus the leakage a t a. given location may be truly negligible while a fraction of an inch from this location it may reach dangerous proportions. This fact should always he kept in mind when checking X-ray diffraction equipment for radiation leakage. The following comment.? have been adapted from Reference ( 2 ) : "Sources of Unwanted Radiation. Xray crystallographic techniques involve the use of an X-ray generator to which is coupled some form of apparatus such as a photographic camera or counter diffractometer. I n operation, dangerous leakage and scattering of radiation can occur in the following ways. (1) Escape of primary X-rays when a tubehousing window is open and apparatus is not in position a t that window. If, while an X-ray tube is energized, a tube-window is open when apparatus is not in the operating position a t that window, there can be an extremely dangerous escape of the primary beam. I t is essential that this must not occur and that some safety interlock device between tuhehousing and diffraction apparatus be used. Safety measures adopted should (a)have a fail-safe characteristic and ( b ) not restrict the responsible operator from making those adiustments recoenized as an es-
beam a t interfaces between an open window of a tube-housing and the diffraction apparatus heing used a t the window. Leakage and scattering of X-rays, from
the region of coupling between an X-ray tube window and the apparatus in use st that. window, ir, one of the most serious hazards, and is likely to give rise to nsrrow but intense heems of stray radiation. (3) Escape of the residual primary X-ray beam from the diffraction apparatus. (4) Leakage, from openings in the diffraction apparatus or through the walls of the apparatns, of radiation (including unwanted fluorescent radiation) scattered by the air or by mechanical parts of the apparatus. (5) Leakage of radiation generated by valve rectifiers in the high-voltage powerunits of X-ray generators. Thermionic rectifiers used in X-ray generators e m sot as powerful sources of penetrating X-reys. Causes include under-running of rectifier filaments, and, in a faulty valve, the passage of some inverse current a t high voltage. Wdls of high-voltage circuit tanks have not allwsys been adequate to prevent this radiation from escaping. bhnitoring of equipment should include this region of the generator. ( 6 ) Penetration of radiation through the walls and (closed) window-shutters of an improperly shielded X-ray tubehousing. "Detection of Straz~Radiation: Personnel Monitoring. I t is essential that any experimental arrsngement be surveyed with a counter-tube sensitive to the soft X-rays normally used in X-ray diffraction. The use of portable survey meters employing counter tubes with low inherent filtration i . . low-absobsorption windows and high quantum efficiency in the lowenergy X-ray region) is recommended far surveying installations. If a thin sheet of lead long enough to cover the counter tube length and extended about 10 cm b~ yond the window is wrapped around the tuhe, the counter will he directionally sensitive and simplify the location of radiatian leaks. Large pieces of film placed around the apparatus may be used to locate narrowly defined leaks, particularly if exposed a sufficiently long time (i.e., 1 hr or more). Film badges worn a n the chest, wrist, or finger may be used for personnel monitoring, but it should he noted bhat they are liable to provide a false sense of security because the stray radiation which may occur in X-rsy diffraction apparatus is often in the f o ~ mof narrow lowenergy beams. The problems involved in complying with local and national safety codes and regulations, insurance, etc., should he taken into consideration in setting up a code of laboratory practice. The a p pointment of a person responsible for the proper instruction of all personnel, and the keeping of inspection records and checking of X-ray units at regular intervals are, however. most imoortant. No one should
being mounted, or existing apparatus modified or realigned, i t is necessary to survey the apparatus. Regular depart mental meetings on safety practices, reviewing the safety of existing apparatus, (Continued a page A10)
Chemical Instrumentation etc., are useful and often necessary to emphnsiae the importance of the problem. "Factors Responsible ,for the Hazards. E a r h of the vmious possible sources of radiation hazard listed above res~dtsfrom oue or more uf the following: (a) Negligenee by the war. ( b ) Failitre by the user to recognize and deal with incompatibility of design when eonpling diffraction appnratrls t o Xray generators of different mnnnfactnring origin. (e) F n d t y eqnipment. ((1) Unsalisfartorv design of e q.u i p ment. T h e use of X-ray 'warning' signs a t the eutrance to an X-ray Iisharatory, small signs attached to the X-ray units, lights indicating what apparatus is in operation, et,c., are usefrrl in minimining the risk of accidental exposure. Wherever possible it is prefernhle to place X-ray unibs in 8. room separate from where personnel do other w ~ r k . " ~ A number of manually and elect,rically operated shutters and conplings have heen desmibed in the literat,ure. Most of these are not commercially available (as yet), hut can be built easily. Iletails of their constrtrction are found in ( l a ) and the references listed in (3). However, the followinp should be kept in mind (la): " I t is realized that metallic lead may e ~ s i l ybe shaped by hand t o satisfy most coupling reqi~irements;however, because it is so readily deformable, it is unaccep t,ahle as a. material for couplings which are to be used over and over again. Even when rlsed on an emergency basis, a lead coupling should be monit,ored with counters or film enrh time i t is used to msure the operator that. excessive radiat,ion leakage dues not occur throughsomesubtle defect dne to inadvertent deformation of the coupling. T h e only exception to this recommendation is if lead is used as au X-ray ahsorbing lining iu a coupling
made, for example, of steel or brass, where tho lend would be protected from eont,act with other materials. These comments do not pertain to the use of lead as n g a r ket material. I n fact,, if properly used, lead is a most satisfactory gasket mat,erinl for purposes of radiation shielding." Finally, any user of X-ray diilraetion equipment should be aware of the fact that the high-voltage (25-100 kvp) transformers are dangeroos. Whenever it is necessary to handle the trallsforrner or cables (e.g., when changing an X-ray tube), always he sure to gn,tnal 1,het,rxnsformer first.
Monitoring Devices
~
' I am thankful t,o Bob Blumerithal (of E~lraf-Nanius)forbringingthefollowing to my altentian: The Dutch government is cousidering revision of the law governing the protection of people working with X-ray eqoipment. Among the recommwded additions are: I. Persons working wilh diffraction appnrat,ns mtwt have a medical check-up t,nice a y e a . 2. Hooms with X-ray apparat,ns m m t have a warning sign on the door. 3. Safety prera,~t,ionsin laboratories for .itudent. and/or inexperienced petsons must he more rigorotrs than for experienced personnel. 4. The X-ray tube window may permit X-raw to "ass onlv when n camera is antomittieally. 5. When X-rays are on, with a camera in place, a. conspicuous light mnst be illuminated. There shall he no X-rays if the hrdb bums out. 6. A radiation hame, which will prevent leakage of secondary radiation from the shut,ter, must be provided.
A10
/
Journal o f Chemical Education
Dosimeters, Cocmlem, ele. A list of companies marketing radiation monitoring equipment is found in the llireatory of Products and Services for Iladioactivity Laboratories (4) under the following headings: dosimeters and personal and radiation survey equipment. Film Badges. A pamphlet d r s r r i b i n ~ the principles of photographic radiation dosimetry and giving detailed information concerning t,heir line of personal monitoring films is available from Eastrnao Kodak (Kodak Sales Service Pamphlet No. P-31). Kodak Personal Monitoring Film, Type 3 is used for detecting aud monit,oring bet,&-, gamma-, and X-radiation. The packet is of the 2-film type; one film is of exceedingly high sensitivity, the other slow. This ernrllsian combination permits therecording of ioniziug radiation dosage over a n extremely wide range (13 milliroent,gens t o 1800 roentgen of radium-gamma radihon). They are available in packages containing 150 2film packets a t $8.35/prtcka,ge. I h P o n t has 8, line of do~imeterfilm packets with one or two films per unit (Types 544, 545, 556, 558); price per package of 150 is approximately $4.50. They may be supplemented with certain added features snch as 5- and &digit consecutive nnmhers indicated on the packet m d visible on lhe film; and lead shield over part of the paoket. Tracerlab offers a commercial film badge service. Their compartmentalized badge not only contains m interchangeable short-term film packet that is replaced and processed by Tracerlab on a weekly, biweekly, or monthly barin, but also a permanent safety back-up film packet. Each year the badge containing the hackup film is replaced. Should the shorL term film become lost or damaged, the back-up film will serve as the permanent record. It may also be nned to verify dosages recorded on the shoreterm film. Eleetronio dat,a processing equipment enables reports to he sent to the suhseriher within 24-48 hr after receipt of the film. Typical prices/uoit are $0.65 for 3-10 units and $0.50 for 61-100 units. Picker alsa offers a film-badge service. Shielding Materials Commericd ssourres of lead bricks, sheet,, shot, tubing, molded shapes, and lead bonded to concrete and wood are found nuder "Shielding Materials: Lead and Compoonds," in llef. (4). One part,icularly interesting material is Chemtrre
31 (Chemtree Corp.). Chemtree 31 is n leaded morta%hich, with the addition of wat,er, may be poured, ea3t or troweled. If i t is self-snpporting, easily bonded to most common struct~lral materials and can be drilled, sawed, and otherwise worked. By weight,, i t is nearly all lead, and thus hai approximately the identical shielding effects against X-rays as does lead. Ilowever, its density (6 g m / w ) is about half that of lead and i t must he twice as thirk as lead for eqml at,tennation. A innnher of other iormnlalions w e alsa ~vailileble; in small quantilie.; the prices are approximately $1/1h. Another itseftd material is the turrgsteti alloy, Kennertium (T. h l . of Ken~mmetd, Inc.). This allov is available in two formulations; one is 50YG more dense than lead, the other 627& It ran he marhined (when carbide tooling is employecl) and joined lo itself or other metals hy silver or copper brasing, soldering, press or shrink fitting, or hy merhenieal at,tachment. Becallsf! this mat,el.ial is quite hard i t does imt possess the disadvantages of lead that were desrribed ahwe. 3M Lend Smlch Tape No. 420 is nu adhesive-backed lead tape which is extremely useful for preverhng radiation leaks fmm tohe hoosiugs and window shutter%
Warning Signs Colorfd signs displaying the i n t e n m tional radiatiuu symbol and containing a warning of specific int,et.est lo users of Xray equipment are available, a t no cost, from Siemens Amerie~,Inr. (Fig. 2).
CAUTION
X-llAY IOYPMINT PRODUCES IIAOIAT8ON I
