Radiation safety in the x-ray diffraction lab

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in the Chemical laboratory

LX. Radiation Safety in the X-Ray Diffraction Lab F. C. WILSON, Plastics Department, E. I. du Pont d e Nemours 8 Co., Inc., Wilmington, Del.

There are numerous factors which contribute to radiation safety in an X-ray diffraction lahorat,ory. By far the most important is a proper attitude of all involved personnel, based on a. knowledge and awareness of the specific hazards involved. No matt,er how many safety devices ark incorporated on X-ray diffraction e q u i p ment, it will be possible for the careless worker to expose himself to radiation and possibly receive a serious burn. Safety devices can only minimize, not eliminate, this hazard. Atthude aside, bhis discussion will be concentrated in two general areas: instrumental design and laboratory and personnel monitoring. In regard i.o both of these, the nat,ure of the hazards should be kept in mind. I n 8. diRraction laboratory, t,he excitation potential is moderate (35 to 50 kv), and there is ususlly present a significant fract,ion of the intenshy as characteristic lines from the target element. T h e two most common elements are copper and molybdenum. I n our work (polymer diffraction) we use copper almost exclusively, and the ehamcteristic K spectrum from copper is soft, corresponding t o &9 kv for the a and @ lines. This "soft" radiation is highly absorbed hy the skin and, hence, direct exposure t o the main beam presents a severe burn hazard. Even a few seconds' exposure t o the direct beam near t,he X-rsy t,uhe can result in a burn which will heal very slowly, if a t all. Cases are known where the amputation of a finger was necessary. A continuous spect,rum of X-rays is also produced in all X-ray tubes. The energy distribot,ion of this continuous spectrum is primarily a function of the excitation potent,ial rather t,han the anode element. Of necessity, this spectrum contains harder (short,er wavelengt,h) X-mys than the chsrncterisbic target lines. For the case of a copper target they are eansiderably harder. Because there is more total energy in the continuous spectrum, and because it is more penetmting, i t const,itutes the major hsaxrd s t moderate dist,ances fl.om the X-rsy h b e . The int,ensity of d l X-ray is diminished by the invewe sqnare of the dist,xrrce from the source. I n addition, scat,t.ering and absal.pt,ion by air forbher redwe the intensity. The harder X-rays of the continuous spectrum are att,enusted less by these two processes, and hence, at, a distance, the remaining X-rays are harder and more penelrating. However, this will generally noa be the typo of exposure expected in the vicinit,y of radioactive sources. Leaks

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Edited by NORMAN V. STEERE, 140 Melbourne Ave., S.E. Minneapolis, Minn. 55414

around diffraction equipment tend to be highly directional and inknsity can vary by orders of magnitade aver very small distances. Because it is difficult to obtain any meaningful reading of radiation intensit,y (e.g., mr per hr), the aim should always he to reduce t.he level of any stray radiat,ion below the limit of detection. This is "easily" possible with these moderate energy sources; and if it is accomplished, one need not worry about the cont,inually lowered "safe levels" of radiation. INSTRUMENTAL DESIGN (ID) All physical factors involved in any diffraction experiment or setup can he conslaerea unaer m e general category of 111s t r ~ ~ m e n tdesign. al It can include, for example, the design and conatruct,ion of the generator, shutter(s), camera or diffractometer, accessory safety devices such as warning lights or interlocks, additional shielding, ete. The interaction of these various components should also be considered. Manufacturers of diffraction equipment have not put sufficient thought in this area. There has been little attempt to standardize certsin features of equipment so that, for example, cameras of one manufacturer could be used with the eeneretar

camera tracks have been standardized. but tube parts and collimators have not been. And i t is not uncommon that instruments from one manufacturer cannot be safely used in an as-received condition. The powder camera and X-ray tube housing, provided by one American manufacturer, can serve as a case in point. The tube housing has no shutter, as such, but only a. cover over a. rotating filter disc. It is not possible to move any camera, into position a t this port when the X-ray tube is on without being exposed to the direct beam. One of the filter positions can be plugged wit,h lead, but with the safety cover over the filter dim one cannot tell whether the lead is blocking the beam without lifting the cover. Assuming that one has "safely" positioned the powder camera in place a t the part, the arrangement is still hazerdoor. The juncture between the camera and t.he tube port is a butt joint, which generally permits serious leakage at, t,his point,. And this is probably the most commonly used powder diffraction setup in this country! Other cases could be cited, but. bhis one suffices. Poor shutter design is the first defect of

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t,his system. There is no proper shut,ter. For today's standards a shutter should be positively acting, positively indicating, and fail-safe. If it is for a camera port, (as opposed to a diffrsctometer porl, where the instrument is seldom moved) it should close automatieally when the camera is removed. In the best designs, t,wo indicating lights are incorporated; one indicating that the port is closed and bhe other indicating that i t is open. These lights should be activat,ed by microswitches responding to shutter position rat.her than by circuit condition if the shut,ter is solenoid actuated. A glance s t the lights will t.hen indicate the position of the shutberopened or closed. I f neither light is on, something is wrong. Either a bulb is bnrned out,, or t,here is some mnlfoncliou. In m y case, appropriate action e m be t,aken. At this point,, t,he at,titude aud awareness of the experiment,er must be such that the malfunction is noted and a p propriate action taken.

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Figure lo.

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Shutter Clored.

Shutter

Open.

13'i~ow I n mrd l b show attch n sh111181., installsd u ~ r a Pickel. generatot.. The large lighls around the t ~ t h eitself indicate t,hat X-rays are being generated. In Figme l b , the left light, which is green, is on, indicating that tho shutter is closed. In

shutter actuating arm is quite apparent it, t,hese two pict,ures. Given a ~ r o o e r l vfunctionine shotter. (Cuntinued on page A981

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tube or shutter port. As mentioned above, a butt joint is not satisfaotary. Proper design a t any juncture requires that a tortuous path be provided for any potential stray X-rays. This is mast satisfactorily accomplished if the shutter element is male (a small "button") and the collimator female. The reverse is more commonly seen, however. The advantage of the male fitting at a port is that an additional 90' "bend" or reflection is required for stray X-rays to escape. In Figure 2 me sketches in cross section of the three types of port-collimtttor combinations discussed above.

Butt J o i n t

Stray X-Rays

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port

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(male) Figure 2.

Port-Collimator Junctionr

Shielding of this type, close ta the source of stray X-rays, is prefersble whenever possible. Another example of poor design for X-ray safety is the Kratky smallangle csmer*diffractometer. As sup plied, it was essentially a butt fit to the X-ray port, for which it w a s presumably designed. Figure 3a gives some indication of the additional lead sheet required to shield this instrument. Not visible are two other additional baffles of lead sheetone on the diffrsctometer and one on the X-ray tube housing around the port. Figure 3b shows this setup in operating status, with the final shield in place and the shutter open, as indicated by the light. This shutter does not, unfortunately, indicate a closed status in a positive manner. Shielding close to the X-ray beam all the way from generation to detection is desirable whenever possible. Such shielding is invariably smaller and generally less expensive and cumbersome than shielding (Continued on page A1001

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Figvre 30.

Addilional Lead Sheiiding ( A r r o w ) .

Figure 3 b .

lnrtrument in Operation.

which blankets an entirc ititricment. There are cases where this appronch is not, practical, however. A wide-angle, threecircle diffractometer is n large, complicated mechanism with a. considerable length of the X-ray beam exposed to the air. Because of the complex motions of this instrument, it is not feasible to shield the beam path, and X-rays are scattered by the ssmple, by the beam stop, and by air. Far this instrument, an overall shield is required snd is shown in Figure 4.

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Figvre 4.

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O v e r d l Shield,

It is fabricated ai dnndmd inlet.layer safety glass and nmt,al. O1.dinary glass suffices since only scattered X-rays from s. pencil-collimeted beam are to be st,tenuated. The beam itself is trapped by alead beam-stop. This shield incorporates an (Continued on page A105)

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"accordion-fold" front and sliding sides for easy access to the instrument, bot these are convenience features rather than sitfetv considerations. .ow se. One laver beam (encountered in a. powder ditrractometer) to a safe level. The cross section of such a beam is much Larger than that of pencil collimated beam, and, as a consequence, the intensity of scabtered radiation is proportionately higher. From the preceding discussion i t should he clear that the basic problem in insbrumental design is shielding of personnel from inadvertent exposure to X-rays, bath direct and scattered. I t is the responsibility of both the manufacturer and the user, but the ultimate responsibility is t,hat of the user. Examples were given as t,o haw this goal may be achieved, but bhey can only be examples. So much variability is encountered in various kinds of equipment that each case must be eonsidered seoaratelv. O w ..rher hni.nnl shodd t,r ~ n e u t ~ ~ m ~ d Thr X - t ~ yrube is nor the tmly s,ww of nldiatim~. The rrrritirr iulm, h ~ u w di n the transformer cme, can also give rise to very intense and hard radiation, partioularly as they age. Thus, this potential source should be monitored or surveyed regularly, since it can be the major source of general exposure of the sort leading to genetio damage.

MONITORING Once a. properly designed X-ray diffraction setup is put into operation, its safe functioning can be determined only by some sort of monitoring procedure. The most important instrument for this purpose is a portable survey meter (shown a t the right in Fig. 5). No setup should he considered safe unt,il i t has been thoroughly surveyed with such an instrument which gives sn instantaneous indication of X-rav intensitv. One of the hazards of x-ray equipment is that stray (unexpected?) leaks tend to be highly directional, and until a given setup has been carefully surveyed one cannot be sure that such leaks are not present. I n diffraction work, as opposed to fluorescence, setups are changed frequently (exchanging one camera for another, etc.), and monitoring should be done with every change, as well

Figure 5.

Loboratory Monitort.

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ss on a routine basis. Proper functiouing of the survey meter call be checked with a low-level radioactive source-a luminous watch dial can serve as such a source-but a safe source usually is furnished with a. survey meter. The inst,rument on the left in Figure 4 is e general laboratory monitor and alarm. This indicates the general level of radiation in the vicinity of its detectors (Geiger tubes) and sounds an alarm if a preset level is exceeded. It is basically a catastrophe alarm, since i t will generally detect only air-scattered X-rays. For t,he alarm to sound, the magnitude of a leak must be high, such as a wide open part or a camera not properly seated. As such, it is a very useful monitor, but it is useless indetecting the typical, highly directional leak which is easily found with bhe portable meter. Film badges are comnlonly used in Xray labs as after-the-fact personnel manitoring devices. I n many areas, t,bere are legal requirements for thier use, but their value in a diffraction laborstory is questionable. As indicated above, stray radiation around diffraction eqiipment tends to be highly direet,ional. One could be exposed to such a leak and have it miss the badge completely, which would lead to a false sense of security. Alternatively, and less likely, the badge could be exposed by such a leak, and one would have no idea of the extent of general body radiation, except far a, highly improvable upper limit. Finger b d g e s could be used, since it is the hauds that are most often in posit,ion to be exposed by the direct beam, but one would have to remember to use them when a p propriate. They would not be on the fingertips, and hence would also miss t,he most likely exposure of the fingertips t,o the direct beam resulting from placing or aligning a sample in a device wit,h an open shutter. With such an exposure, the burn would probably be apparent before the badge was processed.