guished readily from the MSssbauer photons with an N a I detector. Therefore, in this ease, the background contr ibut ion will be relatively large. Pulse-height spectra (PHS) of the BaSn~gmO3 source through an SnO2-A1203 absorber (5 mg S n / e m 2) appear in Fig. 2. With an Mo foil over the detector window, a shoulder appears in the P H S on the left of the maximum. In addition, the full width at half-height is about 30% greater than tha t for the P H S without the foil. The results are tabula ted in Table I. The spectra recorded with and without the Mo foil are shown in Fig. 3 (note the change in the relative intensi ty scale). I t is evident f rom the table tha t there is a 68% decrease in A/B f rom the initial value, whereas A s . o J A s , , remained constant within the experimental error.

For quant i ta t ive applications it is impor tan t to employ concentrat ions of analyte where the analytical response is at a maximum, i.e., the change in absorp- t ion intensi ty per unit change in concentration. In this work the SnO2 concentrat ions correspond to effective thicknesses f rom about 1.0 to 2.0 and the experimental linewidths are about 1.1 mm/see . The decrease in count rate due to the addit ional ~-Sn absorber was on the order of 25%. Although there is some overlap of the SnO2 and/~-Sn absorption lines,

the resolving of the lines via a digital compute r appeared to be adequate for this study. I t would be desirable, if possible, to choose an internal s tandard which does not overlap the analyte absorpt ion line.

These results suggest t ha t internal s tandardiza t ion would also be useful in cases where higher energy precursor radiat ion accompanies the M6ssbauer photons such as in 57Fe spect rometry . An impor t an t advan tage of using this concept is t ha t the background contr ibut ion would be el iminated no m a t t e r if its origin arises f rom the source or analyte absorber. Additional advantages include the el imination of other sources of var iabi l i ty in E such as drift in detector high voltage, single-channel analyzer setting, amplifier gain, and a change in the sou rce - sample - detector geometry. The possibili ty t ha t internal s tandardizat ion m a y be useful for measuring relative recoil-free fractions will be the subject of a future study.

1. P. A. Pella and J. R. DeVoe, Anal. Chem. 42, 1833 (1970). 2. P. A. Pella, J. R. DeVoe, D. K. Snediker, and L. May, Anal.

Chem. 41, 46 (1969). 3. F. C. Ruegg, J. J. Spijkerman, and J. R. DeVoe, Rev. Sci.

Instrum. 36, 356 (1965). 4. J. R. DeVoe, Ed., NBS Tech. Note 501 (1969), pp. 19-28.

Safety Interlock for Raman Spectrometer Laser Beam

A. M. Bartz, C. B. Pratt, and R. O. Kagel

Chemical Physics Research Laboratory, The Dow Chemical Company, Midland, Michigan 48640 (Received 31 December 1970; revision received 3 March 1971)

A Raman spectrometer equipped with a 350 mW laser required a means to confine and reduce the laser beam intensity to avoid eye injury. This is a description of the filte1~shutter system designed to reduce the likelihood of an accidental exposure. INDEX HEADINGS: Laser safety; Raman instrumentation; Apparatus to avoid eye exposure to

laser beam.

I N T R O D U C T I O N

When our Spex Ramalog spect rometer was recently equipped with a Coherent Radia t ion Laborator ies laser with an ou tpu t capabi l i ty of 350 m W in a single line it clearly represented a hazard of inadver ten t eye exposure to the direct or reflected beam b y operat ing personnel. Since it is necessary to see the beam (actually, dust scat tered light) for op t imum al ignment with the sample and instrument , it is not practical to s imply shut the beam off when changing samples but ra ther one wants to a t t enua te it so it is bo th useful and safe. Reference 1 suggests tha t a level of 70 gW of laser radiat ion is a safe max imum for direct eye exposure, so an a t tenua t ion of ~ 104 would be required for our laser. Using two 1% filters in series (104

at tenuat ion) , however, reduced the beam intensi ty so t ha t i t could not be seen adequate ly for a l ignment purposes. An a t tenua t ion of 103 was used with the assumpt ion t ha t the entire beam could not enter the eye by reflection because its divergent aper ture in the sample c o m p a r t m e n t is larger by a factor of 10 t han the pupil of the h u m a n eye for all reasonable positions an opera tor might assume. The intent of the sys tem is to avoid the accidental exposure to the full strength beam by a forgetful or preoccupied opera tor ra ther t han making it absolutely safe for someone to look directly into the laser beam under all conditions. To retain a degree of flexibility as an exper imental piece of equipment it had to have options which make it still dangerous; our s y s t e m merely reduces the odds of an acc iden t

474 Volume 25, Number 4, 1971 APPLIED SPECTROSCOPY

FIG. 2. View of rear of instrument including three-stage beam alternative equipment of the interlock system (exit--right end).

h APPARATUS

Figure 1 is an over-all view of the instrument. The laser, not visible, is directly behind the large main frame on a horizontal ledge, its beam directed into

FIG. 1. Over-all view of laser-Raman instrument.

the main frame at the far left end. A door in the main frame at the left end is hereafter called end door while the sample door is directly above the large micrometer screw at the left end of the spectrometer.

÷24 VDC

SHUTTER

( ~ WriT LITE "POWER"

o C L O 6 E D ~

a ~ 'OPEN / I i

(~) o LOC~D

c ~ c UNLOCRED

RED LITE ~ M ~ E DOOR

IN

/ c o w SHUTTER [

KEY OVER. RIDE

1% FILTER KEY OVER-

RIDE

NC I

f'llO VAC J o No

10~

KNOB

• IN 2UT 1F~~~ ® {~0C]0~D Or/~ OpEN / I N ~! ~ _ _ __

END 1% AUTO ' FILTER OC~D DOOR ~__ SCATTER -~ RED LITE PLATE

~ SAMPLE DOOR OUT

IN SAMPLE

RED LITE ",I...7 SHUTTER ~ MANUAL

RAMAN LASER REAM SAFETY INTERLOCK

END DOOR

FIG. 3. Diagram illustrating mechanical components and electrical schematic of the Ramaa-laser beam safety interlock.

APPLIED SPECTROSCOPY 475

Nomenclature. In means filter or shutter is in the beam, reducing its intensity or blocking it--safe condition, red lights out, solenoids deenergized and down. Out means safety devices, filters and shutter, are out of the beam, not attenuating it, dangerous condition, red light on, solenoids energized and up. Manual filter is in the beam when knob is pulled toward operator and red light is out. Manual filter is out of beanl when knob is pushed forward flush with panel and red light is on.

Operating Instructions. To Open Doors. Doors may be unlocked if the manual 10% filter is in (knob out) and the shutter or 1% filter is in. 1% Filter. The 1% filter may be taken out of the beam if both doors are closed and scatter plate is down by using autofilter switch. Key switch will override above conditions. Key switch initiates flasher.

Shutter. The shutter may be taken out of the beam if both doors are closed and scatter plate is down or 1% autofilter is in by using shutter switch. Key switch will override above conditions. Key switch initiates flasher.

Full Beam Operation. (A) Three toggle switches down--shut- ter out, red light on; 1% autofilter out, red light on; doors closed and locked, red light out. (B) Manual filter out--knob in, red light on.

Standby for night and weekends. (A) All red lights out--safe and no power on solenoids.

Fro. 4. Operating instructions for safety interlock.

The scatter plate with added side shields is a t tached to a slit-viewing per iscope-- the tubular object protruding up at the left end of the spectrometer. The interlock control panel is the one with the three

l ights on in the lower left corner on the front of the instrument.

Figure 2 is a view of the rear of the instrument including the laser with the three-stage beam at tenua- t ion equipment of the interlock system on the exit (right) end. The third compar tment away from the laser houses a rectangular-shaped piece of metal with two holes in it, one of which is covered with a 1% transmit t ing neutral density filter. This filter holder can slide up, placing the hole in the beam path, when the solenoid above it is energized and will slide down by spring action, inserting the filter in the beam, when the solenoid is not energized. When it is in fact down, it activates two position sensing switches (3 and 4 of Fig. 3) used in the logic and control system of Fig. 3.

The middle or second compar tment from the laser looks much like the third except it is ro ta ted 90 ° and the filter holder is moved by a manual coupling (from front control panel). The filter in this compar tment is a 10% transmit t ing neutral density filter so when both are in the beam there is a 10 ~ at tenuation.

The compar tment nearest the laser looks like the third except the rectangular piece has only one hole which is in the beam when the piece is up, or solenoid excited. When it is down the beam is shut off; this is called the shutter . Both filters and the shut ter are slightly ti l ted to avoid returning the reflected beam to the laser where mirror damage could occur.

The laser beam is directed into the ins t rument and ult imately up into the sample compar tment by a system of plane mirrors. Access to the beam is possible by means of the end door and the sample door so these are locked with electromechanical latches and sensing switches used to verify actual locking action.

II. ELECTRICAL-LOGIC CIRCUIT

Figure 3 is a diagram illustrating mechanical components on the right and an electrical schematic to the left of the four solenoids and knob. The 24 V dc supply is on the far left; the solenoids, which need 24 V dc to actuate, are near the right mated to a mechanical fixture. Start ing at the top, the shut ter solenoid can be actuated by the shut ter in -out switch if both doors are closed and the scat ter plate down (glass switch closed) or if the 107o auto filter is in. The shut ter position is sensed by switches 1 and 2, their position in the circuit is shown by like numbers. The shut ter can be moved out of the beam when there is no voltage at the in-out switch by a key override switch; if this is used, however, the relay is actuated which in turn initiates the flasher and flashing light circuit.

The manual 10% filter is coupled to a front panel knob and actuates just one switch (No. 9).

The 1% auto filter solenoid in-out switch will have 24 V dc available only if both doors are closed and the scat ter plate is down. With no voltage the filter stays in the beam. The filter too can be removed with a key override switch which turns on the flasking alarm light.

The doors may be unlocked if the manual filter is in and either the shut ter or the 1% filter is also in. The operator, therefore, cannot open the doors unless the beam is either blocked or reduced to 0 .1%; after the sample door is opened the operator may (having observed the sample and beam condition) remove the manual filter to increase the beam intensi ty to 1%. He may at any t ime have the full intensi ty by using the key override switches.

Figure 4 is a list of operating instructions as posted on the instrument.

I I I . CONCLUSION

The system operates as designed. I t does ~ot make the ins t rument absolutely safe; it is a safeguard intended to prevent the operator from suddenly opening the sample compar tment with the full laser beam on; it does that . I t forces the operator to take a deliberate action (and hopefully to think) when he wants to increase his laser beam intensi ty on the sample from 0.1% to 1% to 10% to 100% with the doors open.

1. W. P. Hanson, Appl. Opt. 6, 1973 (1967).

476 Volume 25, Number 4, 1971