A Co60 gamma irradiator for chemical research

254

Technical notes

TABLE 1. Exposure

Fig. 2 3

4

Film (noscreen) Kodak Kodirex Kodak ultraspeed Dental Film Cea Singul-x

data for the roentgenograms presented

Source to film distances (cm)

Actual exposure times (12 mc I125 source)

Calculated* exposure times for a source of one curie

6

12 min

8 set

7

60 min

40 set

References 1. SPANGENBERC H. D. Engineering Experimental Station News, Vol. 20, p. 48. Ohio State University (1948). 2. MAYNEORD W. V. Lancet 1,276 (1952). 3. SPANGENBERG H. D. and POOL M. L. J. dent. Res. 37,920 (1958). 4. MYERS W. G. and VANDERLEEDEN J. Cl. J. Nucl. Med. July, 149 ( 1960). 5. BERRY P. S. Nucleonics 19, (6), 62 (1961).

A Coao Gamma

Irradiator Research

for Chemical

(First received 19 October 1961 and in final form 17 January 1962) 20

60 min

40 set

* It is possible to make a source of the order of one curie of Pz5 with a diameter of 0.5 mm by our technique. energy is suitable for medical research, and for industrial use. The rapidly decreasing price of I125 supports this statement. Our 12 mc source has also been tried with promising results in connexion with an image intensifier, indicating that sources of higher activities can be of outstanding value during surgical operations. The additional advantages of the monoenergetic radiation from this source as compared with the broader distribution from the usual X-ray tube, for instance the ratio between radiation dose and film blackening, will be studied by some of the authors. Acknowledgments-Suppliers of the activities investigated were Oak Ridge National Laboratory and Nuclear Science and Engineering Corporation, Pittsburgh, U.S.A. The present work was financially supported by Research Grant T-228 from the Swedish Medical Research Council. We are greatly indebted to MR. GEORG ~QUIST for the construction of the exposure mechanism.

Department of Physical Chemistry The Royal Institute of Technology Stockholm, Sweden

P. BERONIUS

Department of Nuclear Chemistry The Royal Institute of Technology Stockholm, Sweden

S. FORBERG

Department of Roentgenolopy The Royal School of Dentistry Stockholm, Sweden Department of Prosthetics The Royal School of Dentistr Stockholm, Sweden

C.-O.

R.

HENRIKSON

SGREMARK

WE HAVE recently constructed a Co60 gamma irradiator which is derived from a design of FIRESTONE and WILLARD(~), but contains also some features of an irradiator described by GREENFIELD et a1.‘2r As compared to the FIRESTONE-WILLARD design, * the present irradiator incorporates the following improvements : (1) it is shielded in all directions except downward, so that it can be placed in most ground-floor locations, (2) it is entirely encased in welded steel for fire protection, (3) it is designed so that the source storage turret serves also as a shipping container, making unnecessary a source transfer at the site of use, (4) it has improved shielding geometry and thickness, and a mechanical interlock, so that it can contain sources of at least 400 c of radiocobalt. The irradiator (Fig. 1) consists essentially of a lead storage turret on top of a lead box in which experiments are placed. A capsule which contains 400 c of radiocobalt is attached to the bottom of a stainlesssteel control rod which passes vertically through the source housing. When in its storage position in the center of the turret, the source is shielded in the upward direction by the source rod and in the downward direction by a lead shutter. In order to put the source in use, the front door is closed, the shutter opened and the source rod lowered into the irradiation chamber. The shutter is similar to a squat stainless-steelstopcock with a lead-filled plug rotating on a shaft which connects with an external handle on the left side of the housing. The hole in the plug is set horizontally for the out-of-use position and vertically, in line with the control rod, to put the source in use. The external handle is mounted on a steel cylinder which rotates in unison with the shutter. A locking bolt slips into a hole in the external cylinder which is set parallel with in this position the source the hole in the shutter; * A second generation of the design, more similar to the device described here, was built at Wisconsin by J. E. Willard, R. J. Neddenriep and R. J. Hanrahan.

FIG. 1. The exposure mechanism used in the pilot studies described in the present paper. The picture shows the 1126-source in exposure position (the iodine source is covered with a plastic) and in non-exposure position. Various types of collimators are shown below.

FIG. 2. Panoramic

roentgenogram

taken with the I 126 X-ray No-screen film.

source

intra-orally

placed.

See note by BERONIUS

et al. 254

FIG. 3. Dental roentgenogram obtained means of the 1125-source described.

FIG. 4. Hand of cadaver taken with the Ilss-source.

by

No-screen film.

Front view of irradiator tubes in place. Shutter handle of the housing, are not visible.

FIG. 1.

with door open and holder for testand mechanical interlock, on left side

See note by HANRAHAN.

Technical notes door can be opened. When the source door is completely closed, the bolt lock can be released and the source lowered. Since the source rod itself prevents moving the shutter when the source is down, this system provides a complete mechanical interlock. In addition, both the source door and the shutter mechanism are equipped with padlocks. The radiation source, a stack 1.6 cm high of cobalt waters 1 cm in diameter, was encapsulated by heliarc welding into the bottom of an 11 in. extension attached by screw thread to the main control rod. In order to use the storage turret as a shipping container the source control rod is removed, a solid cap positioned and bolted down, the external section of the shutter handle removed at a point on the side of the turret section, and a threaded plug inserted through the base plate into an indentation in the shutter, locking it in position. The turret can then be hoisted onto a special steel pallet for shipping. The main shielding section of the housing provides at least 94 in. of lead in all directions from the source except downward. A 2 in. thick lead plate covers the bottom of the irradiation chamber to decrease radiation scattered back from the ground. An extra lead shield in the form of a 6 in. diameter x 6 in. high can is provided on top of the storage turret to decrease radiation scattered upward along the control rod. The side and back walls have “steps” which extend under the turret, supporting it and providing interlocking shielding. Other features of the irradiator include curved 8 in. i.d. access tubes through the side walls, and an aluminum tube sunk 6 ft into the ground directly under the source rod, to provide an emergency storage position. When used as a shipping container for 400 c of Coso, the turret exceeds Bureau of Explosives maximum leakage requirements by about a factor of 10. With the source in use, radiation levels averaging about 2-3 mr/hr at approximately 1 m from the irradiator were found; in addition there were some small areas giving readings as high as lo-15 mr/hr. In order to make the room suitable for continuous occupancy, several layers of lead bricks were placed around the irradiator. This gave average radiation levels at 3 ft of about 0.30.5 mr/hr, with a few spots as high as 2-3 mr/hr. Accidental detachment of the source capsule represents the chief hazard with an irradiator of this type. This problem is minimized in two ways. First, due to the 11 in. length of the capsule, it is possible for a worker to expose the thread above the irradiator for visual inspection and tightening without receiving more than about 5 mr to any portion of his body in the process. Secondly, an area monitor is set so that an increase in the radiation level around the source by as much as 0.3 mr/hr would set off a red warning light and bell. 3

255

Two irradiation geometries are usually used : either 13 x 100 mm test tubes, placed adjacent to the source in a rack, or special annular vessels which surround the source, giving a very high geometrical efficiency. Using the Fricke dosimeter, a dose rate of about 2.4 x 10s r/hr is found for 5 ml samples in the annular vessels, and 0.9 x 10s r/hr for 4 ml samples in the test-tubes. Much larger samples can be irradiated at lower average dose rates. For biological experiments the cobalt capsule can be suspended just inside the irradiation chamber, giving about 3000 r/hr in the central area of the floor. The aluminum holders for the test-tubes or annular vessels can be placed directly into dewars or thermostat baths for irradiations above or below room temperature. As compared with other types of cobalt irradiators, the one described here provides an unusually high radiation intensity per curie of cobalt or per dollar invested. It is also rapid and convenient to work with. The total cost of the irradiator was about $S200 including shipping, gamma alarm system and extra lead brick shielding. Acknowledgments-The author wishes to express his appreciation to MR. DICK RIVERA for making the working drawings, to MR. E. A. J. WARSHYK and the University of Florida Engineering Shops for fabricating the irradiator, and to MR. W. C. BLASKY for aid in assembling and testing it. The work was carried out under the State of Florida Nuclear Science Program. R. J.

HANRAHAN

Department of Chemistry University of Florida Gainesville, Florida, U.S.A. Referqwes 1. FIRESTONE R. F. and WILLARD J. E. Rev. sci. Instrum. 24,904 (1953). 2. GREENFIELD M. A., SILVERMANL. B. and DICKINSON R. W. Nucleonics 10, (12), 65 (1952).

A Simple

Formula

Coefficients

near

for Mass Absorption the K Absorption Edge

(Received 8 February 1962) 1. Introduction IN RADIOISOTOPEX-ray absorption and fluorescence techniques for analysis and measurement of coating thickness, the use of filters to discriminate between Xrays of similar energies is becoming increasingly widespread. These filters operate by virtue of the large discontinuity in mass absorption coefficient as the X-ray energy being transmitted increases through the