US2925509A - Low energy counting chambers - Google Patents

Description

Feb. 16, 1960 P. M. HAYES LOW ENERGY COUNTING CHAMBERS Filed Aug. 10, 1956 0 B MY VA N M L U A P Y B ATTORNEY.

' 2,925,509 LOW ENERGY COUNTING CHAMBERS Paul M. Hayes, Lafayette, Calif., assignor to the United States of America as represented by the United States i Atomic Energy Commission Application August 10,1956, Serial No. 603,462 7 1 Claim. Cl. 313-93) This invention relates, in general, to a superior means for introducing low energy beta particles into a counting chamber and, more particularly, .to window means for such a chamber which provides for the more efficient counting of low energy beta particles and increased functional reliability. V

In conventional practice, diverse types of ionization chambers and counting tubes are. employed for the detection and determination of'ionizing .radiationquantities.

These devices generally comprise a closed chamber wherein there is disposed two electrode surfaces of various configurations. Usually, one electrode is suspended centrally within the chamber and the other is arranged to at least partially encircle the first. t

For measurement and counting of low energy radia 1 tion, especially low energy beta particles (energetic electrons), the structure of the ionization chamber or counter tube (proportional or Geiger Miiller) usually is hellshaped. The walls may be made of metal to serve as one electrode and the central electrode may be supported by a vacuum tight feed-through insulator disposed inthe Wall. Operating conditions depend on the type of gas filling and a variety of electrical circuitsmay be employed therewith in accordance with established practice.

To adapt such a chamber for the detection and counting of low energy beta particles, the mouth of the chamber is provided with a covering of a material which has as low as possible absorptivity for beta radiation. Also,

the covering is made as thin as possible for the same reason. .Thin mica sheets and thin films of various plastics such. as cellophane, polyamide and polyethylene resins have been employed heretoforephowever, these materials leave much to be desired from the standpoint of durability, ease of mounting and radiation transmission characteristics. conventionally, these covered openings are termed windows to indicate the relatively low absorption characteristic or radiation transparency thereof for beta radiation as compared to the remainder of the chamber walls. It will be understood that these windows are a special adaption of counter tubes and chambers to provide for the better measurement and detection of low-energy beta radiation.

It has now been discovered that remarkably superior windows for low energy beta radiation counting devices, of the character described, are provided by utilizing thin films of a polyester resin formed by the condensation of ethylene glycol and terephthalic acid. Such resin film can be fabricated in very thin dimensions which, taken together with excellent radiation transparency characteristics of the material, allows a much larger proportion of low energy beta particles to pass therethrough as compared with previous materials. Moreover, the excellent mechanical characteristics of the film permits improvements in the structure, durability and ease of repair of the counting chamber.

Accordingly, it is an object of the invention to provide an ionization chamber which is superior for the detection and counting of low energy ionizing radiation.

- Another object of the invention is to provide a more durable beta radiation transparent window for an ionization chamber used'for detecting and counting low energy beta radiation.

Still another object of the invention is to utilize films prepared from polyester resins of the ethylene glycol- Q terephth'alate type for fabricating the windows of ionization chambers. l

A further object of theinvention is to provide a win- 3 dow for ionization chambers which is thin and of low absorptive density and is thereby of relatively high transparency ,to low energy beta radiation.

A still further object of the invention is to provide a high radiation transparency window for ionization chambers which is relatively rugged and resistant to perforation. or abrasive contact with radiation emitting surfaces.

One other object of the invention is to provide an ionization chamber structure in which the window may be easily and economically mounted or replaced.

Other objects and advantages will be apparent from the following description taken in conjunction with the accompanying drawing of which the single figure is a sectional view of, an, ionization chamber constructed in accordance with the invention.

Basically, the ionization. chamber of the invention-is provided as a cup-like enclosure having a centrally disposed anode'and a radially disposed cathode element which is similar to those customarily employed in the art. However, there is herein provided a novel covering.

for the end window opening of such enclosure including novel means for mounting and other details of construction providing advantageous operational features.

Constructional details of a practical embodiment of the invention will now be described with reference to the single figure of the drawing. As illustrated therein, the ionization chamber is provided with a cup-like enclosure 11 having an elongated generally cylindrical central sec-.

tion 12, closed at the upper end with a concave closure cap 13 and having the lower end flared outwardly to provide an enlarged rounded rim section 14. The enclosure 11 may be constructed of an appropriate dielectric material such as glass. Ordinarily the enclosure walls will be of sufiicient thickness to be effectively opaque to beta radiation.

An .anode wire 16 is supported coaxially within the enclosure section 12 by means of a glass stem 17 which.

encases a portion thereof, said glass stem 17 being fused to the inner wall of cap 13. The upper end of anode wire 16 is lead exteriorly of the cap 13 for attachment to an external counter circuit (not shown The lower end of anode 16 which terminates near the terminal plane of the rim section 14 of the enclosure 11 is encased in a fused glass bead 18 to prevent spurious discharges and corona effects. Accordingly, a substantial length of anode '16 extends coaxially within the lower length of enclosure section 12. A cylinder 19 formed of copper sheet and split lengthwise is disposed in proximity to the walls Within enclosure section 12 and extending coextensively therewith to serve as the cathode element noted above. A conductor-21 is attached to the upper edge of the cathode cylinder 19 and lead exteriorly through a seal 22 for attachment to the external counting circuit as noted above.

To provide the window for the chamber, a thin film 23 of the aforesaid polyester resin is stretched over the enlarged rim section 14 of the enclosure with the excess edge portions 24 folded back over the lower outer surface of the enclosure section 12. The film 23 may be secured by clamping the folded portion 24 of the film to the outer surface of the enclosure section 12 by appropriate means. A preferred method of securing the Patented Feb. 16, 1960 I 3 film" is" provided by forming a peripheral groove 26 in the" exterior surface of'the" enclosure" 11" in the'" region of the juncture of the flaredgrim 14 and the enclosure portion 12, whereby the iolded portion 24 of the film extends thereover; An elastic'me'mber" such" asth'e ringZTisdispoSedto clampthe film portion 2'4tightly' into' -thegroove 26*and thereby 'elfect'an efficientseal;

Under certain circumstances the ionization" chamber may utilize ordinary air or nitrogen gas fillings; how ever," in order to allow operation with lower'voltages, it

is preferred to employ a raregas such as helium orargon therein. Alternatively, it is preferred to operatein th'e' Geiger-Muller region and to employ a'self-quenching'gas mixture therein as in conventional practice; In this'case appropriate gas mixturesuch as'heliurn or argonwith" various organic vapors includinglow. molecular' weight aliphatic hydrocarbons, alcohols, chlorinated hydrocarbonsor' halogens are employed asthefilling material. I

To obtain maximum sensitivity, the film' is made as thin as possible and films of as thin as 0.0002 5inch areavail'able' commercially under the trade" name Mylar of'theDu Pont Company. Mylar'film is prepared from a polyester resin which is producedby the condensation 'conduit tube 28 sealed into the chamber cap 13 so as to terminate near the inner surface of film 23,- whereby a flushing current of the selected fillinggas-mixture may be directed to impinge upon the inner surface of 'film 23 in the region beneath the beaded tip 18015 the anode wire 16. A stub conduit tube 29 is sealed into the' cap 13-to provide exit means for the flushing gas and a stopcock 31 may be afiixed to the tube 29 for closure pur-' poses in the event that intermittent flushing is employed. Sulficient positive gas pressure is supplied-by'an'external source (not shown) to effect flushing but insuificient to distort or rupture the film 23.

In an operating embodiment employing a sheet of- Mylar resin 0.00025 inch in thickness as the window film' 23 and with appropriate anodevoltagesapplied be-- tween anode wire 16 and cathode cylinder 19 to efiect operation" in the" Geiger" region, counting efiiciencies' of above 20% of the beta particles emitted by carbon 14 have been obtained. Under comparable conditions the best available mica window chambers yielded a maximum of 14% mounting efficiency. Other advantages also were obtained including durability in services requiring rubbing andscraping contact with'rough surfacesbeing monitored or scanned as in paper chromatography tracer applications lin whichnthere is inadvertent but frequent contact with sharpedges, corners,- etc; A gas-mixture of helium and butane was used in a ratio of about 10031 and conventional electronic; counting circuits were employed. Moreover, the essentially plane configuration of the window construction permits' the'closest possible contact with plane surfaces thereby permitting the best counting geometry. In the event of contamination the window film 23 is quicklyv and economically replaced;

While there has been described in the foregoingwhat may. be considered to be preferred embodiments of the invention, various modifications may be made therein without departing from the teachings of the invention and it is therefore intended to cover all such as fall within thescope of the appended claim.

What is claimed is:

An ionization chamber for use in counting low energy beta radiation.comprisinglacup-like enclosure having a cylindrical centralsection, being closed at'oneend by a cap, having a flared rim at the other extremity defining a planar terminal opening with exterior surfaces of said enclosure being grooved radially, a conductive cylindrical cathode sheath disposed in said central enclosure section, an'anode wire'sealed into said enclosure caps and disposed' coaxiallywithin' said cathode sheath, a film of polyethylene terephthalate stretched over said planar en closure opening and overlapping the groove in said exterior'enclosure surface, ahead of extensible material clamping said film into said groove, tubular conduit means for directing an ionizable gas on inner surfacesofsaid film, and means for exhausting. gas from said enclosure;

References Cited in theme of this patent UNITED STATES PATENTS 2,497,376 Swallow Feb. 14, 1950 2,505,919 Simpson May 2, 1950': 2,552,723 Koury ..-May 15, 1951 2,6561476" Firminhac Oct; 20, 1953-

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