IL32654A - Radiation gauging instrument and method - Google Patents

Description

Eadiation Gaging lastrumeai and Method Troxler Electronic Laboratories, Inc.

RADIATION GAUGING INSTRUMENT AND METHOD Ionizing radiation has long been used for testing materials and particularly for determining a physical, characteristic of a material under test, such as density or relative composition, which is capable of detectably modifying a selected type of such radiation. By way of example only, and not by way of limitation, radiation gauging instruments are now in use for determining the concentration of hydrogenous substance, such as water, in a material under test, such as soilo In such determination and measurement of properties and characteristics, reliance is placed on phenomenona collectively referred to herein as the modification of ionizing radiation. It has long been known that ionizing radiation such as neutron radiation, gamma radiation and X-ray radiation, is modified in varying ways by different properties and characteristics of materials, and the use of radiation gauging instruments is a practical application of this phenomenon» In the specific instance here referred to, the moderation of fast neutrons by hydrogenous substances is used to permit determination of the presence and magnitude of moisture content of soil. Still other examples of physical properties which are examined through the use of ionizing radiation technology will readily occur to persons skilled in this art.

With advances in electronic technology, two significant areas of improvement have been opened to the users of radiation gauging instruments, and have been pursued by the manufacturers of such nstruments o First the availabilit of circuit com onents which require relatively simple current supplies has fostered the development of field instruments, permitting the use of radiation gauging instruments outside the laboratory. Second, with the broadening use of such instruments, increasing accuracy has been demanded by those to whom the instruments are supplied and has been met by increasingly sophisticated design of the electronic circuitry employed,, While such advances have thus far substantially broadened both the usefulness and the use of radiation gauging instruments, certain difficulties have become apparent,, More specifically, it is common practice to compensate for certain statistical limitations of nuclear gauging and certain time-temperature drift characteristic of the electronic circuitry in such instruments by following a ratio test procedure. Heretofore in such test procedures, determination of the characteristic under test relies upon the ratioing of a test measurement of the modification of ionizing radiation by the material under test with a previously or subsequently taken standard measurement of the modification of such ionizing radiation by a reference standard, typically a specimen of a material with a stable characteristic „ With the test and standard measurements being taken during different time intervals, it is frequently difficult to assure that the measurements properly reflect the modification of ionizing radiatio by the respective test and standard specimens and do not introduce inaccuracies into the ratio to be determined from the measurements, due to the relatively long period of time required to achieve d n instrumen With the above discussion in mind, it is an object of the present invention to provide a radiation gauging instrument for the non-destructive determina ion of a physical property or characteristic of a material under test, which instrument is self-compensating for factors which heretofore have imposed limitations upon the use of such instruments. In realizing this object of the present invention, radiation detecting means and associated electronic circuitry are provided for receiving ionizing radiation as modified by test and standard specimens and for ratioing the response of a first detector against the response of a second detector, for thereby indicating the previously unknown characteristic of the test specimen material. The electronic circuitry is so arranged that the responses of the two detectors are determined within a common time domain, such as by employing the reaponse of one detector to gate passage of the response of the other detector.

Yet another object of the present invention is to provide a method of determining a physical characteristic of a material under test through exposure of the material to ionizing radiation wherein a test channel specimen and a standard channel specimen are simultaneously exposed to ionizing radiation of a selected type from a common source, and respective test and standard detectors simultaneously receive radiation emitted from the source as modified by the test channel and standard channel specimens. The ratio of the simultaneous responses of the detectors is then measured as indicative of the physical characteristic to be determined. In realizing this object of the present invention, it is preferred to employ detectors which emit trains of pulses, and to measure the ratio of the simultaneous responses by delivering pulses emitted from the standard channel detector to a divider preset for'. acceptance of a predetermined number of pulses and gating the passage of pulses emitted from the test channel detector during the period of time required to accumulate in the divider that predetermined number of pulses „ Some of the objects and advantages of the invention having been stated, others will appear as the description proceeds, when taken in connection with the accompanying drawings, in which -- Figure 1 is a schematic illustration of the elements of an instrument constructed and operating in accordance with the presen invention; Figure 2 is a perspective view of a cabinet structure enclosi the radiation source, specimens, and detectors of the apparatus shown in Figure 1; Figure 3 is a perspective view of an instrument housing adapted for surface characteristic measurements; Figure 4 is a schematic section, in elevation, through the instrument housing of Figure 3, showing one geometrical arrangemen for a radiation source, specimens, and detectors in accordance with the present invention; Figures 5, 6 and 7 are views generally similar to Figure 4, illustrating other geometrical arrangements contemplated for instruments in accordance with the present invention; Figure 9 is a view of the instrument of Figure 8, similar to Figure 4, illustrating a geometrical arrangement wherein a detector is mounted in a probe for penetration into a material under test; and Figure 10 is a schematic illustration of an indicating means which may be substituted into the apparatus of Figure 1.

BACKGROUND OF THE PRESENT INVENTION Before proceeding with the discussion of the present invention with reference to the drawings, it is believed appropriate at this time to point out more fully certain factors relating to . the statistics of radiation gauging instrument operation and to the characteristics of the electronic circuitry involved in such instruments. These areas of interest were briefly discussed above, as contributing to certain difficulties which are now being realized in the application of such instruments.

Statistical Considerations The ultimate or maximum accuracy obtainable by any measuring system involving the detection and/or modification of ionizing radiation, and particularly · those systems wherein the source of ionizing radiation is a radioisotope, is limited by the statistics of energy emission. The probability of emission of radiation energy at any given instant is governed by the fundamental binominal distribution laws pertaining to random events and, with certain restrictions, is represented by the Poisson distribution. Since texts such as "Radioisotope Measurement Applications in Engineering" by R. P. Garder and R. L. Elly, Jr. , published by Reinhold Publishing Com an are available which ive detailed discussions of the statistics and statistical analysis involved, it is not necessary that an extended discussion be given here. However, it should be noted briefly that such analysis, in instances where the measurements are derived from pulse trains originating with radiation detectors, is applicable to systems wherein a physical characteristic of a material under test is measured by relationships between count rates and the property to be determined. If the count rates for two different values of the characteristic under test are known, then the difference in count rate divided by the difference in the property under test indicates the slope of an instrument calibration curve. By application of statistical analysis, the standard deviation (or sigma) of a particular gauging system in terms of the measurements, defining the best precision of the system, becomes equal to the square root of the count rate divided by the slope of the calibration curve. On an assumption that an individual count rate is the true means rate for the system, then the percentage error of a one standard deviation error (also expressed as a one sigma error) may be expressed as the square root of that count total divided by the count total and multiplied by 100. Thus, with a measurement count of 10,000, the percentage error for one standard deviation is equal to one percent. At a measurement count of 100,000, the percentage error for one standard deviation equals 0.316%.

The significance of this discussion of radiation gauging instrument statistics is more apparent when the interaction of this factor with other factors of instrument design is more fully pointed is variation in the mechanical geometry of the instrument in order to permit variation in the slope of a calibration line. However, when the practical limits of the slope of the calibration line have been reached, an alternative procedure for the improvement of accuracy has been an increase in the measurement count accumulated. At some limiting value of the percentage error for one standard deviation, the practical limits of the electronic and mechanical stability of the system exceed the nuclear system statistics and no further improvement in the precision of the system can be obtained by increasing the measurement count. While the upper limi for seeking improvement through increasing the accumulated measurement count has not yet become a major factor in most radiati gauging instrument applications, it has been anticipated for such applications generally and has been actually encountered in certain specific areas.

Circuit Drift Considerations Turning now to the second of the two problem areas which was briefly mentioned above, the electronic circuit elements which are required in order to permit measurement of the pulse trains originated by the* detectors in response to ionizing radiation are subject to drift, which may be either long term or short term. An acceptable division is to include as long-term drift those changes in a system which occur at intervals longer than one operating day. For most purposes, it may be assumed that the rate of emission of ionizing radiation is essentially constant and does not affect long-term, drift, with such drift generally being due to aging chan es in the electronic circuitr and in detection efficienc .

Short-term drift is believed primarily due to the temperature rise in individual electronic components, brought about by the power, dissipation within a component which is required to make the component function. Secondarily, short-term drift is dependent upon ambient temperature changes. Such time- emperature drift is not a linear function, and normally contains a rapid, large change occurring in the first two hours of operation and a longer term, small change which continues for more than an operating day and thus becomes long-term drift. Typical instruments have been found to have drift within ranges of about 0.5% to 1% during the first two hours of operation and about 0.01% to 0.05% per hour thereafter, expressed as changes in count rates. Such percentage variation in count rates may represent very large changes in the material characteristic under test, when compared to the statistical error.

Prior Solutions ? Both of these problems discussed above have been solved heretofore through the use of a reference count ratio. Even before radiation gauging instruments came into wide use, it was learned that frequent calibration would be required if the system were to attain any degree of accuracy. The reference count ratio was devised as a means of extending the interval between calibrations, and consists of a procedure wherein a count rate measurement of the characteristic to be determined is made on a stable material sample of known properties, the system is calibrated on a ratio of test count rate to this reference count rate, and the standard specimen is made available for continued field use with the instrument.

While this ratio technique is quite useful in eliminating most problems caused by long-term drift and mechanical changes in the geometry of an instrument, so long as they affect both the reference and the measurement count totals, serious limitations exist in the ratio technique as it is presently used. More particularly, both the reference and the measurement count totals contain statistical variations as discussed above and when the two numbers are divided to obtain the ratio, the resultant number contains a statistical variation which is always greater than that of each individual count. The resultant degree of randomness becomes the root-mean- ·· square of the original numbers, weighted by the respective magnitude of the numbers. As acceptable practice has been a requirement that the three sigma deviation of the reference count total be less than the one sigma deviation of the measurement count total, in order to prevent significant degradation of the resultant ratio. This requirement has been used both during the initial calibration of a radiation gauging instrument and during use of the instrument.

In following this practice, most reference count rates have been chosen to fall within the range of the measurement count rates of the material under test. As a standard deviation varies as the square root of the count total, it becomes statistically preferable to accumulate nine times the measuremen count total in determining the reference count total. Consequently, the reference count requires nine times the length of time required for the measurement count. In terms of operating time available, this situation quickly becomes impossible, and it is now generally accepted that except in and the reference count period need only extend for four times the measurement count period.

While this apparent waste of operational time is undesirable, it . can be tolerated provided that the taking of a reference count total is not required to be repeated too frequently during any given period of operation. Instruments in which the standard deviation is large with respect to the short-term drift require that a reference coun be made only once during the operating day. Systems with higher' short-term drift may require that a reference count be made twice a day or more frequently.

As the required instrument precision increases, it becomes impossible to obtain a reference count until two hours after turn-on of the equipment, since the initial drift rate can foreseeably cause a change, during the time which is required to make both the reference and the measurement counts, which is larger than the statistical variation during the counting period. At this point, the user is faced with waiting until the drift rate has become stabilized at a usable value below the statistical limitations of the instrument.

The ultimate limitations are thus believed obvious. A point in instrument design is reached where the required precision demands a reference count between every measurement count even after the initial turn-on drift period has passed and four-fifths of the operational time available is wasted. Use of such a radiation gauging instrument for measurement becomes impossible as a practical matter because the drift is greater than the statistical error.

THE PRESENT INVENTION Now referring more particularly to the drawings, it has now been discovered that the difficulties introduced by drift may be avoided by a two-pronged attack, in accordance with which the greatest possible commonality for electronic circuitry employed in a radiation gauging instrument is sought, in order to maintain uniform drift throughout the instrument, and the reference count and measurement count are taken within a common time domain. Thus, in accordance with this invention, a single span of operational time encompasses both the reference count and the measurement count. This is accomplished by employing, in combination with an appropriate source or sources of ionizing radiation and appropriate detectors, an indicating means which is capable of directly measuring the ratio;.-, between reference and measurement counts.

While it is to be understood at the outset of this discussion that the present invention may be based, in function, upon any of the various modifications (including type, energy, intensity or quantity) of ionizing radiation which are characteristic of processes involving such radiation, the present discussion will first proceed with reference to an apparatus configuration in which the modification mechanism is moderation of neutrons _ Proceeding on such a basis, fast neutrons are passed into specimens of material and are moderated by a physical characteristic of the g material, typically a hydrogenous component in the makeup of the material, to become slow or thermal neutrons which are detected by appropriately chosen detector means.

In order to assure that the irradiation of specimens of material is substantially identical, for reasons which will become more fully apparent in the discussion which fellows, it is preferred that an apparatus in accordance with the present invention (Figures 1 and 2) incorporate a source 10 of fast neutrons, which preferably is an' appropriate elemental or isotopic material such as Radium-Beryllium, or Americium-Berylliun In order to directly measure the ratio between reference and measurement counts, and to take the reference count and measurement count within a common time domain, the apparatus of Figure 1 includes both a measurement count channel and a reference count channelo The measurement count channel includes a first detector D-l and associated amplifier A-l receiving pulse trains from the detector D-l and an associated pulse-shaping circuit PS-1 such as a discriminator for shaping the amplified pulses passing from the amplifier Α-1» Similarly, the reference count channel includes a second detector D-2 and associated amplifier A-2 and pulse-shaping circuits PS-2» In accordance with the present invention, the influence. of drift upon determination of physical characteristics made using the apparatus of Figure 1 is minimized through the use of matched circuitry for the amplifiers A-l and A-2 and for the pulse-shaping circuits PS-1 and PS-2. Further, current necessary for the operation of the measurement count channel elements and the reference count channel elements is derived from a common power source, thus avoiding differences in circuit..response otherwise possibly resulting from differences in current supplied thereto.

In order to assure that the detection means, including the detectors D-1 and D-2,' properly responds to modification of the ionizing radiation emitted from the source 10 by the physical characteristic to be determined, the present invention preferably includes means for supporting a test specimen 11 of a material of unknown characteristic and a standard specimen 12 of a material of known characteristic for radiation thereof by ionizing radiation emitted from the source 100 Further, the test channel and standard channel detectors D-1 and D-2 are respectively positioned for receiving radiation as modified by the test and standard specimens 11 and 12, respectively. In the instance where the ionizing radiation emitted from the source 10 and to which the detectors D-1 and D-2 are responsive to fast neutrons, modified by the specimens 11 and 12 to become thermal neutrons, the present invention contemplates that appropriate shielding means such as a cadmium sheet 13 to be interposed between the source 10 and the standard channel specimen 12. While the cadmium sheet 13 imposes no barrier to fast neutrons, and thus permits the irradiation of the standard channel specimen 12 thereby, such a sheet is an effective shield to prevent detection by the standard channel detector D-2 of te thermal neutrons resulting from moderation of fast neutrons by the test specimen 11. Similarly, the shield 13 precludes the detection by the test channel detector D-1 of thermal neutrons moderated by the material of the standard specimen 12, thereby assuring independence of operation of the measurement and reference channels. non-destruetively through the use of the apparatus of the present invention, the apparatus further includes indication means 14 operatively connected to the detectors D-1 and D-2 and responsive to the detection thereby of radiation as modified by the test and standard specimens 11 and 12 for indicating the previously unknown characteristic of the material of the test channel specimen 11» In particular, the indication means includes means for ratioing the response of the first detector D-1 against the response of the second detector D-2, as by measuring the ratio of the simultaneous responses of those detectors „ This indication means preferably includes a divider 15 receiving pulses from the reference channel detector D-2, as amplified and shaped by the associated amplifier A-2 and pulse-shaping circuit PS-2. The divider 15 may be any acceptable digital accumulator, shift register, ring counter circuit or other generally known basic computor circuits, provided that the circuitry may be arranged to accept a predetermined state number of pulses from the detector D-2, amplifier A-2 and pulse-shaping circuit PS-2 before an output pulse is anitted from the divider 15» The choice of the term "divider" for application to the accumulator 15 circuit element will become more clear hereinafter, when the overall operation of the apparatus now being described is more fully pointed out.

The indication means 14 further includes a suitable gate 16, operatively connected to the test channel detector D-1 and to the divider 15, to receive pulses originating with the test channel detector D-1 and amplified and shaped by the associated amplifier A-l and pulse-shaping circuit PS-1 and to receive pulses from the divider 15. Operatively connected to the gate 16 is an appropriate pulse counter 17, on which the measurement count for the test channel specimen 11 is to be displayed. Such pulse counters are well-known and commercially available from known sources , and typically employ gas-tube indicators to permit direct readout of measurement counts. n order to accumulate in the counter 17 a measurement count total indicative of the physical characteristic of the test specimen 11 which is to be determined by the instrument of the present invention, the means for ratioing the responses of the first and second detectors D-1 and D-2 gates passage of the response of the test channel detector D-1 in a predetermined ratio to the response of the standard channel detector D-2. This gating is accomplished through the provision of the divider 15 and the gate 16, and permits passage, of the response of the firs't detector D-1 to the counter 17. That is the passage of pulse trains from the detector D-1 to the counter 17 is permitted through the gate 16 under the control of the divider 15. In performing this function, the gate . 16 may be designed electronically to include solid-state or relay switches or some other circuitry may be turned on and off to contro the passage of current. Other similar elements are known to designers of electronic circuitry. In viewing the cooperation of the divider 15, gate 16 and counter 17, the choice of the term "divider" for the pulse accumulator receiving standard channel pulses may be seen to refer to the fact that the gate 16 is pulsed the divider 15 be preset for the number 250,000, a pulse is passed to the gate 16 upon the first of two hundred fifty thousand pulses emitted from the standard channel detector D-2 reaching the divider 15. Upon receiving a pulse from the divider 15, the gate 16 begins passing pulses originating with the test channel detector D-1 to the counter 17 » Thereafter, the arrival of the last pulse in the train of 250,000 from the standard channel detector D-2 at the divider 15 causes the gate 16 to be turned off, blocking further passage of pulses originating with the test channel detector D-1 to counter 170 It is contemplated that the divider 15 may be preset to control the conduction of the gate 16 in accordance with any of various selected divisors, such as 500,000 or l,000,000o Turning now to the operation of the apparatus as thus far described, it may be seen that the test channel and standard channel specimens 11 and 12 are irradiated with ionizing radiation from the source 10. Modifications of ionizing radiation by the :test and standard specimens 11 and 12 such as are indicative of the properties or characteristics thereof are independently detected by the detectors D-1 and D-2„ As to the standard channel detector D-2, the modification of ionizing radiation by the standard channel specimen is indicative of a characteristic of that specimen which is known stable while, as to the-:test channel detector D-1, the modification of the ionizing radiation by the test channel specimen 11 is indicative of a characteristic of the material under test which is unknown. Through the operation of the divider 15 and the gate 16, receiving pulses originating from the detectors D-1 and D-2 the respective pulse-shaping circuits PS-1 and PS-2, the counter 17 as an indication device is controlled in response to the detection of test specimen radiation modification in accordance with the detection of standard specimen radiation modification, in such a manner as to effect a direct comparison of the modification of radiation by the specimens 11 and 12 and thereby indicate the previously unknown characteristic of the test specimen material. Preferably, the passing of ionizing radiation from a common source into the test specimen and into the standard specimen is performed simultaneously, in order to assure that the statistical variations in the exposure of the material are as nearly sampled from the same statistical population as is possible,, Additionally, the reception at the respective test' channel and standard channel detectors D-l and D-2 of radiation emitted from the source 10 as modified by the test and standard specimens 11 and 12 proceeds simultaneously., This is assured by the use of the divider 15 and gate 16 to take both the reference count total and the measurement count total within the same span of operational time, referred to herein as a common time domain» In order to more particularly point out the significance of common time domain operation, it is appropriate now to refer to the manner of calibrating the instrument of the present invention. More particularly, by pre-establishing the set point of the divider 15 at a selected figure, such as 250,000, the reference count total is thus pre-established. This is accomplished by selecting the material of the standard channel specimen 12 and ..'.determining the geometry of the source 10, shielding material 14, standard channel specimen 12 and associated standard channel detector D-2» Thus, the instrument is calibrated in terms of the measurement count total accumulated in the counter 17 during the period of time required to accumulate in the divider 15 a reference count total of 250,000» Inasmuch as the reference count total and measurement count total are accumulated within a common time domain, and the greatest possible commonality of circuit components and power supplies is present between the amplifiers A-l and A-2 and the pulse-shaping circuits PS-1 and PS-2, the effects of drift on the accumulation of the reference count total and the measurement count total are self-cancelling. The calibration of the instrument is by a direct relation of the measurement count total accumulated in the counter 17 against the physical characteristic to be determined.

In one specific operating embodiment of the apparatus of the present invention, as discussed to this point, the means provided for supporting a test channel sample taken from a material of unknown characteristic and a standard channel specimen of the material of known characteristic for irradiation thereof by. ionizing radiation emitted from the source 10 has comprised a cabinet structure 20 (Figure 2) of generally rectangular parallelopiped form. The source 10 is located within the cabinet at substantially the central point of the cabinet, with the cabinet containing two internal volumes respectively above and below the location of the source. On opposite sides of the internal volumes e re mounted the detectors D-l and D-2. One of standard specimen 12, permanently mounted in position between the source 10 and the corresponding detector D-2, while the other internal volume is normally closed during use of the instrument by a hinged door 21. Test samples of material whose physical characteristic is to be determined may be introduced into the second volume through the door and the door closed during the measurement period.

It is further contemplated by this invention that the technique of taking a ratio- of the test measurement count to a standard measurement count within a common real time domain may additionally be applied to radiation gauges of otherwise generally conventional type. Suitable geometry for such application of the present invention is illustrated by the surface and depth probe gauges shown in Figures 3-9, and now to be described. In all instances, the surface and depth probe gauge instruments shown in these Figures are distinguished from instruments as described above with reference to Figures 1 and 2 principally by the provision of a different geometrical arrangement of the source 10, test channel detector D-l, standard channel detector D-2, and by the fact that no test sample is removed from the material. For these reasons, the reference characters adopted for identifying these and other elements of the instruments in the discussions above have been applied to corresponding elements of the instruments illustrated in Figures 3-9. In recognization of what has become conventional practice in the use of such surface and depth gauges, the instruments as here illustrated include housing means for enclosing the source 10 and the detectors D-l and D-2 with circuitry including the amplifiers A-l and A-2, discrimina ors PS-1 and PS-2, and the chosen indicating device while joined electrically through a cable. In some instances it may be desirable to enclose all of the elements in a common housing. Such arrangements have been generally adopted in order to facilitate field use of the instruments.

The geometrical arrangements illustrated in Figures 3, 4, 5, 6 and 7 all relate to surface gauges, wherein the modification of ionizing radiation is employed to determine the surface physical properties or characteristics of the materials under test. In the arrangement of the Figures 4, 5 and 6, the standard channel specimen 12 is a reference standard formed of a material having a known characteristic. In the instruments of Figures 6 and 7, the sources 10 are provided with shielding means which additionally serves as a collimation means for radiation emitted by the source. That is, radiation emitted from the sources 10 is directed along a predetermined line from the source. Additionally, the test channel detectors D-l are provided with collimating means, limiting the field of view of those detectors to a selected area beneath the surface of the material under test, enclosed within dashed line, and identified as the test channel specimen 11. Such an operation, in which the test channel specimen 11 is located at a determinable distance below the surface on which the instrument rests, is to be distinguished from the arrangements provided in Figures 4 and 5, wherein the test channel specimens 11 are located at the surface on which the instrument rests, but extend at least a certain distance The instrument illustrated in Figure 7 is distinct from the other instruments illustrated herein in yet another respect. More particularly, the standard channel specimen 12 employed for the instrument of Figure 7 is a volume -within the material on the surface of which the instrument rests, at the intersection of the collimated path of radiation emitted from the source 10 and the collimated field of view of the standard channel detector D-2. In an instrument of this specific type, compensation is provided not only for electronic drift and statistical considerations of the nuclear gauging system, but additionally for the effects of local chemical composition of the' material under test. Thus, as to the particular instrument illustrated in Figure 7, the standard channel specimen 12 is a material which is not fully characterized during the time that the gauging operation takes place. Nevertheless, the results obtained from such a gauging instrument provide compensation for electronic drift and- statistical considerations and thus are within a broad scope of application of the present subject invention.

In the application of the present invention to density gauges, wherein direct transmission of ionizing radiation through a specimen of material is employed in determining a physical property or characteristic of that material, a geometric configuration such, as that illustrated in. Figures 8 and 9 may be adopted. In that configuration, the test channel detector D-l is arranged to receive radiation emitted from the source 10 which has penetrated entirely through the test channel specimen 11. In the particular form While a number of variations on the arrangement of the elements of the instruments schematically illustrated in Figure 1 have now been pointed out, it is believed appropriate to restate that the present invention contemplates that the method described herein may be adapted to yet other geometrical arrangements of the instrument elements, while retaining the advantages of self-cancellation of statistical and electronic drift considerations.

While the discussion above has related particularly to the use of instruments as arranged with a counter 17 in the indication means 14, so that the resulting data is in numerical form, the cpresent invention further contemplates that provision may be made for direct, continuous print out or readout, such as by a rate meter or recorder. In obtaining this particular result, the indication means 14 (enclosed within dash lines in Figure 1) may be replaced by an indication means 24 schematically illustrated in Figure 10. In the continuous readout indicating means 24, an analog output signal is obtained which may, for example, be used to drive a direct indicating meter or chart recorder. In order to obtain such an analog output signal, first and second digital to analog conversion circuits 25 and 26 are provided and are operatxvely · connected with the respective amplifiers and pulse-shaping circuits of the test channel and standard channel circuitry. The circui-ts 25 and 26 may be any appropriate type of rate circuit capable of receiving a train of signal pulses' as originated by the detectors D-l and D- 2, · amplified by the respective amplifiers A-l and A-2, and shaped by the respective discriminators PS-1 and PS-2. The analog signals derived by conversion of the digital signals received by the respective circuits 25 and 26 are then delivered to an analog divider 28, which divides the analog signal derived from test channel circuitry by the analog signal derived from the standard channel circuitry to arrive at an output analog signal. Such a signal may then be delivered to an appropriate continuous recording or readout device. - 24 - 32654/2 In tho drawings and specification, there have boon sot forth referred embodiments of the invention, and although specific terns aro employed, thoy are uesd in a generic and deeeriptiv© ce ee only and not for purposes of limitation, the ocopo of the invention being defined in the elains.

Claims (1)

1. Δ nothod of in a naterial under test the presence and of certain characteristics capable of detectable modifying ioniaing radiation while simultaneously electronic drift and considerations comprising the of exposing a test specimen of the material and a dard specimen of a material to have the certain to a degree to ionizing radiation emitted a cocaaon radiation the ionising radiation of a type vhich is aodified by the certain receiving radiation both specimens of material at teat and standard radiation detectors for a period required for the radiation to detect a predetermined number of radiation the while counting an indicating the number of radiation impulses received fron the only during said time method according to Claim receiving of codified radiation for a time period includes accumulating radiation standard specimen and further wherein the counting and indicating of radiation fron the test specimen includes opening gate for passage of radiation impulses therethrough on initiation of accumulation of radiation fron the standard specimen and closing 26 A according to 1 particularly adapted for of content and vherein the of tho and reception of ied radiation include irradiating the neutron radiation and to moderation such neutron radiation by the hydrogenous content of the of the tout and according to 1 particularly adapted for determination of density and therein tho exposure of the and reception of modified radiation include radiation into the and responding to photone scattered the of the and A method according Claim 1 particularly adapted for of density and wherein the exposure of the and reception of radiation include radiation into tho and responding to photons not scattered by the of the and standard An instrument for in a under teat the presence and of certain capable of radiation an radiation emitting a selected type of ionising radiation known to be nodifiod by the certain to be for mounting said radiation eo a teet specimen of the to bo and a standard of the material to the certain to a first detection associated with said radiation source and said means for receiving codified ionising from said test second detection means with said radiation source and said mounting receiving modified radiation from said standard and counting and indicating operatlvely connected to said first and second detection for counting and indicating the number of radiation from the specimen only a time period required for the standard radiation detection to detect a of radiation the standard An instrument according to 6 wherein said counting and indicating means comprises a digital accumulator connected to said second detector for receiving trains of thorofroia and accumulating a predetermined number of such a connected said signal for receiving trains of pulses passed from said first being to said accumulator for passing pulses rom said first detector to said counter only during such period of is for said accumulator to receive said predetermined number of pulses said second detector and said counter being responsive said signal pulses passed by totaling such that the total number 28 the that a of from second to Claim 6 said radiation faet neutron radiation detection means to resulting df fast neutron radiation by content of the of toot standard An instrument according to Claim 6 whorein radiation source emits radiation detection to photons scattered by the density of the of the standard An according to 6 w naid radiation g radiation detection means respond to photons poncing linearly of of and standard A method substantially hereinbefore described with to the An hereinbefore described with reference the Applicants insufficientOCRQuality