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A Method for Measuring Short Period ActivitiesA. D. Schelberg, M. B. Sampson, and Allan C. G. Mitchell Citation: Review of Scientific Instruments 19, 458 (1948); doi: 10.1063/1.1741296 View online: http://dx.doi.org/10.1063/1.1741296 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/19/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in T1 measurement using a short acquisition period for quantitative cardiac applications Med. Phys. 32, 1738 (2005); 10.1118/1.1921668 Electric method for measurements of enzymatic activity Rev. Sci. Instrum. 47, 1394 (1976); 10.1063/1.1134530 A Method of Measuring Short BetaDecay Lifetimes Rev. Sci. Instrum. 24, 391 (1953); 10.1063/1.1770717 Calculations on a ShortTube Method for Measurement of Impedance J. Acoust. Soc. Am. 20, 595 (1948); 10.1121/1.1917010 A ShortTube Method for Measurement of Impedance J. Acoust. Soc. Am. 19, 922 (1947); 10.1121/1.1916643
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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 19, NUMBER 7 JULY, 1948
A Method for Measuring Short Period Activities
A. D. SCHELBERG, lVl. B. SAMPSON, AND ALLAN C. G. MITCHELL
Indiana University, Bloomington, Indiana
(Received AprilS, 1948)
By photographing an oscilloscope it is possible to measure short periods ranging from 0.1 second to 5 seconds. The pulses from a Geiger counter are fed into the x axis amplifier of an oscilloscope using no sweep. An oscillograph recorder is used to photograph the cathode-ray tube so that a record of all the pulses from the counter is obtained. The activated target is moved from the bombardment position to the counting position in about 0.2 second by means of a pneumatic tube. Exposure of calcium to a 23-Mev alpha-particle beam resulted in an activity with a half-life of 0.58±0.04 second. This activity may be assigned to Ti43 produced by the reaction Ca40(a,n )Ti43•
WHEN one attempts to measure the halflives of radioactive isotopes which are
rather short, say less than ten seconds, several difficulties arise. One is the problem of minimizing the time between the end of the activating bombardment and the beginning of the counting period. Another is the problem of handling the high density of pulses issuing from a Geiger counter when it is exposed to a rapidly decaying source. If a conventional scaling circuit and mechanical register are used, it becomes necessary to take photographs of the register, the scaling lights, and a clock. This method is limited by the accuracy to which the clock can be read on the film and the uncertainty of the true number of scaling lights because of the finite exposure time.
The method to be described in this paper allows not more than about 0.2 second to elapse between the end of irradiation and the start of counting. The Geiger counter pulses are fed directly into an oscilloscope and photographed so that a record of all the pulses is obtained.
The radioactive sample was prepared by bombardment in a beam of alpha-particles produced in the 45-inch cyclotron. Because of the high
FIG. 1. A section of one of the records obtained by photographing an oscilloscope with an oscillograph recorder. The time interval between arrows is 0.05 second during which 153 pulses issued from the Geiger counter.
background near the cyclotron it was not feasible to place a Geiger counter beside the target chamber and count the sample as soon as the beam was turned off. It was necessary to place the Geiger counter, a thin walled aluminum type, in a lead block outside the four-foot thick water shielding tanks and move the activated target from the target chamber to the counter. To accomplish this motion of the target a pneumatic tube about 12 feet long was used. This tube passed between two water tanks. The tube was made of i-inch square copper tubing having a wall thickness of l2 inch. Square tubing, rather than round, was used to insure that the irradiated portion of the target would face the Geiger counter after its journey through the tube. The target thus had to be rectangular in shape, slightly under 1
56 inch square and one inch long.
The length was limited by the presence of two bends in the tube required for geometrical reasons. The one end of the tube contained a onemil aluminum "window" through which the alpha-beam could pass and activate the target. At the other end of the tube was a similar "window" through which the beta-rays of the active isotope could pass and strike the Geiger counter.
When a target had been selected and prepared so that it would slide through the tubing, it was placed in the tube and lined up with the beam "window." The tube was evacuated with a mechanical pump, first from the bombardment end so that the target, resting against a stop, would not move out of line with the beam "window." Then a pump at the counter end was started, and the first pump was cut off the system. The second pump maintained a good vacuum in front of the
458
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A MET HOD FOR MEA SUR I N G S H 0 R T PER I 0 D ACT I V I TIE S 459
FIG. 2. Decay curve for A3D showing a half-life of 1.9 second.
o z o ., ... ~ 10
o 80 a: ... Q. 60
~ SO z ~ 40 u
30
20
target when it moved through the tube under the influence of atmospheric pressure. In order to bring the target to quick but reasonably gentl!! stop the following procedure was used. At the Geiger counter end, a small section of rubber tubing was fitted into the square copper tubing. The targets were then made with tapered noses which, on impact, would become wedged in the rubber tubing. Thus the rubber absorbed most of the shock and also held the target at the correct position in front of the counter.
When the tube was evacuated, the alpha-beam from the cyclotron was allowed to fall on the target. The irradiation time was always rather short, a matter of five to ten seconds, since only short half-lives were being sought. Longer exposures would only raise the background on the target and reduce the counting accuracy. At the end of the irradiation period a relay was actuated which sent power to a solenoid valve and also broke the cyclotron oscillator circuit so that the beam was cut off. The solenoid valve admitted air at atmospheric pressure behind the target which then pushed it through the tube to the counting position. The travel time, as mentioned above, was about 0.2 second.
The pulses from the Geiger counter were fed into the x axis amplifier of a Dumont, Model 208 Oscillograph using no sweep. The pulses on the oscillograph screen were separated by photographing the oscilloscope with a General Radio,
Model 651 oscillograph recorder. This camera employs no shutter but moves the film continuously past the lens so that a complete record of all the pulses from the counter is obtained. In order to have a reference time scale on the film, a small 1/25-watt neon bulb was placed on the face of the cathode ray tube and photographed together with the electron spot. The neon bulb was driven by a thyratron oscillator locked in with a Hewlett Packard audio oscillator so that the frequency would remain at some fixed value.
The rate of speed of the film is controlled by adjusting the voltage applied to the camera's motors. It can accommodate up to 100 feet of 35-mm film. For the periods investigated in this paper, SO feet of film allowed to run through in about 17 seconds were adequate. Since the camera takes a little time to get up to speed, it was started somewhat in advance of the end of irradiation. As the intensity of the short period activity died off, the film was slowed down so that the background could be measured over a longer interval for better accuracy.
Figure 1 shows a short section of one of the records obtained. The time scale is 0.005 second per mark. The time elapsed between the arrows is 0.05 second during which 153 counts were recorded, representing an uncorrected counting rate of 183,600 counts per minute. Since the counting rates were so high, it was necessary to
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460 SCHELBERG, SAMPSON, AND MITCHELL
'" 4 ... "1 ... ..J ~
'" 3 ... o .. o ..
2
o
FIG. 3. A plot showing the smooth depentlence of the log of the half-life on the atomic number for the active group of nuclei having one less neutron than proton. The group is split into two families, one for even Z and one for odd Z.
- o.SL-....L4- ....... - 6!-->----!:e-.L..--:-1 ~O---'-:'::12:--J--71 74 ---'---:1:'::6-'--7-1 e::---"'---=2~o--'.l.L2:':2:--'
ATOMIC NUMBER
apply a correction to allow for the particles missed during the dead time of the counter. The expression giving the correct number of counts, no, when n counts are obtained in a time interval, t, IS:
no=n/1- (mit)!
where T is the dead time of the counter, assumed here to be 10-4 second.
The method of analyzing the film was to count up all the pulses in each tenth second interval along the whole length of the film. A plot of this counting rate, counts per tenth second, against the time along the film on semi-log paper gave a first rough decay curve. The correction for counting losses was applied, and then the background curve was extrapolated back under the short period decay portion. After subtraction of the background the final decay curve was obtained.
Figure 2 shows a typical curve, in this case resulting from the exposure of sulfur to an alpha-particle beam. The activity is due to A35 produced by the reaction S32(a,n)A35. A35 is listed in the Segre isotope chart with a period of 1.88 seconds. This activity appeared to be a good one to use for checking the technique described in this paper. The sulfur "rabbit" was prepared by
1 J. D. Kurbatov and H. B. Mann, Phys. Rev. 68, 40 (1945).
melting powdered sulfur and pouring it into a mould. When cold ~t was removed and sanded to the proper size to allow its passage through the tube. Analysis of the data on two runs gave an average value of 1.84 seconds for the half-life of A35.
Cp3 was produced by the reaction S32(d,n)Cl33. The period of this activity was measured once and found to be 2.8 seconds which was somewhat longer than that listed on the Segre chart, namely 2.4 seconds.
Since the method was successful in measuring these short half-lives, it was hoped that it would be a useful tool for finding new short period activities. The regularities in that group of active nuclei having one less neutron than proton, beginning with Be7 and ending with Sc41, suggested a possible isotope of titanium having a period shorter than one second. Extrapolation of the curve obtained by plotting the log of the halflife against the atomic number for all the active isotopes of a given "family" (a "family" refers to all the nuclei with the same neutron excess differing by an alpha-particle), after the method of Konopinski and Dickson,2 indicated a period of 0.65 second for Ti43.
This titanium isotope can be made by the reaction Ca4°(a,n)Ti43. A "rabbit" of metallic
2 G. R. Dickson and E. J. Konopinski, Phys. Rev. 58, 949 (1940).
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A MET HOD FOR MEA SUR I N G S H 0 R T PER I 0 D ACT I V I TIE S 461
calcium was prepared, irradiated, and counted. Analysis of the film showed two short periods and a long period background which was followed in the conventional manner with a scaling circuit. This long period of about 4 hours was attributed to 3.9 hour Sc43 produced by the reaction Ca40(a,p)Sc43. The two short periods were 0.6 second and 9 seconds. It seemed reasonable to associate the 0.6 second activity with Ti43 , but the 9 second period could not be accounted for except on the basis that it was due to some unknown impurity in the calcium.
Since metallic calcium is difficult to preserve once removed from an oil bath, a more durable substance was sought. A desirable material seemed to be crystals of CaF 2 grown from the molten salt. They are chemically pure, inert, and mechanically strong. The possible reactions for alpha-particles on fluorine are listed here to show that they offer no interference in observing the desired activity.
f19(a,p )Ne22-stable F19(a,n )Na22-3 years F19(a,2n)Na21-23 seconds.
Several calcium fluoride crystals were obtained from the Harshaw Chemical Company, Cleveland, Ohio. They were pre-cut to fit the tube except for minor shaping to reduce the possibility of fracturing. Analysis of the data again revealed the short period activity of about 0.6 second and the long 4-hour activity. The 9-second period did
not occur, but there seemed to be a small amount of 23-second Na21 present. Thus it appears quite reasonable to assign this short period to Ti43. The values obtained on four runs are listed below:
Run 1 2 3 4
Half-life (seconds)
0.58 0.67 0.53 0.54
Alphas on metallic calcium
Alphas on calcium fluoride
Average value 0.58±O.04 second.
The result obtained for Ti43 is thus in quite good agreement with the value to be expected according to the analysis of Konopinski and Dickson.2 Figure 3 shows a curve in which 10gTt is plotted against Z for the active group of nuclei having one less neutron than proton. For completeness both the family of odd Z and the family of even Z have been plotted. It will be noticed that not only does each family exhibit a smooth dependence on Z, but so does the group as a whole. The point for Ti43 determined in this experiment has been included.
So far this method has been applied only to solid targets that were readily obtained in the required shape. However, by employing a boat type rabbit, powdered materials could also be investigated. And with some modification the system could be adapted to accept a gas as the target material.
This work was supported by the Office of Naval Research.
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