V.F.Hess (Vienna), About the absorption of gamma rays in the atmosphere. ======================================================================== [Physikalische Zeitschrift XII, 1911, pages 998-1001] According to the findings of Mc.Lenan, Wright, Gockel, Wulf, and others, it seems established that the observed prenetation {penetrating?, through-going} radiation at ground level comes mostly from radioactive substances in the soil {earth} or at the earth's surface. According to the calculations of A. S. Eve, its proportion in the total radiation is more than 95%, and the remainder comes from the radioactive substances distributed in the atmosphere. Therefore, the observed penetration radiation coming from the soil should decrease with increasing altitude above the earth's surface. Eve calculates that the soil {earth} radiation effect is 83% at 10 meters altitude, 36% at 100 meters, and only 0.1% at 1000 meters. But observations by Th. Wulf at the Eiffel Tower and by Gockel in a {free-}balloon by no means found such fast decrease in penetration radiation at higher altitudes. This might have two reasons: first, another, so far unknown ionisator {inonisation matter} could be active in the atmosphere besides the radioactive substances in the soil {earth}. And secondly, it is possible that the absorption of gamma rays in the air is much slower than what Eve assumed in his calculations. It seemed of importance to me to experimentally examine the second reason first, because direct measurments of gamma ray absorption in air - or in gases generally - have not been done yet. The absorption coefficient of gamma rays in air was merely concluded by extrapolation from the law of density /D = constant. Using the Mc.Clelland numbers, one derives the value of = 0.000044. The direct determination of the absorption coefficient of gamma rays in air was performed by me in the following way: On a lawn about 100 meters distance from the Vienna Radium Institute, a Wulf electrometer was installed, attached with an airtight ionisation chamber of 11.1 liter volume and axial dispersal pin. The capacity was 7 centimeters. The sensitivity of the electrometer was 1.95 Volts per segment on the visiual scale. From the position of the electometer, a flat grass meadow streched for about 90 meters length so that the radiation source, a Radium compound, could be set up in any desired distance to up to 90 meters away from the apparatus. The success of the experiment is affected by two challenges: the observed effect decreases proportional with the square of the distance between the {Radium} compound and the ionization chamber. Therefore, saturation currents of very different values {ranges} need to be compared. And secondly, every tiny fluctuation of the natural ionization in the apparatus leads to uncertainties in the measurements of small effects. The measurement is therefore only possible if larger quantities of Radium are available. If the compound is set up to 90 meters distance from the experimental apparatus and one assumes a minimal effect for easy measurement of the above experiment to approx. 20 Volts per hour, then the needed minimal amount of Radium (element) [footnote 1] is calculated to approx. 700 milligrams. A much closer distance of 90 meters is not recommended since the decrease of radiation by absorption through just a 50 meter air layer is just 20%. Five test series were performed: 1020 milligrams Radium Chloride were used for number 1, 920 milligrams each for numbers 2 and 3, and 1420 milligrams each for numbers 4 and 5. The compounds were melted into glass tubes and then were also enclosed into a lead cylinder with 3 millimeters wall thickness in order to completely absorb the beta radiation. The Radium compounds were brought to a room in a basement specifically prepared for the determination of their natural dispersion {decay?, radiaton?}, surrounded by massive {meters thickness} concrete walls and appoximately 100 meters away from the experiment location. The direct line of connection between this storage room and the location of the apparatus goes through 20-30 meters of soil so that a measurable effect of the compound's gamma rays on the ionization chamber was completely excluded. These measurements were performed the following way: The compounds were setup in distances of 10,20,30,... up to 90 meters from the electrometer. Each time the saturation current was determined by averaging over 5-10 individual readouts. The natural dispersion {decay, radiation} was measured accurately before and after each experiment and then subtracted from the observed effects. If we define the collected ionization currents as i0, i1, i2, etc. and their respective distances r0, r1, r2, etc. then obviously the following equations apply i1 r1^2 = i0 r0^2 * exp(- (r1 - r0); i2 r2^2 = i0 r0^2 * exp(- (r2 - r0) etc., from where the absorption coefficient of the gamma rays in air can be calculated as ln (i0 r0^2) - ln (i1 r1^2) = --------------------------- . r1 - r0 The results are listed in the following table: experiment No. 1 . . . . . . . . 0.458 * 10^-4 No. 2 . . . . . . . . 0.493 * 10^-4 No. 3 . . . . . . . . 0.415 * 10^-4 No. 4 . . . . . . . . 0.479 * 10^-4 No. 5 . . . . . . . . 0.389 * 10^-4 Average value = 0.447 * 10^-4 The relatively large deviations between the individual values can be are caused from the unavoidable fluctuations of the natural ionization during the several hours duration of the measurement times. The average temperature was 22 deg C +/- 3 deg during the experiment, and the average barometric pressure 750 millimeters +/- 2 millimeters. The resulting average value agrees within 2% of the absorption coefficient calculated by Mc.Clelland from measurements with solid and fluid materials. Although this agreement should be declared coincidental, considering the uncertainty of the values for lead, it can be concluded from my measurements that the apsorption property of for gamma rays by Radium C in air is indeed what Eve assumed in his calculations: that the soil's {earth's} penetration radiation must decrease quickly with altitude, and that only a few percent of surface radiation can already be expected at an altidute of 500 meters. In recent times {days} I also had the opportunity to perform measurments of penetration radiation at different altitudes during {free-}balloon flights. A Wulf radiation apparatus served a measurement instrument, manufactured manufactured by Guenther & Tegetmeyer and made completely airtight and with temperature compensation for the sole purpose for balloon flights and underwater measurements. The wall thickness of the instrument is 2 millimeters so that it can easily withstand all pressure differences occuring during balloon flights. The instrument's capacity is 1.6 cm, and the volume is 2039 cubic centimeters. A voltage drop by 1 Volts is therefore analog to a ionisation value q = 1.56 iones per cubic centimeters and second (with the elementary quantum assumed to be e = 4.65 * 10^-10 E.S.E.). Several days before the ascent {balloon flight start} I made radiation measurements with this instrument at the club property of the Austrian AeroKlub, a flas grass meadow located in the Vienna Prater (park). Here, the following values of q were found: August 23 9am - 12pm q = 35.9 iones per cubic cm and second " 26 10am - 12pm q = 35.4 " " " " " " " 27 10:30am - 12:30pm q = 34.9 " " " " " " " 28 6:30am - 7:45am q = 32.3 " " " " " " The last value was observed immediately before the ascent. The relatively high values of q are mostly caused by the remaining {residual} radiation of the instrument walls. In order to deternine it, I performed several exoeriments about the shielding effect of external penetration radiation by submerging the instrument under water and by measuring above water level. These experiments - which are not finished yet, however - resulted in a value of approx. q = 25 iones per cubic centimeter and second for the residual radiation {Eigenstrahlung = "own radiation", literally translated} of the container walls. Thus, the external radiation at the location of the {balloon's} ascent [casually] gererated 10 iones in average [per ccm and sec]. The balloon flight occured during the nice weather period in the second half of August. The barometer already showed from August 21 a tendency of slowly rising air pressure. On August 23 and 24, short thunderstorms went over Vienna, but on the afternoon of August 25 the sky was totally clear again, followed by nearly cloudless midsummer days with mostly southerly winds. On August 28 at 8am, the balloon {with name} "Radetzky" (1200 cubic meters) by the Austrian Aeroklub was lifted [off the ground] with Herrn Oberleutnant S. Heller as flight leader and me as the only passenger, by the Military-Aeronautical Institute commanded by Herrn Hauptmann Hoffory. Since there was practically no wind, it {the balloon} stayed straight above the club's property [air field] at approx. 150 meters altitude immediately after take-off, and but after a quarter hour it started to drift slowly back and forth over the Prater {park}. The wind turned shortly before 9am; the balloon, which by now had risen to 390 meters level due to the effect of the sunshine, crossed the Danube River and took on the North to Northwest course along the river above {North of} the Inundations area {?, probably name of Vienna suburb}. Above Viena and Wienerwald {Vienna Forrest, name of the place} was a light layer of fog in the morning which started to disappear around 9:30am. At 9:20am we passed over Klosterneuburg at an altitude of 550 meters after another crossing over the Danube River, then we rised to 800 meters level and flew again in a more or less Northern direction. The Danube River was flown over a third time; near Spillern we were at 900 meters altitude and finally we reached above Schoenborn-Mallebarn an altitude of 1070 meters (1300 meters above sea level). The landing occured smoothly after 11am near Goellersdorf. The flight route was approx. 45 km. The sky was cloudless during the entire trip. The temperature was 17 degrees {C} at 7am at the starting point, and the relative humidity {was} 88%. At the highest elevation reached during the flight, the temperature in the shadow of the balloon's basket was 28 degrees {C}, and the relative humidity at 24%. I started the measurements in the balloon at 8:14am and continued until 10:48am. After the landing, the measurements were immediately carried on at the ground. The attachment of the instrument at the rim {wall} of the basket proved to work very well: the instrument was screwed onto a small console which was hung on the basket rim {wall} via overlapping hooks. An adjustment screw allowed exact horizontal levelling of the console board. The observations could be made with the same ease as done on the ground. The isolation of the instrument was as good {same good quality} before and after the ride {flight}; the charge loss was 0.24 Volts per hour. The results of the observations are summarized in the following table. time location altitude observed radiation ------------------------------------------------------------------------- 06:30-07:45 club property of the Aeroklub 0 32.3 iones/ccm/sec (in the Prater) 08:14-08:59 over Vienna, along Danube River 150-440 m 28.1 " 08:59-09:44 over Inundations area and 440-800 m 34.7 " Klosterneuburg 09:48-10:18 over Leitzersdorf, Hatzenbach 800-900 m 34.3 " 10:18-10:48 over Hoebersdorf, Schoenborn- 900-1070 m 35.5 " Mallebarn 11:11-12:12 on the strubble field bei 0 34.9 " Goellersdorf (after landing) So, in general, the radiation at {higher} altitudes was not substantially different from the radiation at ground level. The radiation decrease of 4.6 iones found in the second measurement interval is, what I believe, not of much importance as individual value {single data point}. My results, which in contrast to the last balloon flights by Gockel were performed with airtight instruments, agree very well with Gockel's results. If corrected to normal air pressure, Gockel's measurements even show a slight increase of radiation with altitude. According to the results so far needs one to think that there are other ionisators {ionisating particles}, besides radioactive substances of the soil {earth}, with penetration radiation in the air and that their effect rises with increasing altitude. There is an obvious objection that the balloon is covered with radioactive inductions {?} during the flight caused by the strong surrounding electrical field, which could cause the increasing radiation levels. But, in my opinion, it doesn't apply because the measurements made after the return on ground level next to the balloon jacket {cover, hull} showed by all means a normal radiation value. To possibly further clarify the question about the source of this penetration radiation at higher altitude levels, I will undertake one more balloon flights in near future which will last during a night. In conclusion, at this point I'd like to cordially express my thanks to the chairmanship of the Austrian Aeroklub, especially Herrn Dr. Baron von Economo for providing the flights free of charge and to Herrn Oberleutnant Siegfried Heller of the Austrian Airship Division for his kind support during the flight.