NEU'FRONS BY ALPHA-PARTICLE BOMBARDMENT OF LIGHT ELEMENTS1 C. S. COPELAND* A N D S. C . . L I N D
School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received February 6, 19$8
In view of the widespread interest in neutrons, it was felt that it might be of interest to examine the light elements under the same experimental conditions with respect to their neutron emission under a-particle bombardment, using radon as the source of a-particles. For detection of the neutrons, it was decided to use the radioactivity induced in iodine. This element is one which is strongly activated by neutrons, and forms a single radioactive isotope, 112*, with a very convenient half-period of 25 minutes. By the use of ethyl iodide and the method of Szilard and Chalmers (5), the activity induced in a very large quantity of iodine can be concentrated and easily measured. It must be admitted that the activity so obtained is not an absolute measure of the number of neutrons emitted by a given source, nor is the ratio of two activities produced by two different sources necessarily the ratio of the numbers of neutrons emitted by the two sources. This is because iodine (or any detector element) is not 'activated to the same extent by neutrons of different energies, and the energy spectrum of the neutrons emitted by diffaent elements may vary widely. For a discussion of the difficulty of determining the absolute number of neutrons emitted by a given source, the reader is referred to the paper by Amaldi, Hafstad, and Tuve (1). EXPERIMENTAL
The element, or compound of the element, to be examined was sealed, except where otherwise stated, with radon in a soft-glass bulb approximately 7 mm. in diameter. This bulb was kept in a soft-glass tube, sealed at one end, which could be dipped into ethyl iodide. The radioiodine was prepared by placing the neutron source at the This paper was abstracted from a thesis submitted by C. S. Copeland to the Graduate Faculty of the University of Minnesota in partial fulfillment of the reiuirements for the degree of Doctor of Philosophy, June, 1937. * Present address: Department of Chemistry, University of Southern California, Los Angeles, California. 567
568
C. 8. COPELAND AND S. C . LIND
center of a 500-ml. volumetric flask which contained 490 ml. of ethyl iodide plus approximately 27 mg. of free iodine. To increase the amount of radiokdine produced, the flask was cast in paraffin in a 4-liter beaker. After a measured time of irradiation the neutron sourc? was removed and the free iodine was extracted with 30 ml. of water containing a little sodium bisulfite. The iodide so formed was precipitated as silver iodide, filtered, washed, dried with alcohol and ether, and then examined for radioactivity. The radioactivity of the silver iodide was measured by means of an ionization chamber connected to a linear amplifier, The ionization chamber consisted of a brass cylinder 13 cm. long and 7.7 cm. in diameter fitted
FIG.1. Diagram of the circuit. rl, resistance of such value that the potential drop across the filament is 2.5 volts; r2, rd, 10,000-ohm potentiometers; r3, fixed resistance, 30,000 ohms; r6, 500-ohm rheostat; rb, 100-ohm rheostat; A, ionization chamber; G, galvanometer, 10-9 to 10-10 amperes per millimeter, 1500 ohms resistance; V, 0 t o 10 voltmeter. at one end with an aluminum window 0.1 mm. thick and 5.2 cm. in diameter. The central electrode was a brass wire 1 mm. in diameter and approximately 9 cm. long. A brass rod mounted in an amber plug in the side of the chamber served t o support the central electrode and also t o connect the electrode with the amplifier circuit. To prevent any electrical leakage from the case to the central electrode across the amber, the amber plug was mounted in a grounded brass tube which was in turn mounted in hard rubber. By means of six 45-volt B-batteries, the case was kept at $270 volts with rrspect to the central electrode. The ionization chamber was filled with carbon dioxide at pressures ranging from 1to 3 atmospheres. The chamber was mounted with the aluminum window face down so that
NEUTRONS BY (Y-PARTICLE BOMBARDMENT OF LIGHT ELEMENTS
569
the filter paper holding the silver iodide could be placed directly under the window. The linear amplifier circuit was a standard circuit using the General Electric FP 54 Pliotron tube. The circuit is shown in figure 1. The circuit was shielded and the Pliotron tube was mounted in a brass case which could be evacuated. This case was equipped with a contact key (K in figure I), so that the control grid of the tube could be grounded or floated a t will. The floating grid method of measurement was used. The radioactivity of the radioiodine was measured with respect to the activity of a standard radium tube (0.0036 mg. of radium) taken as unity. The radium standard was used in order to eliminate error arising either from variations in the sensitivity of the circuit or from variations in the pressure in the ionization chamber. It was proved experimentally that the relative activity of the radioiodine was independent of the pressure in the ionization chamber over the range of pressure used. The value of the relative activity of the radioiodine, cyo, a t the time of removal of the neutron source was obtained by graphical extrapolation. The half-period was taken as 25 minutes. From cya, a quantity K , proportional to the neutron emission per millicurie per second, was calculated by means of the equation
where t is the time of irradiation of the ethyl iodide and Mo is the number of millicuries of radon present a t the start of the irradiation. AI and X are the decay constants of radioiodine and radon, respectively. Since in most cases compounds and not pure elements were used as neutron sources, it was necessary to calculate the neutron emission which would have been observed if the pure emitting element, and not its compound, had been used. The appropriate conversion factor, p, to make this calculation was obtained from the equation
where ni is the subscript of the ith element in the formula of the compound and ne is the subscript of the emitting element. 2 is the atomic number, DISCUSSION OF RESULTS
The results obtained for a number of substances are shown in table 1. In the column headed “neutron source” are given the elements or compounds which were sealed in glass bulbs with radon. I n the case of compounds, the neutron emission given is for the compound as a whole. The last column gives the neutron emission 011the basis that the emission for
TABLE 1 Neufron emission by various substances when bombarded by a-particles from radon
-
NEUTRON BOURCE
IYE OP IR IADIATIOJ OP ETHYL IODfDE
Mo
K
minutea
Beryllium . . . . . . . . . . . . . . . . . . . . . . .
218 22
55
34.6 32.2 23.5
Lithium carbonate
196 223 83 137
60.0 47.7 46.3 40.6
2.74 1.82 2.04 2.13
Boron (amorphous)
154 235 93
63 .O 49.4 47.9
5.20 4.13 3.54
15.0
Calcium fluoride. . . . . . . . . . . .
224 27 1 2912
79.8 55.7 53.7
8.70 6.35 3.80
19.8 20.7 17.9
19.5
12.4
Sodium carbonate. . . . . . . . .
397 426 164
04.3 88.9 83.4
4.35 2.96 3.15
7.69 6.18 6.85
6.9
4.4
Magnesium oxide, . . . . .
227 315 266 256
80.6 76.8 37.3 31.2
1.33 1.18 0.51 0.48
2.99 2.81 2.5 2.8
2.8
1.8
Aluminum (100-mesh filings)
293 100 164 400
69.1 66.6 64.7 56.9
2.55 2.50 2.45 2.00
6.71 7.12 6.86 6.48
6.8
4.3
Phosphorus (red). . . . . . . . . . . . . . . .
370 354
59.1 48.4
0.49 0.40
1.6 1.5
1.6
1.o
Potassium chloride . . . . . . . . . . . . . .
324 200
59.1 39.2
0.56 0.26
1.9
1.7
1.1
Potassium carbonate. . . . . . . . . . . . .
281 179
73.3 70.1
0.12 0.16
0.3 0.4
0.4
0.3
Calcium carbonate . . . . . . . . . . . . . .
316 138
79.6 66.2
0.17 0.15
0.4 0.4
0.4
0.3
30.0 12.2 17.0
57
157
100
50
64 8.28 8.36 8.75 8.04
8.4
5.4
14.8
9.43
15.1 14.2
1.4
-
M o is the initial number of millicuries of radon in the neutron source. a . is the radioactivity of the silver iodide a t the time of removal of the neutron source. K is proportional to the neutron emission per unit time per millicurie of radon. ;is the average value of K. 570
NEUTRONS BY PARTICLE BOMBARDMENT OF LIGHT ELEMENTS
571
beryllium is 100. I n addition to the substances listed in table 1, silica, sulfur, zinc, carbon, and paracyanogen were examined. Silica was examined by sealing about 30 millicuries of radon in a fused quartz bulb. A very weak emission was observed. As nearly as could be judged, after allowing for the amount of radon used, the emission was of the same order of magnitude as the emission observed with the potassium and calcium carbonate sources. Sulfur and zinc were examined under conditions somewhat different from those used for the other substances investigated. Flowers of sulfur and zinc dust were sealed with radon in Pyrex bulbs, and the ethyl iodide was irradiated in a 500-ml. round-bottom flask. The results so obtained were compared with the result observed when a Pyrex bulb containing aluminum and radon was employed. Both the sulfur and zinc bulbs showed a weak emission of about the same order of magnitude. The emission was certainly less than one-tenth that obtained with an aluminum bulb. The paracyanogen and carbon were both sealed in fused quartz bulbs. The ethyl iodide was irradiated according to the standard procedure, but the activity of the radioiodine was measured with a Geiger tube counter. This activity was compared with that produced by a Pyrex bulb filled with beryllium and radon. Taking the neutron emission for beryllium as 100, the emission for paracyanogen was approximately 0.9 and the value for carbon was approximately 0.4. The above value for the neutron emission of paracyanogen is certainly too low. It was discovered after the radon had been introduced that insufficient paracyanogen had been used to fill the bulb, and, furthermore, a globule of mercury was accidentally introduced at the time the radon was collected. Since an emission of about the same order of magnitude was obtained with calcium carbonate, potassium carbonate, sulfur, and zinc sealed in glass, carbon sealed in quartz, and an empty quartz bulb, it seems rather difficult to ascribe any significance to this emission from t h e standpoint of any particular element involved. Indeed, in those cases where glass bulbs were used, a weak residual emission might be expected, sirice an empty soft-glass bulb filled with radon shows a neutron emission of about 1.5 (beryllium = 100). On the whole, in view of the results obtained, it would seem best to consider that there is a general background, and that all drfinitely positive results should be corrected for this background. Table 2 gives the values for the substances listed in table 1 after this correction has been made. In addition, in table 2 the corrected values of the neutron emission for the compounds listed have been converted into values for the active element involved. These results are shown graphically in figure 2.
TABLE 2 Corrected values of the neutron emission for the substances Eisted in table I
___.
NENTRON SOURCB
x
____
s___l_
Lithium carbonate . . . . . . . . . . . . . . . . . . . . . . . . Beryllium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Paracyanogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
..................... Sodium carbonate, . . . . . . . . . . . . . . . . . . . . . . . . Magnesium oxide, . . . . . . . . . . . . . . . . . . . . . . . Aluminum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphorus., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potassium chloride. . . . . . . . . . . . . . . . . . . . . . . .
5.1 100 9.1 0.5 12.1 4.1 1.5 4 .O 0.7 0.8
ACTIVP: ELEMENT
Ir
Y'
Li Be B
4.68
N F Mi3
1.90 1.85 2.55 1.76
P c1
1.93
24 100 9.1 1 22.4 11 2.6 4.0 0.7 1.5
Na
AI
is proportional to the neutron emission per second per millicurie of radon. the conversion factor by which the value of the neutron emission for a compound is multiplied to obtain the neutron emission of thc active element in the compound. K
1 is
'K
= gK.
I
.I "e.!
Fro. 2. Neutron yields from certain light elements (rare gases not investigated) 572
NEUTRONS BY a-PARTICLE BOMBARDMENT OF LIGHT ELEMENTS
573
In the case of the oxides and carbonates listed in table 2, the neutron emission is ascribed to the metal involved, because of the weak neutron emission observed with potassium carbonate, calcium carbonate, and silica. The neutron emission observed with paracyanogen is due to the nitrogen, since a similar carbon-filled bulb is a decidedly poorer neutron emitter. This is in agreement with the observation of Wertenstein (7) that nitrogen under RaC’ a-particle bombardment produces radiofluorine. In the case of potassium chloride, the active element is undoubtedly chlorine, because of the appreciable difference in neutron emission shown by potassium chloride, and potassium carbonate. If potassium were the active element, we should expect the two compounds to show about the same neutron emission, since the conversion factor to potassium is 1.93 for potassium chloride and 2.07 for potassium carbonate. The choice of chlorine as the neutron emitter in potassium chloride is given added weight by the recent work of Pollard, Schultz, and Brubaker (3), who showed that both argon and chlorine emit neutrons’under ThC’ and RaC’ a-particle bombardment. The chlorine disintegration has been further studied by Hurst and Walke (2) and by Ridenour and Henderson (4),using artificially accelerated 11 MeV. and 9 MeV. a-particles, respectively. With regard to a neutron emission from potassium, Zwy (8) has shown, by studying the radioactivity induced in potassium chloride, that potassium must disintegrate with neutron emission under RaC’ a-particle bombardment. Walke (6) has also investigated this reaction, using 11 MeV, a-particles, and has shown that both K39and K41 disintegrate with neutron emission. Our experiments would seem to show that the neutron emission from potassium under RaC’ a-particle bombardment must be slight, since we were unable to detect any difference between potassium and calcium carbonates. Even if we assign all of the emission observed with potassium carbonate to potassium, the emission for potassium would only be approximately 0.5 per cent of that observed for beryllium. SUMMARY
All of the elements from lithium through calcium, with the exception of the rare gases, have been investigated for neutron emission under a-particle bombardment. The a-particle source used was radon, and in each case the neutron yield was determined by measurement of the radioactivity induced in iodine. Positive neutron emission was observed for lithium, beryllium, boron, nitrogen, fluorine, sodium, magnesium, aluminum, phosphorus, and chlorine. The neutron emission from carbon, silica, sulfur, potassium carbonate, calcium carbonate, and zinc is less than 0.5 per cent of the emission from beryllium.
574
C. 8. COPELAND AND 8. C. LIND
REFERENCES (1) AMALDI,HAFSTAD, AND TUVE:Phys. Rev. 61, 896 (1937). (2) HURSTAND WALKE:Phys. Rev. 61, 1033 (1937). (3) POLLARD, SCHULTZ, AND BRUBAKBR: Phys. Rev. 61, 140 (1937). (4) RIDENOUR AND HENDERSON: Phys. Rev. 62, 889 (1937). (5) SZILARD .4ND CHALMERS: Nature 134, 462 (1934). (6) WALKE:Phys. Rev. 62, 400 (1937). (7) WERTENSTEIN: Nature 133, 564 (1934). (8) ZWY: Nature 134, 64 (1934).
