30lun*^ 
 
 MDDC 1069 
 
 UNITED STATES 
 ATCMIC ENERGY COMMISSION 
 OAK EIDGE 
 TENNESSEE 
 
 ARTIFICIAL RADIOACTIVE ISOTOPES OE POLORIDM, BISMUTH, AND LEAD 
 Chapter 17: A New Reaction with High Energy Deuterons 
 
 by 
 David H. Templeton 
 
 University of California 
 
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MDDC-1069 
 
 AXTIFICIAL HADIQACnVI ISOTOPES OF H1L0IIUM, BISMDTH, AHD LEAD 
 
 Dissertation for the Depree of Doctor of Kiilosophy 
 
 David H. Ibmpleton 
 
 TJnlverBity of California, Berkeley, California 
 
 Chapter IV 
 A New Reaction with Hiph Energy Deuterons 
 
 211 
 The fonnatfon of At has been observed when bismuth was bombarded with hiph 
 
 eijeriry deuterons. This cannot be explained by impurities in the targ-et or helium in 
 
 •ttie ion source. A reaction of the type (d,p~) is unreasonable for theoretical 
 
 reaaons . A reaction of the type (d,;u') where /i" represents a negative mesotron, 
 
 cannot explain the phenomenon in every case. An explanation which is consistent with 
 
 the observed facts assumes that secondary alpha particles produced by the incident 
 
 209 211 
 
 deuterons in turn <au8e the Bi (a,2n)At reaction. Reasonable cross sections for 
 
 bhe two steps in this mechanism account for the observed yield. 
 
 yihen bismuth was bombarded with 200 Mev deuterons the polonium fraction, sepa- 
 rated by distillation, contained a strong alpha activity of 7.5 hours half-life. 
 
 Alpha -pulse analysis showed two groups of alpha particles at about 5.9 and 7.4 Mev. 
 
 210 
 This activity carried on tellurium when reduced with sulfur dioxide, while Po 
 
 under the same conditions did not carry. It was concluded that the activity was 
 
 At , which is known to have these characteristic properties. No known reaction 
 
 211 209 
 
 of deuterons can produce At from Bi . Ihis same type of reaction has, however, 
 
 'leen confirmed in other parts of the periodic table by notine the formation of iodirc 
 
 activities from antimony, tellurium from tin, and gallium from copper. 
 
 Bombardments were made with various energies of deuterons by setting ihe probe 
 
 t different radii in the cyclotron, and astatine was separated from a portion of 
 i-ach target. Ihe deuteron energies were estimated from magnetic field measurements. 
 
 The yields of astatine from these bombardments are listed in liable 1 as calculateu 
 
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 assujninp a current of 0.3 microampere of deuterons . Die yield from a 400 Mev helium 
 ion bombardment, assuminp 0.015 microampere, is also listed. Ihese values -are onljr 
 approximate because of large uncertainties in -the chemical yield, in the peometriocl 
 distribution of the beam on the target, and especially in the values of the beam 
 current. 
 
 Table 1 
 Yields of Astatine from Bismuth 
 
 Projectile 
 
 Cross-section 
 
 400 Mbv a 
 200 Msv d 
 
 150 Lfev d 
 90 Mev d 
 70 Mev d 
 50 Mev d 
 30 Mev d 
 
 2 X 10"^^ om^ 
 1 X 10-29 
 4 X 10-2° 
 6 X 10-21 
 
 6 X 10-21 
 4 X 10-21 
 
 7 X 10-21 
 
 211 
 If the compound nucleus Po were to decay by emittlnp a negative beta par- 
 ticle of very hirh enerpy, leavinr insufficient energy for any other particle to b< 
 
 211 
 ejected. At would result. If one calculates the time required for this prooesi 
 
 by extrapolation of the known half-life vs enerp-y relationship for beta emitters, 
 
 tne resulting half-life is many powers of ten lon/rer than the expected lifetime of 
 
 An excited nucleus. As the excitation energy is increased, the time required for 
 
 l.eavy particle emission deoreases much faster than that required for beta-particle 
 
 fcir.ission. 
 
 A second hypothesis is that the compound nucleus emits a negative mesotron, 
 
 211 
 
 forming At . Ihis mechanism should be possible only at hiph energies, because 
 
 about 100 Mev -nust be supplied for the mass of the mesotron. Considering the 
 
MDDC-1069 
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 blndlni? enerey of the deuteron and the uncertainty in the value of the mass of the 
 mesotron, 90 Mev deuterons mifht have just enoufrh energy for this reaction. The 
 amall but finite yields at 70, 50, and 30 Mev show that some other explanation is 
 necessary. 
 
 In order to obtain further evidence for or against a mechanism involving meso- 
 tron formation, lead was bombarded with 400 Mev helium ions. 
 
 207 211 
 
 Pb plus a helium ion forms the same compound nucleus, Po , as deuterons 
 
 on bismuth, lihen lead was bombarded with 400 Mev helium ions, very little if any 
 astatine was formed. Ihus the formation of the astatine by mesotron formation or by 
 any other mechanism involving this compound nucleus is very improbable. In the case 
 of the helium bombardment of lead the excitation of the compound nucleus was much 
 greater than for deuteron boiibardments of bismuth, so •that the argument is not com- 
 pletely riporous. 
 
 Ihe astatine was not formed from thorium or uranium impurities in the target, 
 because of the absence of short-lived radium isotones known to be formed from these 
 materials under these conditions. Ihere is no known isotope between thorium and 
 
 bismuth of long enough life to have been present undetected in the target in ap- 
 preciable concentration. 
 
 Helium Ion contamination in the ion source is not a possible explanation for 
 the astatine at high enereies. When the target is at radius 81 inches, the yield 
 of astatine per unit time is about the same with deuterium as with helium in the ion 
 source, even when the tank had been pumped out repeatedly to remove helium. Ihe 
 higher cross-section for formation by helium ions is compensated by the lower efi 
 fioiency of the ion source to produce helium ions. Since the cross-seotion for for- 
 nation of astatine by helium ions is much greater at lower energies, helium eontam- 
 
 nation may have contributed to the observed yields in the lower enerpy deuteron 
 bombardments . 
 
MDDC-1069 
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 Ihe only slnrle mechanism which is fairly consistent with all the facts is 
 
 that the astatine is produced by secondary alpha particles. It is likely that these 
 
 alpha particles would be expelled from the heavy element nuclei with about 30-40 Mev 
 
 enerpy. It is known experimentally that the a,2n orosa-seotion for alpha particles 
 
 -25 2 
 of this energy range is about 5 x 10 om . If it is assumed that this cross-aecticn 
 
 ia effective, on tlie average, over the first 0.025 cm of the alpha particle path in 
 
 211 
 bismuth, one such alpha particle in 3000 will produce At . Usinp this figure and 
 
 211 
 the observed yields of At , the cross-section for formation of the secondary alpha 
 
 particles was calculated. Ihese cross-sections are listed in Ifeble 2. %erefore if 
 
 the cross -sections for formation of secondary alpha particles are as hiph as the 
 
 values listed in Ibble 2, the phenomenon is explained. It should be noted that most 
 
 of these cross-sections are but small fractions of the geometrical cross-section for 
 
 -24 2 
 bisipulii, which is of the order of 10 om . 
 
 Ihble 2 
 Cross -sections Required for Secondary Alpha Formation to Account for 
 
 the Astatine Formation 
 
 Projectile 
 
 Cross-Section 
 
 400 Mev a 
 
 6 X 10"^^ om^ 
 
 200 Nfev d 
 
 3 X 10"^^ 
 
 150 Mev d 
 
 1 X 10-2« 
 
 90 Mev d 
 
 2 X 10-27 
 
 70 Itev d 
 
 2 X 10-" 
 
 50 Mev d 
 
 1 X 10-" 
 
 30 Mev d 
 
 2 X 10-" 
 
 It may be possible to verify this su''pested mechanism in a number of ways. If 
 several very tiiin foils of different thicknesses (much less than the range of a 30 
 Mev alpha particle) are bombarded with deuterons, the observed overall cross -sect: on 
 
MDDC-1069 
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 should be a function (approximately linear) of the thickness of the foil. For a 
 one-step mechanism, the cross-section should be independent of target thickness. 
 Alternate approaches are to try to observe the secondary alpha particles by cloud 
 chamber or photographic emulsion tracks, or by inducing reactions in secondary tar- 
 gets outside of the incident deuteron beam, but within the ran^e of secondary alf^a 
 particles. None of these experiments has yet been attempted. 
 
MDDC-1069 
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 References 
 
 1) D. R. Corson, K. R. iiacKenzie, and E. Segre, Phys . Rev. 58, 672 (1940). 
 
 2) R. H. Goeckermann, private communication. 
 5) M. Lindner, private coiranunioation. 
 
 4) D. R. Miller, private communication. 
 
 5) P. R. O'Connor, private communication. 
 
UNIVERSITY OF FLORIDA