88-Inch Cyclotron Research Articles

Mass-Ratio Method Applied to the Measurement ofL-Meson Masses and the Energy Balance in Pion Decay

We report a comprehensive series of measurements made on the masses of $L$-mesons produced by the 184-inch cyclotron. A general ratio principle of measurement is employed which largely eliminates systematic errors. The particular method that we have developed is described in detail. The theory of nonequilibrium particle orbits in the cyclotron field is worked out to provide formulas from which momenta may be calculated, and to obtain the momentum distribution functions determined by the target and detector dimensions. The energy-loss processes in nuclear track emulsion, which is used as a stopping material and detector, are studied and the range-momentum exponent $q$ is found. Several small corrections to the mean range are made. A number of range straggling effects are evaluated. The theoretical distribution of the quantity $R{p}^{\ensuremath{-}q}$ ($R$ being the range and $p$ the momentum) is studied, and the first three moments of the distribution are calculated explicitly. The distribution is found to be closely gaussian. From the theory of the distribution of $R{p}^{\ensuremath{-}q}$, the best estimate and the statistical uncertainty of the mass ratio (e.g., of meson to proton) are evaluated. A number of effects influencing the ratio are studied, but all the corrections found are very small. The measurement of the momentum acquired by the muon when a pion decays has also been treated. Several important relations connecting this quantity with the particle masses are then introduced. Apparatus developed for the application of the ratio principle is described. A number of experiments in which mesons and protons of similar velocities were detected in the same nuclear track plate are reported. Each experiment was repeated a number of times. The measurements of particle ranges and orbit parameters, the magnetic field measurements, the dimensional tolerances, the calculations, and other important details are discussed.The following mass ratios are reported: $\frac{{\ensuremath{\pi}}^{+}}{\mathrm{proton}}={0.1488}_{7}\ifmmode\pm\else\textpm\fi{}0.00011$, $\frac{{\ensuremath{\pi}}^{\ensuremath{-}}}{{\ensuremath{\pi}}^{+}}=0.998\ifmmode\pm\else\textpm\fi{}0.002$, $\frac{{\ensuremath{\pi}}^{+}}{{\ensuremath{\mu}}^{+}}=1.321\ifmmode\pm\else\textpm\fi{}0.002$. The center-of-mass momentum acquired by the muon in positive pion decay was measured as 29.80\ifmmode\pm\else\textpm\fi{}0.04 Mev/c and its energy, 4.12\ifmmode\pm\else\textpm\fi{}0.02 Mev. All the results are consistent if the rest mass of the neutral particle in the pion decay is zero. With this assumption, the measurements further imply that the positive pion-muon mass difference is 66.41\ifmmode\pm\else\textpm\fi{}0.07 electron masses. The derived masses, in units of the electron mass, are: ${\ensuremath{\pi}}^{+}=273.3\ifmmode\pm\else\textpm\fi{}0.2$, ${\ensuremath{\pi}}^{\ensuremath{-}}=272.8\ifmmode\pm\else\textpm\fi{}0.3$, ${\ensuremath{\mu}}^{+}=206.9\ifmmode\pm\else\textpm\fi{}0.2$.

Principles and Techniques of Mixed Radiation Dosimetry: Application to Acute Lethality Studies of Mice with the Cyclotron

Because of the increasing population of nuclear machines that subject man to yradiation and neutrons, it is important to study the biological effects of various radiations on animals. The common denominator for judging the amount of radiation put into the system under observation has been the physical dose. The objective of this discussion is to enumerate some of the factors governing the physical dose and to elaborate on some of the best methods for measuring them. As an illustration of the application of the principles and techniques, a careful study was made of the dose inside an animal exposure facility at the Oak Ridge National Laboratory 86-inch cyclotron. A companion paper (1) describes an acute lethality study which was made on mice, the cyclotron being used as a neutron source.

Thirty-Six-Atmosphere Diffusion Cloud Chamber

A 36-atmosphere diffusion cloud chamber has been constructed for use with the 184-inch cyclotron and with the bevatron. Minimum-ionizing particles leave dense, sharp tracks of good contrast with hydrogen filling at design pressure. In the operation described, one event is obtained every 15 minutes for a reaction cross section of 30 mb for negative pions on hydrogen. Among the unique features of the chamber are (a) the high pressure; (b) the high magnetic field; (c) the simple unit preassembly of the chamber and the upper pole piece; (d) the distortion-free photography afforded by the small camera windows of good optical quality directly in front of the camera lenses and the absence of a large top glass near the sensitive region; (e) the thin window for collimated beams; (f) the supported cast Astrolite windows for illumination; and (g) the satisfactory sensitive layer obtained despite the large height-diameter ratio of the pressure vessel.

Analysis of Secondary Particles Resulting from High-Energy Nuclear Bombardment

Experimental studies have been made of the low-energy yields of secondary particles resulting from high-energy bombardment. For this purpose, thin foils of Be, Al, Ni, Ag, Au, and U have been bombarded by an internal beam of 375-Mev alphas, 332-Mev protons, and 187-Mev deuterons. Secondary particles emerging from the disintegration of the nucleus at 0\ifmmode^\circ\else\textdegree\fi{} to the incident beam direction are magnetically analyzed and detected in nuclear emulsions located beneath the median plane of the 184-inch cyclotron. There are three specific positions for these emulsions corresponding to energies of the secondary protons and of alpha particles of approximately 6, 10, and 20 Mev. This apparatus was modified to make a special study of the secondary alpha and proton yields by including two extra plates so that 10 energy points could be taken in the range from approximately 5 to 22 Mev. The angular distribution of secondary particles has also been measured for 240-Mev alpha bombardment of Be, Al, Ni, and Ag. Here, secondary particles emitted at 0\ifmmode^\circ\else\textdegree\fi{}, 45\ifmmode^\circ\else\textdegree\fi{}, and 135\ifmmode^\circ\else\textdegree\fi{} are detected in nuclear emulsions at positions for which the secondary proton and alpha energy is approximately 6 Mev. A secondary particle is identified by measurement of its radius of curvature upon entering the emulsion and its range and specific ionization in the emulsion. A considerable yield of hydrogen and helium isotopes as well as of particles of higher atomic number is found. The relative yields of the secondary protons and alpha particles for each element and for each bombardment are shown as a function of energy. The results, while found to be consistent with the predictions of an evaporation model, also indicate that a large percentage of secondary protons and alphas are directly knocked out of the nucleus.

Studies on the Isomeric Pair,Nb89mandNb89

By bombardment of zirconium, niobium, yttrium, and silver targets in the 184-inch cyclotron it has been possible to produce the isomer pair Nb 89m and Nb 89 . The isomerism arises from the odd 41st proton below the 50-proton closed shell of the single-particle nuclear model. Nb 89 (g 9/2 ) is a 1.9-hour activity emitting 2.85-Mev positrons to produce Zr 89 . The yield of the Nb 89m (p ½ ) is about 30-100 fold less and its radiations were not directly observed. By scintillation spectrometer measurements on zirconium daughter activity it is shown that Nb 89m decays with about a 2-hour half-life to 4.4-minute Zr 89m (p ½ ). The M4 gamma transition between the two niobium levels is not observed, indicating that the level spacing is small and the upper level deactivates primarily by positron emission. The logft values are larger than expected and are discussed in terms of the even-even core rearrangement considerations of A. De-Shalit and M. Goldhaber [Phys. Rev. 92, 1211 (1953)]. Nb 89 was also prepared by bombardment of NaBr with accelerated carbon ions in the 60-inch cyclotron.

Comparison of Nitrogen- and Proton-Induced Nuclear Reactions

Excitation functions for ($p, 2p$), ($p, \ensuremath{\alpha}$), and ($p, 2\ensuremath{\alpha}$) reactions on ${\mathrm{Mg}}^{25}$ were measured with the internal 22-Mev proton beam of the 86-inch cyclotron. The results are compared with the excitation functions obtained for the reactions produced by 29-Mev nitrogen ions on carbon in the 63-inch cyclotron. Both sets of reactions may proceed via the same compound nucleus, ${\mathrm{Al}}^{26}$. After corrections for the barrier penetrations of the incident particles, the (N, $2p$) and ($p, 2p$) results are quite comparable. The corrected (N, $\ensuremath{\alpha}$) and (N, $2\ensuremath{\alpha}$) cross sections are larger than those for the corresponding proton-induced reactions by factors of about two and four respectively, indicating that processes other than conventional compound nucleus formation are involved in these nitrogen-induced reactions. The shape of the excitation functions for the nitrogen-induced reactions indicates that the protons and alpha particles are emitted from nearly spherical nuclei with charge equal to that of the compound nucleus; direct interactions such as stripping are thus excluded.

New Chain Barium-126—Cesium-126

The neutron-deficient chain ${\mathrm{Ba}}^{126}$-${\mathrm{Cs}}^{126}$ has been produced from nitrogen-ion bombardments of indium in the 60-inch cyclotron, by the reaction ${\mathrm{In}}^{115}(\mathrm{N}, 3n)$. Studies have been made of this new chain with a 50-channel scintillation spectrometer, a scintillation coincidence spectrometer, and a time-of-flight mass spectrograph. Element assignments and genetic relations have been verified chemically, and the mass number assigned with the isotope separator. ${\mathrm{Ba}}^{126}$ decays principally by orbital electron capture with a half-life of 96.5\ifmmode\pm\else\textpm\fi{}2.0 minutes, and its daughter ${\mathrm{Cs}}^{126}$ is a positron emitter of 1.6\ifmmode\pm\else\textpm\fi{}0.2 minute half-life and electron capture branching of 18\ifmmode\pm\else\textpm\fi{}4 percent. The decay of ${\mathrm{Cs}}^{126}$ proceeds by allowed transitions, \ensuremath{\sim}62 percent to the ground state of ${\mathrm{Xe}}^{126}$ and \ensuremath{\sim}38 percent to the first excited state at 385 kev. The positron spectrum has a maximum energy of 3.8\ifmmode\pm\else\textpm\fi{}0.4 Mev. On the basis of its decay properties, ${\mathrm{Cs}}^{126}$ appears to have a groundstate configuration of (1+).

Scattering of 10-Mev Protons on Carbon and Magnesium

In order to study the mechanism of excitation of nuclear levels by inelastic scattering, a scattering chamber of 36-inch diameter was constructed. The deflected beam of the Berkeley 60-inch cyclotron was brought out of the shielding by means of strong focusing magnets. Particles from the nuclear reactions were detected by either of two systems, a quadruple proportional-counter telescope, or a coincidence crystal spectrometer. Energy resolution was of the order of 2.5 percent. The angular distribution of protons from the reactions ${C}^{12}(p, {p}^{\ensuremath{'}}){\mathrm{C}}^{12*}$, $Q=\ensuremath{-}4.43$ Mev, and ${\mathrm{Mg}}^{24}(p, {p}^{\ensuremath{'}}){\mathrm{Mg}}^{24*}$, $Q=\ensuremath{-}1.38$ Mev, were found to be peaked in the forward direction. Since the compound nucleus is excited to about 13 Mev, where the level density is expected to be high and the statistical theory of the nucleus should hold, it is proposed that two processes of excitation occur. One, the formation of the compound nucleus and its subsequent decay---symmetrically about 90\ifmmode^\circ\else\textdegree\fi{}; two, direct collision of the incident proton with a nucleon on the surface shell of the nucleus. The elastic scattering of protons on the above nuclei was also studied and interference maxima were found. Three new levels are believed to have been confirmed in magnesium.

An Indirect Measurement of then−nInteraction

The gamma-ray spectrum from the process ${\ensuremath{\pi}}^{\ensuremath{-}}+d\ensuremath{\rightarrow}2n+\ensuremath{\gamma}$ has been remeasured using a high resolution 110-channel pair spectrometer. The negative pions produced in an internal target by 335-Mev protons of the Berkeley 184-inch cyclotron, are captured at rest in a high-pressure deuterium vessel. Comparison of the spectrum with the gamma-ray spectrum from the process ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}n+\ensuremath{\gamma}$ measured with the same apparatus shows the effect of the $n\ensuremath{-}n$ interaction as a broadening of approximately three times the resolving width. In spite of the low counting rates and resulting statistical inaccuracy, it is possible to establish limits for the scattering length "$a$" for $S$-state neutron-neutron scattering. The experimental data are analyzed by evaluating a weighted first moment of the spectrum. Comparison of the weighted first moments of the theoretical spectra with that of the data leads to a value of $a=\ensuremath{-}15.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm with limits, based on the probable error, of $a=\ensuremath{-}8.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm and $a=\ensuremath{-}\ensuremath{\infty}$ cm. The value $a=\ensuremath{-}15.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm corresponds to an unbound state (\ensuremath{\sim}160 kev positive) for the hypothetical dineutron. The bound dineutron of more than 50 kev is less than 0.1 percent probable. This result is consistent with the $n\ensuremath{-}p$ and $p\ensuremath{-}p$ scattering data and is in agreement with the theory of charge independence of nuclear forces.

The accumulation and destructive action of astatine 211 (eka-iodine) in the thyroid gland of rats and monkeys.

INTRODUCTION IN 1940 Corson et al. isolated a radioactive element, the chemical, physical and nuclear properties of which established it to be element 85, the last of the halogen group (1). This element, now named astatine (from the Greek aararos, “unstable”) was produced by the transmutation of bismuth by alpha-particles accelerated at 30 Mev1 in the 60-inch cyclotron at the Crocker Laboratory, University of California at Berkeley. Hamilton and Soley compared the metabolism of I131 and At211 in the guinea pig and demonstrated that At211, like I131, was accumulated by the thyroid gland. This work included the use of not only normal animals but also guinea pigs in which thyrotoxicosis had been produced by the administration of the thyrotropic hormone (2). Later, a more detailed report was presented by Hamilton et al., which was concerned with not only the metabolism but also the biologic effects of astatine in rats and monkeys. In this work, changes due to the destructive action of astatine in the thyroid ...

Nuclear Radii from Inelastic Cross-Section Measurements

Inelastic cross sections for high-energy protons, deuterons, ${\mathrm{He}}^{3}$ particles, and alpha particles accelerated in the 184-inch cyclotron were measured by an attenuation technique. All the cross sections vary as the square of the nuclear radius and indicate that if the nucleus has a fringe it increases as ${A}^{\frac{1}{3}}$. The derived value of the nuclear radius constant ${a}_{0}$ depends on the nuclear model and the particular bombarding particle. The effect of the nucleus is found to extend at least to a radius given by ${a}_{0}=(1.68\ifmmode\pm\else\textpm\fi{}0.04)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm. Deuteron stripping cross sections were derived and found to vary approximately as the square root of the mass number.

Elastic Scattering of Intermediate-Energy Alpha Particles by Heavy Nuclei

The deflected alpha-particle beam of the University of Washington 60-inch cyclotron has been used to study the elastic scattering of 13- to 42-Mev alpha particles by Ag, Ta, Pb, and Th at 60\ifmmode^\circ\else\textdegree\fi{}. The alphaparticle energy was decreased in steps of approximately 1 Mev by placing remotely controlled absorbers in the cyclotron beam. Elastically scattered alpha particles were selected and counted by a differential-range coincidence proportional-counter telescope.Up to a certain critical energy ${E}_{0}$, which increases with $Z$, the cross section for each element decreases with increasing alpha-particle energy in agreement with the Rutherford formula. At higher energies, the dependence is given by the empirical formula $\ensuremath{\sigma}(E)=\ensuremath{\sigma}({E}_{0})\mathrm{exp}{\ensuremath{-}K(E\ensuremath{-}{E}_{0})}$. The slope parameter $K$ is about 0.26 ${\mathrm{Mev}}^{\ensuremath{-}1}$ and decreases slightly as $Z$ increases. Interpretation of results in terms of a semiclassical strong absorption model leads to values of the sum of nuclear radius and alpha-particle radius best fitted by the empirical formula $D=(1.50{A}^{\frac{1}{3}}+1.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm.

Research Notes

The strong focusing principle; Negentropy; The study of color centers; For centuries; The new 60-inch cyclotron.

(p, pn) and (p, αn) Excitation Functions

Excitation functions for ($p, \mathrm{pn}$) and ($p, \ensuremath{\alpha}n$) reactions were measured with the internal, 23.5-Mev proton beam of the ORNL 86-inch cyclotron. Methods of obtaining homogeneous and known incident energy are described. The results indicate that, as found previously, emission of charged particles is much more probable than predicted by the statistical theory of nuclear reactions but that this cannot be explained, as has been proposed, by the lowering of the Coulomb barrier due to oscillations of the compound nucleus or by difficulties with the energy level density formula at low excitation energy.

Angular Distributions of Fission Fragments from 22-Mev Proton-Induced Thorium Fission

The angular distributions of barium, strontium, zirconium, ruthenium, and silver fission products from thorium fission induced by 22-Mev protons were measured using the internal, circulating beam of the ORNL 86-inch cyclotron. Within the accuracy of the measurements, all angular distributions are symmetric about 90\ifmmode^\circ\else\textdegree\fi{} and may be well fitted to $I(\ensuremath{\theta})=a+b{cos}^{2}\ensuremath{\theta}$. For Ba, Sr, Zr, Ru, and Ag fission products, for which the fission mass ratios are 1.53, 1.52, 1.37, 1.19, and 1.04, the anisotropy ($\frac{b}{a}$) is 0.26, 0.25, 0.19, 0.15, and 0.10, respectively.

The University of Washington Sixty-Inch Cyclotron

The new constant frequency 60-inch cyclotron at the University of Washington is described. Its principle mechanical features are the following: (1) A 20-inch diffusion pump is placed close to the vacuum chamber in order to provide high pumping speed; (2) The rf conductors are almost free of unsoldered joints in order to provide a high ``Q'' circuit; (3) The positions of the shorting ``spiders'' can be readily changed without breaking the vacuum; (4) The dees and dee stems are removable as one unit from the vacuum envelope; (5) The cyclotron vacuum chamber is constructed of welded aluminum alloy in order to reduce accumulated radioactivity; (6) A unique side-loading target chamber can be used for ordinary bombardments without disturbing apparatus more remote from the cyclotron; (7) The exit strip and other parts of the dee system subject to beam erosion are made of graphite. The oscillator uses a single ML-5681 tube in a grounded-grid self-excited circuit. A 2-kw frequency-modulated booster oscillator drives the main oscillator above the ``multipactoring'' threshold. A cathode bias resistor is used in place of the familiar constant-current network. Ions are drawn out of the dees by means of a dc-biased rf deflector. Four degrees of freedom of the deflector are adjustable from the console. An account is given of the major troubles encountered and the steps taken to achieve satisfactory operation. The cyclotron produces external beams of 125 to 200 μa at a nominal deuteron energy of 21 Mev with an oscillator power input of 100–125 kw.

Measurement of Angular Distributions of Nuclear Reaction Products with an Internal Cyclotron Beam

Angular distributions of elastically-scattered protons, and of deuterons, tritons, and alpha particles from (p,d), (p,t), and (p,α) reactions are being measured with the internal 22-Mev proton beam of the Oak Ridge National Laboratory 86-in. cyclotron. The particles are detected by observing the activities induced by them—Cu(p,n)→38 min for protons, Pb(d,p)→3.3 hr for detuerons, Co(t,p)→1.6 hr for tritons, and P(α,n)→34 min for alphas. Energy discrimination is obtained by absorbers. The energy and energy distribution of the incident beam are found by measuring the excitation function for Cu(p,n) and comparing with published data. The angle of incidence and its inhomogeneity are determined by beam-locating experiments, and checked by the Rutherford law for elastic scattering from heavy elements at small angles. The current is monitored by neutron counters, and measured by counting the activity in the target after bombardment. This is calibrated in a separate run against the published Cu(p,n) cross section. The background is reduced (to <5 percent at most angles) by various types of carbon shielding. Several projects based on these methods have been carried out using the ORNL 86-Inch Cyclotron, and extensions to other types of measurements are in progress.

Angular Distributions of 22-Mev Protons Elastically Scattered by Various Elements

Angular distributions of 22-Mev protons elastically scattered by fifteen elements from beryllium to thorium were measured with the internal, circulating beam of the Oak Ridge National Laboratory 86-inch cyclotron. The experimental methods, which are somewhat unconventional, are described. The results show all the characteristics of diffraction scattering including at least two maxima and minima for each element. The angles at which these occur can be traced from element to element through the periodic table, following a $\frac{\ensuremath{\lambda}}{R}$ dependence with fairly good accuracy.

Production of Negativeμ-Mesons and Their Behavior in Nuclear Research Emulsions

A beam of negative $\ensuremath{\mu}$-mesons produced inside the tank of the 184-inch cyclotron has been studied. 2.7\ifmmode\pm\else\textpm\fi{}1 percent formed one-prong stars when stopped in Ilford C2 emulsion. Two two-prong stars with prong ranges of 2 or 3 microns were observed which appear to be fission-like disintegrations induced by ${\ensuremath{\mu}}^{\ensuremath{-}}$ capture. 32 percent of the ${\ensuremath{\mu}}^{\ensuremath{-}}$-mesons have clusters of grains about 1 micron in diameter or two pronglets about one micron long at their termini. These are interpreted as Auger electrons accompanying capture by a heavy nucleus, or in some cases as fission of the capturing nucleus. An upper limit on the cross section for direct production of negative $\ensuremath{\mu}$-mesons, $\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}\mathrm{dp}}$, in the forward direction at $pc=129$ Mev, has been evaluated to be 0.015 times the cross section for production of negative $\ensuremath{\pi}$-mesons in the same interval.

Possibilities of Heavy Ion Bombardment in Nuclear Studies

Bombardment of nuclei by multiply charged ions of medium atomic weight such as ${\mathrm{C}}^{12}$ or ${\mathrm{O}}^{16}$ is considered as a possible means of studying nuclear structure. Estimates are made regarding the approximate magnitude of: (a) distortion effects in target nuclei produced by the incident particles, (b) consequences of the distortion such as effects on thresholds of reactions having their origin in Coulomb barriers, (c) stimulation to fission, (d) effects characteristic of the leakage of neutrons and protons out of the two colliding nuclei by wave-mechanical penetration of the regions of negative kinetic energy; an exploration of these effects should amount to a study of the halo of neutrons and protons surrounding the more compact nuclear interior and might be helpful in determining the number of nuclear particles at the nuclear surface having a given energy. The treatment is qualitative and the mathematical discussion involves many approximations. General design characteristics of a 60-inch cyclotron that should be capable of imparting the necessary energy to multiply charged ions are considered.