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LETTERS TOLindner. 3 The Mg 28 for this work has been prepared by spallation of Si and of K2SO4. Previous to the measurements in the beta-ray spectrometer, the isotope was studied and it was confirmed radiochemically that it is a Mg isotope and the activity of its daughter Al 28 was separated and counted; so it was checked that the assignment as Mg 28 was correct. The value of 20.8±0.5 hr was determined for its half-life.The samples for the beta-ray spectrometer were made by irradiating a few grams of the chemicals Si or K2SO4 in a copper vial at 420 Mev in the University of Chicago synchrocyclotron for about one hour. The targets with about 1 or 2 mg of Mg carrier were dissolved, hold-back carriers of likely active impurities added, and the Mg 28 activity cleaned by precipitating impurities as sulfides in acid and basic solutions and by precipitating ferric hydroxide with ammonia. Finally, the magnesium was precipitated as the hydroxide with NaOH or was precipitated as the magnesium ammonium phosphate.The samples for the beta-ray spectrometer were mounted in a thin Zapon backing; for that the magnesium was converted to the chloride or mounted directly as magnesium ammonium phosphate. The thinnest samples had thickness of about 0.2 mg/cm 2 .For the low energy part of the spectrum a Geiger counter with a window of Formvar E and a thickness of 0.2 mg/cm 2 was used. It was supported by a grid and filled in situ to 10 cm of Hg pressure with a mixture of 20 percent ethyl alcohol and 80 percent argon. For the high energy part of the spectrum a Geiger counter with a mica window of 1.3 mg/cm 2 was used.A typical example of the results obtained is shown in Fig. 1. The high energy beta of Al 28 shows an allowed shape from its end 0 500 1000 1500 2000 2500 3000 E (Kev) FIG. 1. The Fermi plot for the beta-spectra of Mg 28 and Aps.point up to where the activity of Mg 28 begins to appear. The extrapolated activity of Al 28 for a given momentum was subtracted from the observed activity to obtain the activity of Mg 28 . The Fermi plot of Mg 28 obtained this way is shown also in Fig. 1, and it has an allowed shape for energies greater than 100 kev. Below 100 kev the usual experimental difficulties distort it. Most of the measurements gave similar results, and it can be concluded that both Mg 28 and Al 28 have allowed spectra. The most probable values for the maximum energies obtained are 418dbl0 kev for Mg 28 and 2850db50 kev for Al 28 . The value of log// for the decay of Mg 28 comes out then to be 4.25. No other group of beta-activity could THE EDITOR 331 be detected, and it can be estimated that no less than 90 percent of the decay of Mg 28 goes through the 418-kev beta. I am indebted to H. L. Anderson for making available to me the beta-ray spectrometer and the facilities of the University of Chicago synchrocyclotron, and to R. K. Sheline for letting me know the results of his experiments before publication. Thanks are due L. Kornblith, Jr., C. Bordeaux, and the crew of the synchrocyclotron for their cooperation during the ir...

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Furthermore it was found that Rb 91 has an isomeric state. Both these states of Rb 91 (half-lives 100 sec and 14 min, respectively) decay to the well-known Sr 91 9.7 hr which again decays to the 60-day and the 50-min isomers of Y 91 . All these radioactivities were found in the samples of mass number 91.A more detailed account of the experiments will be published elsewhere. 2 We wish to thank Professor N. Bohr for his interest taken in our work and Dr. J. Koch for help and advice during this investigation.1 O. Kofoed-Hansen and P. Kristensen, Kgl. Danske Videnskab. Selskab Mat.-fys. Medd. (to be published); see also Phys. Rev., preceding letter.

479value of the spin of Ti of either 3.1"±0.4 or 3.7±0.4, depending upon whether the isotopic abundance is taken to be that of Ti 47 or Ti 49 , respectively. This would indicate that the spin is probably 7/2 and that the signals are probably due to Ti 49 instead of Ti 47 , although this measurement is not conclusive because of uncertainties in the degree of dissociation. From the observed optimum signal to noise ratios of the Ti resonances in various samples we can also estimate the spin to be 7/2±l. Since we have observed the magnetic moment to be negative, this indicates an ft/2 state for the odd neutron, which is not in disagreement with the predictions of the nuclear shell model. 6 Assuming, then, that the spin is 7/2 we find from (1) that the diamagnetically uncorrected value of the magnetic moment of Ti 49 (or perhapswhere we have taken the proton moment to be 2.7925 nm. We have searched over a wide region for the resonance signal from the other odd Ti isotope, but have as yet failed to find it, probably because the gyromagnetic ratio is small. We have observed the nuclear magnetic resonance of As 75 in a 1.2 molar aqueous solution of NasAsS4 and also in a basic aqueous solution of Na3As04. The Na3AsS4 sample gives a large signal, with a half-width of about 0.7 gauss; the line width is limited by quadrupole broadening, which is not excessive in this case because the sample is well dissociated into symmetric (AsSJ" ions. On the other hand, it was found that in the NasAs04 sample the As resonance signals were obliterated by excessive quadrupole broadening unless the sample was made basic (by the addition of NaOH) to a pK>12. This is in agreement with the chemical evidence that Na 3 As04 does not dissociate into (ASO4) ions, except in basic solution. 7 We have not detected a " chemical shift" 5 between the As resonance frequencies in (ASS4) and (ASO4) . The ratio of the As 75 resonance frequency to that of protons in H 2 0 in the same magnetic field has been found to be *'AS75AH = 0.17129±0.00003.(3)The spin 8 of As 75 is known to be 3/2, and we have verified this by comparing the As 75 signal from Na 3 AsS4 to the D 2 signal in D2O; our experimental result is 7(As 75 ) = 1.5±0.2. Taking the proton moment to be 2.7925 nm, we find from (3) the diamagnetically uncorrected value of the moment of As 75 to be /x(As 75 ) = + (1.4350±0.0003) nm.This value is in agreement with that recently reported by