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The value of v for 246Cm was found to be 3.20+ 0.22 in a BF3 proportional counter calibrated with a Cf standard.The average number of prompt neutrons v emitted per fission has been measured for the actinides in several different ways. ' ' Most measurements have been on induced fission of the isotopes U, 2 U, and Pu, which are important to reactors. Although the spontaneous fissioning of some even isotopes has been studied, and values have been obtained for isotopes through mass 254, there is little reported on the isotopes between mass 244 and mass 252. ' Measurements were made with a~' Cm sample enriched in an electromagnetic separator at Oak Ridge National Laboratory. Composition of the sample determined by mass spectrometry is shown in Table I. Two 5-ml aliquots from a solution of 9.948X10' dis/(min ml) with 93.05 n o/ '4'Cm (measured by absolute n counting and n-pulse analysis) were counted in a BF, proportional counter. The a halflife of '~Cm was taken as 18.099+0.015 yr, ' and the a-energy distributions given by Lederer, Hollander, and Perlman' were applied. The ' Cm samples were counted at two different times, and the counting efficiency was determined by two methods.In the first method, counting efficiency was determined with. a '"Cf solution obtained from a sample whose neutron emission rate had been determined to +3% in a manganese sulfate counter calibrated with a National Bureau of Standards neutron standard.A 10pl aliquot of the original Cf solution was diluted to 5 ml, and a 500pl aliquot of the dilution was counted. Neutron emission rate of the~6 2Cf standard was 3.705+ 0. 190X 10' n/min, including original uncertainty in emission rate and errors in pipetting and dilution. The averages of 15 five-minute counts of two '4'Cm samples were 31972+ 57 and 32 041+ 74 counts/min; the average of 15 five-minute counts of the '"Cf standard was 38 624 +74 counts/min; and background counts of 30 and 60 min gave 10+ 0.6 and 10+ 0.4 counts/min, respectively. The '"Cf was counted the day after its neutron emission was determined, so that decay calculations were unnecessary.The total emission rate for "'Cm was found to be 3.071+0.172X 10' n/min, with the counting efficiency of 0.1042 +0.0056 and the average count rate of the "'Cm samples. The v for "'Cm was then calculated from this emission rate and from the half-life, and the v values are shown in Table II.In the second method, two 100pl aliquots from a californium solution of 1.753 x 10' dis/(min ml) with 78. 1 a% '"Cf (measured by absolute o. counting and n-pulse analysis and corrected for fissions) were diluted to 5 ml and counted in the BF, proportional counter. From the 0. half-life of 2.7,31 yr, ' from the a particle to fission ratio of 31.3+ 0. 2, ' and from the v shown in Table II, a neutron emission rate of 1.658X 10' n/min was calculated for these samples. The averages of 10 fiveminute counts of two "'Cm samples were 31371 + 44 and 31378+90 counts/min; averages of two '"Cf samples were 17471+ 61 and 17471+80 counts/min (a 30-min background count show...

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