ISSN:0021-8979    

DOI:10.1063/1.1721974

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

By concentrating uranium into lumps, one can decrease its resonance absorption as compared with the resonance absorption of the same amount of uranium spread out uniformly in a moderator. It is shown here that this decrease is caused principally, not by the diminished over‐all density of the neutrons at the lump, but by their changed energy distribution. This shows deep minima at the resonance lines, i.e., where the uranium absorbs most strongly. The considerations here presented show that the decrease in absorption is largest for the low energy resonances of U238which are responsible for the bulk of the absorption in the case of uniform distribution. As a result, the resonance absorption of uranium lumps extends over a very large energy region–up to about 5000 ev. It can be decomposed, approximately, into two parts: the mass absorption, proportional to the number of U238atoms present; and a surface absorption, proportional to the surface of the lump. The total decrease of the resonance absorption produced by lumping (the ``advantage factor'' of lumping) can be estimated to reach, under favorable conditions, a value as high as 20.

ISSN:0021-8979    

DOI:10.1063/1.1721975

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The total resonance absorption of neutrons by spheres of uranium and uranium oxide imbedded in graphite and bombarded with essentially adE/Espectrum is found to be given by 1/&agr;=c[1+(&kgr;/&rgr;R)] where &agr; is the advantage factor, that is, the ratio of absorption of uranium when distributed uniformly throughout graphite to the absorption when it is lumped in a sphere.Ris the radius of the sphere and &rgr; is its density.cand &rgr; were measured in various parts of a large graphite block exposed to cyclotron neutrons from the reaction Li (p,n). Values ofcand &rgr;, respectively, are 0.051 and 5.1 g/cm2for U3O8. In a newer notation,A= 10.9 barns and &mgr; = 1.95 g/cm2.

ISSN:0021-8979    

DOI:10.1063/1.1721976

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

In the expression 1/&agr;=c[1+(&kgr;/&rgr;R)] for the advantage (&agr;) of decreased resonance absorption of uranium in adE/Eneutron spectrum brought about by lumping UO2into spheres, the temperature dependence ofcis measured and &kgr; is measured also. &rgr; is the density of UO2in g/cm3;Rthe radius of the sphere in cm. In the range 20° to 1000°C, &Dgr;c/&Dgr;Taverages 1.8×10−6per degree centigrade and is positive as expected. This temperature dependence decreases by about a factor of three from the lowest to the highest measured temperatures. &kgr; is 5.06 g/cm3leading to a value of &mgr; in the expression [Phys. Today5, 32 (May, 1952)]&sgr;(E)dEEeff=A1+&mgr;SMof 1.69 g/cm2.&Dgr;A&Dgr;T is 4×10−4barns∘C.

ISSN:0021-8979    

DOI:10.1063/1.1721977

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

Tensile creep measurements beginning in a tensile strain region of about 10−6have been made upon a set of seven single crystals of zinc of 99.99+percent purity. Measurements were made both by means of electrical resistance strain gauges and an optical lever system. Some of the measurements made in the latter apparatus lasted 100 hours or more, and in both systems, measurements were made at several temperatures.The creep results obtained in this work can be best described by the single empirical relation&eegr;=btmin which &eegr; is the shear strain,tis the time after completion of the application of stress to the crystal, andbandmare constants for a given creep test under given experimental conditions. It is found thatbincreases with the stress but does not vary greatly with temperature. The values ofmare practically independent of stress but vary with temperature, and appear to fit best the equation,m=m0exp(−T0/T)in the temperature interval investigated. For these crystalsT0= 1580°K,m0=120, and the temperature at whichm=1 is 58°C. Values ofm>1 are interesting in dislocation theory.

ISSN:0021-8979    

DOI:10.1063/1.1721978

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The plastic extension of zinc crystals in the region just beyond the elastic limit is found to obey the empirical law, &eegr;=btmin which &eegr; is the resolved shear strain,tis the time,mis a constant with a value close to 0.5 for initially unstrained crystals, andbis dependent on the resolved shear stress. Application of a theory due to Koehler shows thatbshould be related to stress (&sgr;) in such a way that a plot of ln(mb&sgr;2/&Dgr;&sgr;) against 1/&sgr; should be a straight line of slope −cG. In this, &Dgr;&sgr; is the stress increment from one creep curve to the next,Gis the shear modulus, andcis the concentration of anchor points which determines the lengths of segments of dislocation lines which generate according to the Frank‐Read mechanism. The experiments are in fair agreement with this theory and values ofcobtained are roughly equal to the concentrations of metallic impurities. The small amount of dissolved gases appears to play no significant part in the creep process.

ISSN:0021-8979    

DOI:10.1063/1.1721979

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

Spectral modifications of Cs137gamma radiations produced by penetration and diffusion through water have been observed. A single crystal of NaI(Tl) was used to observe the pulse‐height distributions produced by radiation from both plane parallel and point isotropic sources. Inversion of the pulse‐height distribution was carried out by use of an analog computer to obtain the spectral distribution of radiation.

ISSN:0021-8979    

DOI:10.1063/1.1721980

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The results obtained by applying the current‐distribution method to the problem of approximating the scattering cross section, when the transmitter and receiver are separated, are presented for several simple geometric configurations. The method is applied for the case in which the transmitter is located on the axis of revolution and the ratiol/&lgr; is large, wherelis a ``characteristic dimension'' of the body and &lgr; is the wavelength. These results indicate that in most of the cases considered the cross section increases as the angle between the receiver and the transmitter increases.

ISSN:0021-8979    

DOI:10.1063/1.1721981

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The first two terms of Kline's asymptotic expansion are obtained for the scattering of a plane wave incident along the axis of a perfectly reflecting semi‐infinite body of revolution. When this method is applied to the paraboloid theexactelectromagnetic solution is obtained in closed form. The accuracy of the method of physical optics is studied by using the asymptotic expansion.

ISSN:0021-8979    

DOI:10.1063/1.1721982

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The value of the nose‐on back scattering cross section of a semi‐infinite cone is determined by the exact methods of electromagnetic and acoustical theory, and by physical optics. It is shown that, to the degree of approximation used, the electromagnetic value and the physical‐optics value are equal. The acoustical value is found to be less than the electromagnetic value by a factor which depends only on the cone angle; both are proportional to the square of the wavelength. It is shown that the electromagnetic and physical‐optics answers agree with experimental data to within a factor of two. The electromagnetic theory results obtained hold for the cases in which the half‐cone angle is close either to 0 or to &pgr;/2.

ISSN:0021-8979    

DOI:10.1063/1.1721983

出版商:AIP    

年代:1955

数据来源: AIP