LETTERS TO THE ED I TOR

These considerations show that by placing our source anddetector exactly on extended tangents to the mean radiuswe have been using the instrument so that many particleswere lost by hitting the grounded outer plate. Increasedintensity should result from moving S and F to thecalculated positions, that is, about 2 mrn nearer the centerof curvature.

SAMUEL K. ALLISON

LESTER S. SKAGGS

NIcHoLAs M. SMITH, JR.Ryerson Physical Laboratory,

University of Chicago,Chicago, Illinois,

February 20, 1940.

' J. Mattauch, preceding Letter to the Editor.' F. T. Rogers, Jr., Rev. Sci. Inst. 11, 19 (1940).3 S. K. Allison, L. S. Skaggs, N. M. Smith, Jr., Phys. Rev. 54, 171

(1938);L. S. Skaggs, Phys. Rev. 56, 24 (1939).

no correction for nonparallelism of the neutron beam wasconsidered necessary.

It was believed desirable to measure the transmission ofthe quartz crystal for C neutrons since the earlier measure-ments on quartz were made with small samples, andfurthermore, it is of interest to know whether the crosssection per molecule varies appreciably with thickness ofsample. This particular piece of crystal is 4.2 cm thick andhas 10.97 g/cm'. The parallel faces are perpendicular to theprincipal or optic axis of the crystal, and are of such sizethat a usable cylindrical section 4.2 cm long and 10.70 cmin diameter was available. The 'results of these measure-ments, expressed in terms of cross sections per molecule,are shown in Table I.

TABLE I. Total cross sections for quartz crystals (&(/024 cm 2).(Detectors used are shozvn in parenthesis. )

COLD NEUTRONS(BoRQN)

RES. NEUTRONS C NEUTRONS C NEUTRONS(IN. RH.) (IN. RH.) (BQRQN)

The Transmission of Neutrons of Different EnergiesThrough Quartz Crystals

Recent experiments' 2 on crystals of iron, nickel, quartz,and nickel-iron alloys (Permalloy) have shown that singlecrystals are very transparent to the slow neutrons which

I

are readily absorbed by cadmium. This high transparencyfor C neutrons has been attributed' ' to the fact that only afew narrow bands in the neutron spectrum satisfy the wave-length conditions for coherent scattering. The neutrons in

these bands are scattered into the appropriate diffractionpatterns by the nuclei in the crystal lattices while theneutrons in the other parts of the spectrum go through thecrystal practically unhindered, except for possible captureor collision resulting in incoherent scattering. Additionalmeasurements have been made using a large single crystalof quartz and neutrons of both higher and lower energiesthan were used before. .The observations on quartz werealternated with similar ones on carbon.

The neutrons of greater than thermal energies wereobtained from 600 milligrams of radium mixed withberyllium placed about 2 cm below the top surface, of aparaffin cylinder 15 cm in diameter. Two boron carbidedoughnuts 15 cm in diameter with central holes 8 cm indiameter were used to improve the geometry. One of thesewas placed directly on top of the paraffin cylinder and theother directly under the sample. The scattering samplesand the detectors were placed 12 cm and 27 cm, respectively,above the top surface of the paraffin.

The transmissions of the samples for C neutrons and forthe neutrons which pass through 0.4 g/cm' of cadmiumwere measured using indium and rhodium as detectors.These cadmium filtered r1eutrons which activate rhodiumand indium have energies of approximately 1.5 volts. '

The transmissions of these same samples of quartz andcarbon were measured using the neutrons from paraffincooled with liquid air. The temperature of the paraffin waschecked by a system of thermocouples which indicated aconstant value of about 100'K. A boron chamber connectedto a linear amplifier was used as a detector of the neutronsin this case. The geometry of the apparatus was such that

2.3 %0.7 7.2 &1.2 3.2 &1.2 3.0~0.7

These results afford additional proof that the increasedtransparency of material in the form of single crystals is dueto interference effects, since this high transparency largelydisappears when the wave-length of the neutrons isdecreased by a factor of about seven. The cross sectionobtained for this quartz for C neutrons is lower than thevalue of 4.3 &0.6)&10 "cm' found by Whitaker and Beyerusing thinner samples. While this difference may not besignificant, it is in the direction to indicate a decrease ininteraction cross section with increase in thickness ofcrystal. The results with neutrons from cooled paraffin

'

show that the changes which take place in the energydistribution of the neutrons are not very important inchanging the transmission of the quartz. This result wasnot unexpected. The carbon sample showed no change incross section with change in the energy of the neutrons.The average value obtained was 4.9&(10 ' cm'.

The total C neutron cross section of single crystal quartzgives an upper limit for the sum of the cross sections due toincoherent scattering and capture. 4 The latter is believedto be negligible. This upper limit of the incoherent scatteringcross section must be considerably higher than the actualvalue because of the high energy tail (extending up to thecadmium cut-off at about 0.3 volt) on the neutrondistribution.

It is a pleasure to acknowledge financial support of thiswork by the Permanent Science Fund of the AmericanAcademy of Arts and Sciences through a grant to one ofus {M.D. W.).

M ARTIN D. WHITAKER

WILLIAM C. BRIGHT

EDGAR J. MURPHY

New York University,University Heights,

New York, New York,February 17, 1940.

' M. D. Whitaker and H, G. Beyer, Phys. Rev. 55, 1101 (1939);55, 1124 (1939).

2 H. G. Beyer and M. D. Whitaker (to be published).~ H. A. Bethe, Rev. Mod. Phys. 9, 69 (1937).4 Halpern, Hamermesh and Johnson, Phys. Rev. 55, 1125(A) (1939).