A d escription is give n of th e mec ha ni'c a l des ign and ope ra tion uf all a utumati c ga mma-ra y ~uint Source c a li brator.The use of st ati s ti c al d es ign in e xpe rim e nt s fo r e va lu atin g pe rfo rm a nce fac to rs, s uc h as int e rc ha ngeabi li ty of s tati o ns a nd run diffe re nc es u sin g th e s a me d a ta obta in e d in co mp ari so ns of th e s ources, is de sc rib ed in d e ta il.Key Word s : S tati sti c a l expe rime nt de s ign, te s tin g equipm e nt , routin e tes tin g, radi oac tivit y s ta nd a rd iza tio n mea s ure me nt s, gamm a-ray po in t sourc e s
Design and Performance of an Automatic Gamma-Ray Point-Source CalibratorRece ntly , in res pon se to a need for s tandard s for work ers in the fi eld of gamma-ray s pec tr om e try , a gamma-ray "kit" for point-source radioac tivity s tandards has been de veloped [Hu tc hin son , 19601. These sources are pre pared from solution s' whi c h are standardized e ith e r by coi ncide nce countin g or, as in th e ca se of cesium-barium-137, by meas ure m,e nts usin g the NBS calibrated 477-y-ionization c hamber. ' The sources are pre pared by de pos iting either 0.05 or 0.1 ml of the calibrated radioac tive s olution onto mounts c on sisting of a 0.006-ce ntimeter-thi c k polyester tape whi ch is s upported by an alum inum annulu s (3.8 c m I.D., and 5.4 cm O.D.), as shown in figure 1. As it is de sirable for all of these sources to be nominally the same strength and the same size, the solution is dispen sed with an ultramicroburet [NBS eire . 594, Mann and Seli ger, 1958]. After drying, the sources are covered with another layer of the same kind of polyeste r tape . The sources are the n intercom pared with se ve ral accurately standardized sources , for the purpose of individual calibration.For sev eral years these calibrations were performed manually; that is, the sources we re placed , one at a tim e, in a jig whic h was he ld in a fix ed pos ition relative to a sc intillation counte r, and th e count rates were intercom pared . As part of th e program to in crease the acc urac y of the standards, it was d ecid ed to design and construct an automatic s ampl e c hange r with the goal of attainin g source i·ntercomparison s with a precision of the order of 0.1 perce nt. Th e c han ge r is a round turn-table of 1/4-in.-thic k a luminum alloy having a diame ter of 24 in. , with so urce po sition s spaced at 18° intervals on the circ umfe re nce of a c ircle 20 in. in diamete r (fi g. 2). These positions have l-in.-diameter holes in which rigid plastic sample c arriers rest. The gamma-ray poin t sources are held firmly in place on top of the carriers by the pressure of phosphor-bronze springs. There are 20 indexing hol es equally spaced around the table as shown in figure 2, the center of each one radially in line with the cen ter of a sample carrier and the center of the table, and 3/8-in. in from the edge of the table. These holes, in conjunction with a solenoid-plunger pin , are used for positioning the sources above the det...