The reaction e+e~e+e m. m has been analyzed using 97 pb ' of data taken with the Crystal Ball detector at the DESY e e+ storage ring DORIS II at beam energies around 5.3 GeV. For the first time we have measured the cross section for yy~m. m. for n m invariant masses ranging from threshold to about 2 GeV. We measure an approximately flat cross section of about 10 nb for 8'=m 0 0 (0.8 GeV, which is below 0.6 GeV, in good agreement with a theoretical prediction 'tr n' based on an unitarized Born-term model. At higher invariant masses we observe formation of the ft(1270) resonance and a hint of the fo(975). We deduce the following two-photon widths: I rr(f, (1270)) =3.19+0. 1620 z, keV and I "(fo( 975)) (0.53 keV at 90% CL. The decayangular distributions show the m~system to be dominantly spin 0 for W &0.7 GeV and spin 2, helicity 2 in the f, (1270) region, with helicity 0 contributing at most 22% (90% C.L.).
The evaluation of the agreement between measured and calculated dose plays an essential role in the quality assurance (QA) procedures for intensity modulated radiation therapy (IMRT). Film dosimetry has been widely adopted for this purpose due to excellent film characteristics in terms of spatial resolution; unfortunately, it is a time-consuming procedure and requires great care if film has to be used as an absolute dosimeter. If this is not the case, then an independent ionimetric measurement is mandatory to assess the absolute dose agreement. Arrays of detectors are now replacing films for routine IMRT QA, since they permit very simple verification procedures. They show excellent characteristics in terms of linearity, repeatability, and independence of the response from the dose rate, but at the same time present a poor spatial resolution, due to the limited number of detectors available. In our institution, a diode matrix (MapCHECK, provided by Sun Nuclear) is adopted for routine QA. The aim of this work is to compare the performances of absolute film dosimetry with this matrix in QA procedures and to investigate the origin of possible discrepancies between the two methods. The results we present show a very good agreement between the two detectors when used to assess the mean dose deviation between calculated and measured doses (in both cases 0.2%). If the y matrix method is adopted, MapCHECK response shows a slightly better agreement with computed dose distribution than film dosimetry (mean percentage of points satisfying the constraint y < or = 1: 96% versus 94%). This difference is shown not to depend on the different field sampling, but on the detectors' capabilities. Moreover, we show that the diode matrix is able to identify eventual delivery errors as well as film. Our conclusion is that the diode matrix may effectively replace both film dosimetry and ionimetric measurements in routine IMRT QA.
100 and 3000 MeV. Formation of~, g, and g' mesons is observed, and the following meson partial widths are obtained: I 0-7.7+0.5+0.5 eV, I "»-0.514+0.017+0.035 keV, and 'tl I "yy=4. 7+0.5+0.5 keV. No other narrow resonances are observed, and upper limits are given for the product I p yyB& yy. 38 1365 Q~1988 The American Physical Society 1366 D. A. WILLIAMS et al. 38 the range 10 '-10 ' sec which is difficult to measure: they have both a short decay length and a small natural width. The best determinations of the q and g' lifetimes are obtained by using measurements of I & and the branching ratio for X~yy to derive the full width. ' A precise measurement of the m. lifetime using the decaylength technique was recently achieved by Atherton et al. In this paper we describe a study of the reaction e+e~e+e yy~e+e yy, where each y* is a quasireal photon radiated from one of the beam particles. The virtual photons scatter to produce a final state consisting of two real photons. The final-state leptons scatter at very small angles and are not detected. The sum of the transverse momenta of the final-state photons with respect to the beam axis is essentially zero. %e observe three peaks in the invariant-yy-mass spectrum corresponding to the formation of m, g, and g' mesons. This is the first observation of n. production using this technique, first suggested by Low in 1960. The data used for this analysis were collected with the Crystal Ball detector running at the DORIS II e+e storage ring at DESY. The e+e center-of-mass energy varied from 9.4 to 10.6 GeV, with most of the data taken on the Y(1S), Y(2S), and Y(4S) resonances. The sample used for the study of g and g' production has an integrated luminosity of 114 pb ', of which about 40% has
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