1、Designation: E2426 10Standard Practice forPulse Counting System Dead Time Determination byMeasuring Isotopic Ratios with SIMS1This standard is issued under the fixed designation E2426; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision
2、, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice provides the Secondary Ion Mass Spec-trometry (SIMS) analyst with a method for determining
3、 thedead time of the pulse-counting detection systems on theinstrument. This practice also allows the analyst to determinewhether the apparent dead time is independent of count rate.1.2 This practice is applicable to most types of massspectrometers that have pulse-counting detectors.1.3 This practic
4、e does not describe methods for precise oraccurate isotopic ratio measurements.1.4 This practice does not describe methods for the properoperation of pulse counting systems and detectors for massspectrometry.1.5 This standard does not purport to address all of thesafety concerns, if any, associated
5、with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E673 Terminology Relating to Surface Analysis2.2 ISO Standards:3
6、ISO 21270 Surface Chemical AnalysisX-ray photoelec-tron and Auger electron spectrometersLinearity of in-tensity scale; and references 1, 2, 10, 13 and 14 therein.3. Terminology3.1 Definitions:3.1.1 See Terminology E673 for definitions of terms used inSIMS.3.1.2 See Terminology ISO 21270 for definiti
7、ons of termsrelated to counting system measurements.3.1.3 isotopic ratio, nwritten asm2X/m1X, for an elementX with isotopes m1 and m2, refers to the ratios of their atomicabundances. When it is a value measured in a mass spectrom-eter it refers to the ratio of the signal intensities for the twospeci
8、es.3.1.3.1 DiscussionThe notation Dm2X or dm2X refers tothe fractional deviation of the measured isotopic ratio from thestandard ratio or reference. In this practice, Dm2X will refer tothe fractional deviation of the measured ratio, uncorrected formass-fractionation (see 3.1.4) and dm2X will refer t
9、o thefractional deviation of the measured ratio that has beencorrected for mass-fractionation. An example for magnesium(Mg) is:D25Mg 525Mg/24Mg!Meas25Mg/24Mg!Ref 1 (1)where:(25Mg/24Mg)Ref= 0.126634.3.1.4 mass-fractionation, nsometimes called “mass-bias,”refers to the total mass-dependent, intra-isot
10、ope variation in ionintensity observed in the measured isotopic ratios for a givenelement compared with the reference ratios. It can be expressedas the fractional deviation per unit mass.3.1.4.1 DiscussionThe mass of an isotope i of element X(miX) shall be represented by the notation mi, where “i”is
11、aninteger.1This practice is under the jurisdiction of ASTM Committee E42 on SurfaceAnalysis and is the direct responsibility of Subcommittee E42.06 on SIMS.Current edition approved June 1, 2010. Published July 2010. Originally approvedin 2005. last previous edition approved in 2006 as E2426 05 (2006
12、). DOI:10.1520/E2426-10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from International Organiz
13、ation for Standardization (ISO), 1, ch. dela Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http:/www.iso.ch.4Catanzaro, E. J., Murphy T. J., Garner E. L., and Shields W. R., “AbsoluteIsotopic Abundance Ratios and Atomic Weight of Magnesium,” Journal ofResearch of the National Bureau
14、 of Standards, Vol 70a, 1966, pp. 453458.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4. Summary of Practice4.1 This practice describes a method whereby the overalleffective dead time of a pulse counting system can be deter-mined
15、by measuring isotopic ratios of an element having atleast 3 isotopes. One of the isotopes should be approximatelya factor of 10 more abundant than the others so that a first orderestimate of the dead time can be calculated that will be close tothe true value. The efficacy of the method is increased
16、if thesample is flat and uniform, such as a silver (Si) wafer or apolished metal block so that the count rate of the isotopesvaries minimally during the individual measurements.5. Significance and Use5.1 Electron multipliers are commonly used in pulse-counting mode to detect ions from magnetic secto
17、r massspectrometers. The electronics used to amplify, detect andcount pulses from the electron multipliers always have acharacteristic time interval after the detection of a pulse, duringwhich no other pulses can be counted. This characteristic timeinterval is known as the “dead time.” The dead time
18、 has theeffect of reducing the measured count rate compared with the“true” count rate.5.2 In order to measure count rates accurately over theentire dynamic range of a pulse counting detector, such as anelectron multiplier, the dead time of the entire pulse countingsystem must be well known. Accurate
19、 count rate measurementforms the basis of isotopic ratio measurements as well aselemental abundance determinations.5.3 The procedure described herein has been successfullyused to determine the dead time of counting systems on SIMSinstruments.5The accurate determination of the dead time bythis method
20、 has been a key component of precision isotopicratio measurements made by SIMS.6. Apparatus6.1 The procedure described here can be applied to anymass spectrometer, including SIMS, with a pulse countingsystem.7. Procedure7.1 Choose a sample of the appropriate material to make themeasurements simple a
21、nd uncomplicated by issues such ascharging or geometry. The material should be a conductor, orsemiconductor. It should be polished flat and mounted in asuitable manner for analysis in the SIMS instrument. Theelement to be measured should have at least 3 isotopes withone being approximately 10 times
22、more abundant than theothers. The signal obtained from the most abundant isotopewill be used as the denominator to form all of the isotopicratios. Examples of this kind of element are: Si, Mg, andtitanium (Ti).7.1.1 The dead time of a counting system can be character-ized as either retriggerable (pa
23、ralyzable, or extendable), non-retriggerable (non-paralyzable, or non-extendable), or a com-bination of the two. In a retriggerable system the length of thediscriminator output pulse is increased if a pulse arrives at thediscriminator input before the output has returned to itsquiescent state. Some
24、systems have an additional recovery timeafter the output has returned to its quiescent state during whichthey will not react to input pulses. This time just adds to thesystem dead time. In such a system the dead time is given by:CMeas5 CTruee2tCTrue(2)where:CMeas= the measured count rate,CTrue= the
25、true count rate, andt = the dead time.7.1.1.1 In a non-retriggerable system a pulse is simplyignored if it arrives before the discriminator output hasreturned to its quiescent state (plus some recovery time). Insuch as system the dead time is given by:CMeas5CTrue1 1 CTrue3t(3)7.1.1.2 These two equat
26、ions, for each type of system, yieldthe same result to first order. Thus, for the purposes of thisguide we will assume that Eq 4 adequately describes thesystem in question:CMeas5 CTrue3 1t3CTrue! (4)7.1.2 The procedure for computing the dead time fromisotopic ratio measurements involves effectively
27、computingtwo quantities: the dead time, and the mass fractionation. Inorder to do this, the fractional deviations (see 3.1.1.1) for eachof the minor isotopes is computed and then fitted to a line as afunction of mass. Fig. 1 shows a representation of a plot for atypical 3-isotope system where, in th
28、is case, isotope 1 is usedas the reference isotope.5Fahey, A. J., “Measurements of Dead Time and Characterization of IonCounting Systems for Mass Spectrometry,” Review of Scientific Instruments,Vol69, 1998, p. 1282.FIG. 1 Representation of the Effects of Dead Time and MassFractionation on Measured I
29、sotopic Ratios Expressed asFractional DeviationsE2426 1027.1.2.1 For this system the distance “f” shown in the plot isgiven by:f5Dm2X 1 m2 m1!Dm2X Dm3X!m3 m2!(5)where:m1,m2, and m3= the masses of each of the measuredisotopes.7.1.2.2 This equation takes into account mass bias, andcomputes the magnitu
30、de of the dead time effect on the majorisotope. The first order estimate of the dead time can be simplycomputed from:f5m1XMeas3t (6)where:t = the dead time, given the assumptionthatm1Xmeasm1XTrue.7.1.2.3 This approximation could be iterated to achieve amore precise estimate of the dead time. In addi
31、tion, massfractionation dependencies other than linear could be used.7.1.3 This procedure can be extended to systems with morethan 3 isotopes so that a fit can be performed to the minorisotope ratios, such as with an element like Ti.7.1.4 Uncertainties can be assigned to the computed deadtime throug
32、h normal propagation of errors. This uncertaintycan then be used in the computation of measured isotopic ratiosfor other elements. The uncertainty in the dead time may be asignificant factor in some high-precision isotopic ratio mea-surements.7.1.5 The dead time can be measured as a function of thec
33、ount rate of the major isotope by making isotopic ratiomeasurements at various count rates of the major isotope. Tocontrol the count rates one can vary the width of the massspectrometer entrance slit or by changing the source intensityin some other way. Ideally one expects the dead time to beindepen
34、dent of count rate. This may not be the case if, forexample, the pre-amplifier is AC-coupled to the discriminatorand the input voltage to the discriminator changes as a functionof count rate. Thus, a measurement of the dead time as afunction of count rate can give the analyst an indication ofwhether
35、 the counting system is functioning as expected.8. Keywords8.1 dead time; isotopic ratios; pulse counting; SIMSASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advise
36、d that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revised, either re
37、approved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, which you may attend. If y
38、ou feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individua
39、l reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org). Permission rights to photocopy the standard may also be secured from the ASTM website (www.astm.org/COPYRIGHT/).E2426 103
