1、Designation: E 2426 05Standard Practice forPulse Counting System Dead Time Determination byMeasuring Isotopic Ratios with SIMS1This standard is issued under the fixed designation E 2426; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisi
2、on, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 determin
3、ing 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 prac
4、tice does not describe methods for precise oraccurate isotopic ratio measurements, or both.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,
5、 associated 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:2E 673 Terminology Relating to Surface Analysis2.2 IS
6、O Standard:3ISO 21270 Surface chemical analysis X-ray photoelec-tron and Auger electron spectrometers Linearity ofintensity scale; and references 1, 2, 10, 13 and 14 therein.3. Terminology3.1 Definitions:3.1.1 See Terminology E 673 for definitions of terms used inSIMS.3.1.2 See Terminology ISO 21270
7、 for definitions 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
8、 the twospecies.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 guide, Dm2X will refer to thefractional deviation of the measured ratio, uncorrected formass-fractionation (see 3.1.4) and dm2X wi
9、ll refer to thefractional deviation of the measured ratio that has beencorrected for mass-fractionation. An example for 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-iso
10、tope variationin ion intensity observed in the measured isotopic ratios for agiven element compared with the reference ratios. It can beexpressed as 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”i
11、saninteger.4. Summary of Practice4.1 This practice describes a method whereby the overalleffective dead time of a pulse counting system can be deter-mined by measuring isotopic ratios of an element having atleast 3 isotopes. One of the isotopes should be approximatelya factor of 10 more abundant tha
12、n 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 if thesample is flat and uniform, such as a Si wafer or a polishedmetal block so that the count rate of the isotopes variesminimally during the i
13、ndividual measurements.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 Nov. 1, 2005. Published January 2006.2For referenced ASTM standards, visit the ASTM website, www.astm.org,
14、 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 Organization for Standardization (ISO), 1 rue deVaremb, Case postale 56, CH-1211, Geneva 20, Switzer
15、land.4Catanzaro E. J., Murphy T. J., Garner E. L., and Shields W. R., “AbsoluteIsotopic Abundance Ratios and Atomic Weight of Magnesium,” J. Res. Nat. Bur.Stand., 70a, 1966, pp. 453-458.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States
16、.5. Significance and Use5.1 Electron multipliers are commonly used in pulse-counting mode to detect ions from magnetic sector massspectrometers. The electronics used to amplify, detect andcount pulses from the electron multipliers always have acharacteristic time after the detection of a pulse after
17、 which noother pulses can be counted. This characteristic time is knownas the “dead time.” The dead time has the effect of reducing themeasured 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, suc
18、h as anelectron multiplier, the dead time of the entire pulse countingsystem must be well known. Accurate 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 de
19、ad time of counting systems on SIMSinstruments.5The accurate determination of the dead time bythis method 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 co
20、untingsystem.7. Procedure7.1 Choose a sample of the appropriate material to make themeasurements simple and 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 in
21、strument. Theelement to be measured should have at least 3 isotopes withone being approximately 10 times 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, an
22、d Ti.7.1.1 The dead time of a counting system can be character-ized as either retriggerable (paralyzable, 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 puls
23、e arrives at thediscriminator input before the output has returned to itsquiescent state. Some 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 th
24、e dead time is given by:CMeas5 CTruee2tCTrue(2)where:CMeas= the measured count rate,CTrue= the 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 tim
25、e). Insuch as system the dead time is given by:CMeas5CTrue1 1 CTrue3t(3)7.1.1.2 These two equations, 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
26、The procedure for computing the dead time fromisotopic ratio measurements involves effectively 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 afunct
27、ion of mass. Fig. 1 shows a representation of a plot for atypical 3-isotope system where, in this case, isotope 1 is usedas the reference isotope.7.1.2.1 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 mea
28、suredisotopes.5Fahey, A. J., “Measurements of Dead Time and Characterization of IonCounting Systems for Mass Spectrometry,” Review of Scientific Instruments, Vol 69,1998, p. 1282.FIG. 1 Representation of the Effects of Dead Time and MassFractionation on Measured Isotopic Ratios Expressed asFractiona
29、l DeviationsE24260527.1.2.2 This equation takes into account mass bias, andcomputes the magnitude 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 T
30、his approximation could be iterated to achieve amore precise estimate of the dead time. In addition, 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
31、 as with an element like Ti.7.1.4 Uncertainties can be assigned to the computed deadtime through 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-p
32、recision isotopic ratio mea-surements.7.1.5 The dead time can be measured as a function of thecount 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
33、 changing the source intensityin some other way. Ideally one expects the dead time to beindependent 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 measu
34、rement of the dead time as afunction of count rate can give the analyst an indication ofwhether 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
35、connection with any item mentionedin this standard. Users of this standard are expressly advised 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 res
36、ponsible technical committee and must be reviewed every five years andif not revised, either reapproved 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 caref
37、ul consideration at a meeting of theresponsible technical committee, which you may attend. If you 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
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