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Anatomy of an IRMS Lecture 7 – Detectors 1 Required Reading

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Anatomy of an IRMS Lecture 7 – Detectors 1 Required Reading Anatomy of an IRMS Lecture 7 – Detectors Required reading: Wade, Organic Chemistry, pp59-68. 1) Introduction a) Job of detectors is to take incoming ions and turn them into a stable electronic signal (either V or I) that can be measured, recorded, etc. b) Possible to do this either digitally (ie, record every ion) or in analog (record current). Best solution depends on magnitude of ion current. c) Detectors can be in two classes: Faraday cups (no gain) and electron multipliers (gain) 2) Faraday Cups a) Basic idea is a metal plate. When an ion strikes it, it will be neutralized by an e- flowing into the detector, thus positive ion current is balanced by negative electron current, which is what we measure. b) Problems i) reflection – ions can strike surface and reflect, rather than being neutralized ii) secondary electrons – When charged ions strike a metal surface, they can cause emission of electrons (called ‘secondary electrons’). This emission of e- looks like absorption of positive ion in terms of current. Emission depends on cup material, ion mass and energy (velocity), incident angle iii) small signal – ion currents typically in fA to pA range, very easily perturbed by stray electrical and magnetic fields. How to safely transmit and measure it? c) Solutions i) Shape – make cups long and deep, so that reflected ions and SE more likely to strike wall than to escape ii) low emission material (carbon coatings) iii) magnetic field around cup (steers electrons sideways); only practical on single detector iv) repeller plates; negatively charged shield at entrance to cup; typically a few tens of volts 3) Electrometers a) Next problem is the very low current. Obvious solution is to amplify it, but note that for typical ion currents we need a 109 to 1012 –fold amplification. Must minimize noise and drift, and have absolutely linear response. Turns out to be a difficult problem. Many different types of amplifiers in electronics, ones that are specialized for this function called an “electrometer”, ie a circuit designed specifically for measuring very tiny currents. Converts current to a propotional voltage. b) Op amp. Electrometer built around an operational amplifier. Many versions, with varying characteristics. Not a single component, but integrated circuit based on multiple transistors. All share these common features: i) Two inputs, one regular and one inverting. Both are summed to yield input voltage. ii) Infinite input impedance, so no current loading. iii) Zero output impedance, so no power limitation. iv) Amplifier has ~infinite gain. Without feedback, any input leads immediately to infinite positive or negative voltage (limited by power supply; typically +/- 15V, or 50V, etc). Can use this as a threshold comparator. v) With a feedback loop to negative input, op amp will adjust output to keep two inputs at the same voltage. (1) Aside: voltage follower (signal buffer): connect signal to positive input, feedback loop with no resistor to negative input; to keep inputs the same, output adjusts to exactly the same as input, such that Vout = V+. Because the op amp has plenty of power, this output signal will not be loaded by downstream resistance. 1 Anatomy of an IRMS Lecture 7 – Detectors c) d) Principle of operation i) V+ = V- = ground (0V) (kept that way by operation of op amp). The voltage drop across the input resistor will thus be Vin-V- = Vin = Iin*Rin. ii) Input impendance is essentially infinite, so Ia = 0. Since Ia = Iin + If, output current will adjust to balance, thus Iin = -If. Remember that we are at the inverting input, so feedback current will be negative. (1) Similarly, Vout-V- = Vout = If*Rf iii) by substitution into eqn 2 (Iin = -If) (1) Vin/Rin = - Vout/Rf (2) thus the output voltage is Vout = -(Rf/Rin)Vin (3) ratio of Rf/Rin determines the gain of the electrometer; since Rin is typically very small, this is close to Rf. Thus the common approximation that the gain of the amplifier equals the resistance of the feedback resistor. In fact, the current flowing through the feedback resistor is exactly equal to that of the ion current, which is why we use V=IR to calculate ion currents. e) Noise. Two types to consider: random noise, and “shot noise” i) Random noise. Caused by thermal perturbations of the charge carriers in resistors. Termed Johnson Noise. 2 (1) V = 4kBTR (a) kB is Boltzman constant (b) T is absolute temperature (K) (c) R is resistance (ohms); this means will be dominated by noise in biggest resistor (2) at 300K, for a 1011 ohm resistor, is of order 10,000 counts per second. Sets lower limit on how small a current we can measure. ii) Shot noise (possibly named after Walter H Schottky), also sometimes called Poisson noise or just counting statistics. (1) Ions hitting detector represent discrete events. Because they are randomly (not evenly) spaced, their arrival at any point in space varies subtly with time, causing a fluctuation. Only observable at very small currents/voltages. (2) In mass spectrometry, this noise source is usually considered as part of “counting statistics”, which we discuss next week. iii) Time Response of detector a) Notice that the op amp works by having current flow through a very large resistor. If the input current is stable, it will reach a steady-state output, but what happens when the input is varying? Takes some time to adjust. b) Amount of time that takes to adjust is determined by the RC product of the feedback loop. All circuits have some “stray capacitance” due to wires, connections, etc, and since 2 Anatomy of an IRMS Lecture 7 – Detectors R is very large this becomes non-negligible. Typically include a fixed capacitor to stabilize this, but recognize we need to build the circuit to make C as small as possible to allow fast response times. c) Equation: τ = RC (t in seconds, R in ohms, C in farads). Gives the time to reach 1/e adjustment, thus often called the “e-folding time”. 2) Secondary electron multipliers. a) What do we do when analog currents are too low for stable amplification by an electrometer circuit? Need to use an amplifying detector. Several types, all make use of the phenomenon of secondary electron emission. b) Discrete dynode SEM. i) Multiple discrete plates (12-14 typical), each held at successively higher potential (1- 2 kV overall). ii) Ion hits first plate and releases secondary electrons. These are accelerated towards the second, which release more electrons, etc. Results in a cascade of electrons. Typical gain of 104 to 108. iii) Can be operated in analog or digital (ion counting) modes. iv) One issue is that ions of different energy (mainly mass, but also V) produce different numbers of SE’s (emission efficiency). v) Second issue is deadtime. Each e- cascade reduces voltage at plate, takes a fixed (short) period of time to recharge. Net effect is that sensitivity goes down as input current rises. vi) Dark Noise. Amount of current generated with no incident ion beam due to stray charge movement. Lower is better. vii) Typical deadtime correction: (1) N = Nm/[1-(Nmτ/T)] (2) Nm = number of measured ions (3) τ = dead time (4) T = time interval over which measurement is made c) Conversion dynode multiplier. i) The first dynode in the chain is a specialized material (often Cu-Be allow) held at very high potential (-10kV) that provides more even SE emission as function of 3 Anatomy of an IRMS Lecture 7 – Detectors mass. Drawback is that this material is sensitive to air, must be protected. Also larger, more expensive. d) Continuous-dynode EM (Channeltron). i) Horn-shape that is lined with semiconductor, gives a continuous voltage gradient. ii) Main advantage is that they can be made very small, to fit closely spaced along focal plane. 3-4mm wide channeltrons used in the original Neptune/Triton configuration. Also less sensitive to air than conversion-dynode EM’s. Perform roughly half as well as discrete-dynode detectors. iii) Disadvantage is that they tend to drift, especially as they get older, because of breakdown/damage in the semiconductor coating. Can be destroyed by relatively high ion currents. iv) 3) Latest Thermo innovation is Compact Discrete Dynodes (CDD), which are about 7mm wide (twice the size of a Faraday cup). Can be stack fairly closely, but have the better sensitivity and stability of a discrete dynode detector. 4 .
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