Photon multiplier detector

Fig. 1.34 In the photon multiplier detector ions are transformed into photons which are detected by a photomultiplier.

Photon multiplier detector. (Reproduced from Ref. 3 with permission of John WQey Sons)... 

An incident ion beam causes secondary electrons to be emitted which are accelerated onto a scintillator (compare this with the operation of a TV screen). The photons that are emitted (like the light from a TV screen) are detected not by eye but with a highly sensitive photon detector (photon multiplier), which converts the photon energy into an electric current. 

At the electron multiplier detector, each arriving ion starts a cascade of electrons, just as a photon starts a cascade of electrons in a photomultiplier tube (Figure 20-12). A series of dynodes multiplies the number of electrons by 105 before they reach the anode where current is measured. The mass spectrum shows detector current as a function of mlz selected by the magnetic field. 

The detector converts ions of a given m/z value into a measurable electrical signal whose intensity is proportional to the corresponding ion current. With beam instruments (sectors, quadrupoles, or TOF analyzers) and the quadru-pole ion trap, the ions are first separated according to their m/z value before detection, usually by an electron multiplier or a photon multiplier. The operation of these most common detectors is briefly outlined below. 

A scintillator, sometimes known as the Daly detector, is an ion collector that is especially useful for studies on metastable ions. The principle of operation is illustrated in Figure 28.4. As with the first dynode of an electron multiplier, the arrival of a fast ion causes electrons to be emitted, and they are accelerated toward a second dynode. In this case, the dynode consists of a substance (a scintillator) that emits photons (light). The emitted light is detected by a commercial photon... 

Soft X-ray absorption measurements are done at low-energy synchrotron X-ray facilities such as the UV ring at NSLS or the Advanced Photon Source (APS) at Lawrence Berkeley National Laboratory (LBNL). The beam size is typically 1 mm in diameter. The electron yield data are usually obtained in the total electron yield (EY) mode, measuring the current from a channel electron multiplier (Channeltron). Sometimes a voltage bias is applied to increase surface sensitivity. This is referred to as the partial electron yield (PEY) mode. Huorescence yield (EY) data are recorded using a windowless energy dispersive Si (Li) detector. The experiments are conducted in vacuum at a pressure of 2 X 10 torr. 

The lifetime of channel electron multipliers is ca. 1-2 years. Neutrals or photons hitting the detector also increase the noise of the detection. 

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

The ideal high-throughput analytical technique would be efficient in terms of required resources and would be scalable to accommodate an arbitrarily large number of samples. In addition, this scalability would be such that the dependence of the cost of the equipment to perform the experiments would scale in a less than linear manner as a function of the number of samples that could be studied. The only way to accomplish this is to have one or more aspects of the experimental setup utilize an array-based approach. Array detectors are massively multiplexed versions of single-element detectors composed of a rectangular grid of small detectors. The most commonly encountered examples are CCD cameras, which are used to acquire ultraviolet, visible and near-infrared (IR) photons in a parallel manner. Other examples include IR focal plane arrays (FPAs) for the collection of IR photons and channel electron multipliers for the collection of electrons. 

Yet, quantum events do take place at the detectors. No-event is an event. The quantum state relevant under these circumstances is the physical quantum states equivalent to Eq. (19). First use Eq. (18) with state [1 0] indicating one-energy quantum in the system. This statement means everywhere inside the setup. The quantum state impinging at D1 is a direct product [0 0][l 0], whereas at D2 it reads [0 -1]<8>[1 0]. Using Eq. (3), we see that the root amplitude multiplying the photon transition is zero at Dl, is zero always, whereas at D2 the root amplitude is —1 which ensures the possibility that the photon is transferred (emitted) thereby prompting for an event. 

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