7.5: Radiation Transducers

Introduction

In Nessler’s original method for determining ammonia (see Section 7.4) the analyst’s eye serves as the detector, matching the sample’s color to that of a standard. The human eye, of course, has a poor range—it responds only to visible light—and it is not particularly sensitive or accurate. Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. Ideally the detector’s signal, S, is a linear function of the electromagnetic radiation’s power, P,

\[S=k P+D  \]

where k is the detector’s sensitivity, and D is the detector’s dark current, or the background current when we prevent the source’s radiation from reaching the detector. There are two broad classes of spectroscopic transducers: photon transducers and thermal transducers, although we will subdivide the photon transducers given their rich variety. Table \(\PageIndex{1}\) provides several representative examples of each class of transducers.

Transducer is a general term that refers to any device that converts a chemical or a physical property into an easily measured electrical signal. The retina in your eye, for example, is a transducer that converts photons into an electrical nerve impulse; your eardrum is a transducer that converts sound waves into a different electrical nerve impulse.

Table \(\PageIndex{1}\). Examples of Transducers for Spectroscopy

transducer	class	wavelength range	output signal
photovoltaic cell	photon	350–750 nm	current
phototube	photon	200–1000 nm	current
photomultiplier	photon	110–1000 nm	current
Si photodiode	photon	250–1100 nm	current
photoconductor	photon	750–6000 nm	change in resistance
photovoltaic cell	photon	400–5000 nm	current or voltage
thermocouple	thermal	0.8–40 µm	voltage
thermistor	thermal	0.8–40 µm	change in resistance
pneumatic	thermal	0.8–1000 µm	membrane displacement
pyroelectric	thermal	0.3–1000 µm	current

Photon Transducers

A photon transducer takes a photon and converts it into an electrical signal, such as a current, a change in resistance, or a voltage. Many such detectors use a semiconductor as the photosensitive surface. When the semiconductor absorbs photons, valence electrons move to the semiconductor’s conduction band, producing a measurable current.

Photovoltaic Cells

A photovoltaic cell (Figure \(\PageIndex{1}\)) consists of a thin film of a semiconducting material, such as selenium sandwiched between two electrodes: a base electrode of iron or copper and a thin semi-transparent layer of silver or gold that serves as the collector electrode. When a photon of visible light falls on the photovoltaic cell it generates an electron and a hole with a positive charge within the semiconductor. Movement of the electrons from the collector electrode to the base electrode generates a current that is proportional to the power of the incoming radiation and that serves as the signal.

Figure \(\PageIndex{1}\): Schematic diagram of a photovoltaic cell that consists of a semiconducting thin film of selenium placed between two electrodes. When a photon strikes the semiconductor, it generates an electron and a hole that carries a positive charge. The movement of the electron from the collector electrode to the base electrode generates a measureable current.

Phototubes and Photomultipliers

Phototubes and photomultipliers use a photosensitive surface that absorbs radiation in the ultraviolet, visible, or near IR to produce an electrical current that is proportional to the number of photons that reach the transducer (see Figure \(\PageIndex{2}\)). The current results from applying a negative potential to the photoemissive surface and a positive potential to a wire that serves as the anode. In a photomultiplier tube, a series of positively charged dynodes serves to amplify the current, producing 10⁶–10⁷ electrons per photon.

Figure \(\PageIndex{2}\).Schematic of a phototube (left) and a photomultiplier tube (right). In both, a photon strikes the photoemissive cathode producing electrons. In the phototube, the electrons that are drawn toward a positively charged anode, generating a current. In the photomultiplier tube, the electrons accelerate toward a positively charged dynode. Collision of these electrons with the dynode generates additional electrons, which accelerate toward the next dynode. A total of 10⁶–10⁷ electrons per photon eventually reach the anode, generating an electrical current.

Silicon Photodiodes

Applying a reverse biased voltage to the pn junction of a silicon semiconductor creates a depletion zone in which conductance is close to zero (see Chapter 2 for an earlier discussion of semiconductors). When a photon of light of sufficient energy impinges on the depletion zone, an electron-hole pair is formed. Movement of the electron through the n–region and of the hole through the p–region generates a current that is proportional to the number of photons reaching the detector. A silicon photodiode has a wide spectral range from approximately 190 nm to 1100 nm, which makes it versatile; however, a photodiode is less sensitive than a photomultiplier.

Figure \(\PageIndex{3}\): Illustration of silicon photodiode. See the text for a description of how the deterctor works.

Multichannel Photon Transducers

The photon transducers discussed above detect light at a single wavelength passed by the monochromator to the detector. If we wish to record a complete spectrum then we must continually adjust the monochromator either manually or by using a servo motor. In a multichannel instrument we create a one-dimensional or two-dimensional array of detectors that allow us to monitor simultaneously radiation spanning a broad range of wavelengths.

Photodiode Arrays

An individual silicon photodiode is quite small, typically with a width of approximately 0.025 mm. As a result, a linear (one-dimensional) array that consists of 1024 individual photodiodes has a width of just 25.6 mm. Figure \(\PageIndex{4}\), for example, shows the UV detector from an HPLC. Light from the deuterium lamp passes through a flow cell, is dispersed by a diffraction grating, and then focused onto a linear array of photodiodes. The close-up on the right shows the active protion of the photodiode array covered by an optical window. The active width of this photodiode array is approximately 6 mm and includes more than 200 individual photodiodes, sufficient to provide 1 nm resolution from 180 nm to 400 nm.

Figure \(\PageIndex{4}\): The photo on the left shows the UV detector from an HPLC and the photo on the right shows a close-up view of the its linear photodiode array deterctor. The bright line, from reflected light, is the photodiode array. The active portion of this detector is covered by an optically transparent window.

Charge-Transfer Devices

One way to increase the sensitivity of a detector is to collect and store charges before counting them. This is the approach taken with two types of charge-transfer devices: charged-coupled detectors and charge-injection detectors. Individual detectors, or pixels, consist of a layer of silicon dioxide coated on top of semiconductor. When a photon impinges on the detector it creates an electron-hole pair. An electrode on top of the silicon dioxide layer collects and stores either the negatively charged electrons or the positively charged holes. After a sufficient time, during which 10,000-100,000 charges are collected, the total accumulated charge is measured. Because individual pixels are small, typically 10 µm, they can be arranged in either a linear, one-dimensional array or a two-dimensional array. A charge-transfer device with 1024 x 1024 pixels will be approximately 10 mm x 10 mm in size.

Note

There are two important charge-transfer devices used as detectors: a charge-coupled device (CCD), which is discussed below, and a charge-injection device (CID), which is discussed in Chapter 10. Both types of devices use a two-dimensional arrays of individual detectors that store charge. The two devices differ primarily in how the accumulated charges are read.

Figure \(\PageIndex{5}\) shows a cross-section of a single detector (pixel) in a charge-coupled device (CCD) where individual pixels are arranged in a two-dimensional array. Electron-hole pairs are created in a layer of p-doped silicon. The holes migrate to the n-doped silicon layer and the electrons are drawn to the area below a positively charged electrode. When it is time to record the accumulated charges, the charge is read in the upper-right corner of the array with charges in the same row measured by shifting them from left-to-right. When the first row is read, the charges in the remaining rows are shifted up and recorded. In a charge-injection device, the roles of the electrons and holes are reversed and the accumulated positive charged are recorded.

Figure \(\PageIndex{5}\): Schematic diagram of a charge-coupled device (CCD). The figure on the left shows a single detector, or pixel. When a photon impinges on the detector it generates an electron-hole pair in the p-doped layer. The electrons migrate and accumulate beneath a positively charged electrode. Individual pixels are arranged in an array, as shown on the right. This array may be a single row (a linear, or one-dimensional array) or in a series of rows. The charge in each pixels is recorded by shifting the pixels from left-to-right and by moving rows of pixels up.

Figure \(\PageIndex{6}\) shows an example of spectrophotometer equipped with a linear CCD detector that includes 2048 individual elements with a wavelength range from 200 nm to 1100 nm. The spectrometer is housed in a compact space of 90 mm x 60 mm

Figure \(\PageIndex{6}\): Example of a spectrophotometer equipped with a linear CCD detector.

Thermal Transducers

Infrared photons do not have enough energy to produce a measurable current with a photon transducer. A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of infrared photons increases a thermal transducer’s temperature, changing one or more of its characteristic properties. A pneumatic transducer, for example, is a small tube of xenon gas with an IR transparent window at one end and a flexible membrane at the other end. Photons enter the tube and are absorbed by a blackened surface, increasing the temperature of the gas. As the temperature inside the tube fluctuates, the gas expands and contracts and the flexible membrane moves in and out. Monitoring the membrane’s displacement produces an electrical signal.