GM-COUNTER (Geiger-Muller Counter)

GM-COUNTER (Geiger-Muller Counter)A Geiger counter, also called a GeigerMller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a GeigerMller tube. Each particle detected produces a pulse of current, but the Geiger counter cannot distinguish the energy of the source particles. Invented in 1908, Geiger counters remain popular instruments used for measurements in health, physics, industry, geology and other fields, because they can be made with simple electronic circuits.

DescriptionGeiger counters are used to detect ionizing radiation, usually beta particles and gamma rays, but certain models can detect alpha particles. An inert gas-filled tube (usually helium, neon or argon with halogens added) briefly conducts electricity when a particle or photon of radiation makes the gas conductive. The tube amplifies this conduction by a cascade effect and outputs a current pulse, which is then often displayed by a needle or lamp and/or audible clicks.

Modern instruments can report radioactivity over several orders of magnitude. Some Geiger counters can be used to detect gamma radiation, though sensitivity can be lower for high energy gamma radiation than with certain other types of detectors. The density of gas in the device is usually low, allowing most high energy gamma photons to pass through undetected. Lower energy photons are easier to detect, and are better absorbed by the detector. Examples of this are the X-ray and Beta Pancake Geiger Tube.

Good alpha and beta scintillation counters also exist, but Geiger detectors are still favored as general purpose alpha/beta/gamma portable contamination and dose rate instruments, due to their low cost and robustness. A variation of the Geiger tube is used to measure neutrons, where the gas used is boron trifluoride or Helium 3 and a plastic moderator is used to slow the neutrons. This creates an alpha particle inside the detector and thus neutrons can be counted. Types and applications

A modern digital Geiger counter is used with applications ranging from nuclear medicine, mining, contamination monitoring, and national security.

A Geiger counter and metal detector combined for detecting both metal and radioactive materials for security purpose.

The configuration of GM tubes determines the types of radiation that it can detect. For example, a thin mica window on a GM Tube (shown here) will allow for the detection of alpha radiation, whereas GM Tubes without a thin mica window are too thick for the alpha and low energy beta radiation to pass through and be detected.

A GM instrument is one of many different types of radiation detectors. The GeigerMller tube is one form of a class of radiation detectors called gaseous detectors or simply gas detectors. Although useful, cheap and robust, a counter using a GM tube can only detect the presence and intensity of radiation (number of particles detected in an interval of time, as opposed to energy or wavelength). The GeigerMller counter has applications in the fields of nuclear physics, geophysics (mining), and medical therapy with isotopes and x-rays. Some of the proportional counters have many electrodes and are called multi-wire proportional counters or simply MWPCs.

Geiger plateauThe Geiger plateau is the voltage range in which the GeigerMller counter operates. If a GM tube is exposed to a steady radiation source and the applied voltage increased from zero, at first the count rate increases rapidly; at a certain voltage the rate of increase flattens out (only changing a few per cent for every 100 volts increase).

Depending on the characteristics of the specific tube (manufacturer, size, gas type etc.) the exact voltage range may vary. In this plateau region, the potential difference in the counter is strong enough to ionize all the gas inside the tube, upon triggering by the incoming ionizing radiation (alpha, beta or gamma radiation). Below the plateau the voltage is not high enough to cause complete discharge; a limited Townsend avalanche is the result, and the tube acts as a proportional counter, where the output pulse size depends on the initial ionization created by the radiation. Higher voltages give a pulse size independent of the initial ionization energy. If the applied voltage is too high, a continuous glow discharge is formed and the tube cannot detect radiation.

The plateau has a slight incline caused by increased sensitivity to low energy radiation, due to the increased voltage on the device. Normally when a particle enters the tube and ionizes one of the gas atoms, complete ionization of the gas occurs. Once a low energy particle enters the counter, it is possible that the kinetic energy in addition to the potential energy of the voltage are insufficient for the additional ionization to occur and thus the ion recombines. At higher voltages, the threshold for the minimum radiation level drops, thus the counter's sensitivity rises. The counting rate for a given radiation source varies slightly as the applied voltage is varied; for standardization of the response of the instrument, a regulated voltage is used to maintain stable counting characteristics. [3]GM tubesThe usual form of GM tube is an end-window tube. This type is so-named because the tube has a window at one end through which ionizing radiation can easily penetrate. The other end normally has the electrical connectors. There are two types of end-window tubes: the glass-mantle type and the mica window type. The glass window type will not detect alpha radiation since it is unable to penetrate the glass, but is usually cheaper and will usually detect beta radiation and X-rays. The mica window type will detect alpha radiation but is more fragile.

Most tubes will detect gamma radiation, and usually beta radiation above about 2.5 MeV. GeigerMller tubes will not normally detect neutrons since these do not ionise the gas. However, neutron-sensitive tubes can be produced which either have the inside of the tube coated with boron or contain boron trifluoride or helium-3 gas. The neutrons interact with the boron nuclei, producing alpha particles or with the helium-3 nuclei producing hydrogen and tritium ions and electrons. These charged particles then trigger the normal avalanche process.

Although most tubes will detect gamma radiation, standard tubes are relatively inefficient, as most gamma photons will pass through the low density gas without interacting. Using the heavier noble gases krypton or xenon for the fill effects a small improvement, but dedicated gamma detectors use dense cathodes of lead or stainless steel in windowless tubes. The dense cathode then interacts with the gamma flux, producing high-energy electrons, which are then detected.

Quenching and dead timeThe ideal GM tube should produce a single pulse on entry of a single ionising particle. It must not give any spurious pulses, and must recover quickly to the passive state. Unfortunately for these requirements, when positive argon ions reach the cathode and become neutral argon atoms again by obtaining electrons from it, the atoms can acquire their electrons in enhanced energy levels. These atoms then return to their ground state by emitting photons which can in turn produce further ionisation and hence cause spurious secondary pulse discharges. If nothing were done to counteract it, ionisation could even escalate, causing a so-called current "avalanche" which if prolonged could damage the tube. Some form of quenching of the ionisation is therefore essential. The disadvantage of quenching is that for a short time after a discharge pulse has occurred (the so-called dead time, which is typically a few microseconds), the tube is rendered insensitive and is thus temporarily unable to detect the arrival of any new ionising particle. This effectively causes a loss of counts at sufficiently-high count rates.

External quenching uses control electronics to temporarily remove the high voltage between the electrodes. Self-quenching or internal-quenching tubes stop the discharge without external assistance, by means of the addition of a small amount of a polyatomic organic vapor such as butane or ethanol; or alternatively a halogen such as bromine or chlorine.

If a poor diatomic gas quencher were introduced to the tube, the positive argon ions, during their motion toward the cathode, would have multiple collisions with the quencher gas molecules and transfer their charge and some energy to them. Neutral argon atoms would then be produced and the quencher gas ions would reach the cathode instead, gain electrons in excited states which would decay by photon emission, thereby producing spurious tube discharge as before. However, effective quencher molecules, when excited, do not lose their energy by photon emission but by dissociation into neutral quencher atoms. No spurious output pulses are then produced.

Halogen tubeThe halogen GM tube was invented by Sidney H. Liebson in 1947,[4] The discharge mechanism takes advantage of a metastable state of the inert gas atom to more-readily ionize a halogen molecule, enabling the tube to operate at much lower voltages, typically 400600 volts instead of 9001200 volts. This type of GM tube is therefore by far the most common form now. It has a longer life than tubes quenched with organic compounds, because the halogen ions can recombine while the organic vapor is gradually destroyed by the discharge process (giving the latter a life of around 108 events)

Radiolabeling and Isotopic Markers - Introduction

NomenclaturePurpose

Radiolabeling and Isotopic Markers

All elements can exist as two or more isotopes that differ in the number of neutrons in the nucleus. Some isotopes are stable indefinitely, while others are unstable (radioactive). Radioactive isotopes decay with a defined half-life, and primarily through release of helium nuclei ( particles), electrons or positrons ( particles), and radiation. The ready detection of this emitted radiation, even on a very small scale, underlies the utility and high sensitivity of a radioactive label.

This website focuses on the isotopes primarily of interest to organic chemists, which include the non-metal Main Group elements. Once prepared, radiolabeled compounds meet a variety of fates, but the goal is ultimately to detect the labeled molecule, fragment or metabolite: a suitable radiolabel or isotopic marker should allow normal chemical or biochemical processes to be monitored without causing any interference

This article addresses radiolabeling and isotopic labeling in the context of organic synthesis. In general, this labeling involves the application of known synthetic methods to target molecules in which at least one atom (or a statistical portion thereof) is present as an isotope other than its naturally most abundant one. Molecules that contain such an isotope are referred to being labeled because such isotopically distinct atoms serve to mark the molecule (or a fragment thereof) for later detection by various means.

All elements can exist as two or more isotopes that differ in the number of neutrons in the nucleus. Some isotopes are stable indefinitely, while others are unstable (radioactive). Radioactive isotopes decay with a defined half-life, and primarily through release of helium nuclei ( particles), electrons or positrons ( particles), and radiation. The ready detection of this emitted radiation, even on a very small scale, underlies the utility and high sensitivity of the radioactive label.

This article focuses on the isotopes primarily of interest to organic chemists, which include the non-metal Main Group elements. The isotopes of metals within the Main Group, as well as alkali metals, alkaline earths, transition metals, lanthanides and actinides have many important applications in medicine, geology, inorganic chemistry, but are outside the scope of this discussion. The nuclei considered here are listed in the table below.

Minor Main-group IsotopesFamilyNucleusNatural abundanceHalf-life

Hydrogen2H0.0156%stable

3Htrace45008 d

Carbon11C~20 m

13C1.1%stable

14C