Scintillation Detectors

The use of gross counting systems in laboratory work allows for the detection of radioactivity in an object. They work by determining the amount of radionuclides present. They can be used to measure the presence of many types of radiation such as gamma rays, beta rays or x-rays. Their job is to essentially convert radiation energy into some kind of signal for the purpose of measuring. As a device, gross counters tend to consist of a detector, a preamplifier, an amplifier, a discriminator and a counter.

 

There are many types of radiation detectors, but they all work on two basic principles to convert radiation energy into an electrical signal: ionization and excitation. In ionization, the electrons are removed from the atom and then become electrically charged. They then can be influenced by an electric field, which produces a current that can be measured. Gas proportional detectors such as Gieger Muller and 3He neutron are examples of ionization detectors. With excitation, the ions are “excited” to a higher level until electromagnetic radiation is emitted.

 

One of the more common types of detectors is a gas filled radiation detector. Gas filled detectors, such as a He-3 neutron detector, measures radiation by reading the amount of ionized molecules existing in a gas or in the air. Gas fills the inside of the detectors cylinder. A charged voltage source is placed between two areas of the gas filled cylinder. The positive ions and free electrons will be attracted to the cathode and anode sides of the chamber respectively and passed through a wire leading to the detector, which are displayed as a signal. The more radiation that passes through, the more current is displayed. In addition to the gas filled detectors, the next widely used detection instrument is a scintillation detector.

 

The scintillation detector was originally developed, as we know it today, in the 1950’s and 60’s to solve the issue of low efficiency rates that the gas filled detectors had. Half a century prior to that though, in 1895, Wilhelm Roentgen, while experimenting with electrical currents, accidentally discovered the principles of the scintillation detector. He produced high voltages in an evacuated gas tube, which then began to give off a glow. The platino-barium cyanide crystals that he was working with also started to glow. He made the correlation between the two phenomenons and formulated that he was dealing with invisible electro-magnetic waves. Not knowing what to call these new waves, Roentgen named them “X-Rays”.

 

As opposed to gas, a scintillation device uses a special material that lights up when radiation is detected in a process called luminescence. This “special material” is usually a sodium-iodide crystal, the scintillator, which interacts with radioactive emissions. The amount of light emitted is dependent on the intensity of the radiation waves. The crystal is very delicate and must be protected by an aluminum casing. When light strikes the inside of the aluminum surface it is reflected to the PMT (photomultiplier tube). The PMT is made up of a photocathode, a focusing grid, dynode and an anode. The photocathode works to convert any flashes made by radiation hitting the crystal into electrons, which are then directed to the dynode where they are multiplied and shot towards the anode. The electrons then create an electrical current.

 

Scintillation detectors are often lightweight and portable, but can also be used as a stationary system. The thickness of the scintillator will determine which radiation waves can be absorbed. A thin scintillator is perfect for absorbing low energy waves, as the crystal isn’t too thick to block the waves out. A thicker crystal works well to detect high-energy waves such as gamma rays. In fact, scintillation detectors are a very popular device in the measuring of gamma rays due to its reliability and affordable cost. They are also fast and have little “dead time”, meaning the time it takes for a device to recover from a one measurement to the next.

 

Scintillation devices have been used as gamma ray detectors in many situations, including on board spacecraft on missions to observe cosmic gamma rays. They are particularly helpful in monitoring radiation levels in the sky as well as being used in nuclear physics, astrophysics and many other scientific applications.