M. E. Taylor Engineering, Inc. has been making the finest quality scintillators for over 30 years. Our original scintillators were P47 phosphor deposited onto a BK7 glass substrate. These scintillators are inexpensive and so reliable that they are still used in OEM instruments today and remain our best sellers. However, over the years other types of scintillators have made their way into our product line for their use in specific applications. Some examples are inorganic single-crystal scintillators such as YAP, YAG and our LXSR as well as organic plastic scintillators. There is no best scintillator - each one has advantages and disadvantages. Advice on how to choose a scintillator for your personal application can be found below.
Our new ITO-GOLD (pronounced eye-to-gold) SEM scintillator consists of a quartz substrate that is plated with indium tin oxide (conductive and transparent) to which a gold plated ring is electrically bonded. The P47 phosphor is then deposited. The advantages we have experienced include the gold retaining ring making better electrical contact with the scintillator without cutting into the phosphor material. The signal-to-noise ratio is better due to a decrease in the electronic noise created by a good electrical bond. There is a conductive layer under the phosphor that uniformly applies the bias voltage (10 KV usually). An aluminum overcoat is not required. In the event a pinhole develops in the phosphor, the exposed glass surface will not charge up. It is easier to handle during installation, and it is recoatable. All this leads to a longer time between scintillator changes and a cost savings overall.
Theory of Scintillation
A scintillator is a material that accepts incident high-energy electromagnetic or charged particle radiation and in turn uses that energy to fluoresce photons whose peak emission wavelength is longer than the wavelength of the incoming radiation. In the case of a Scanning Electron Microscope (SEM), the scintillator disc collects the secondary electrons that are produced as the electron beam scans the surface of the sample. These electrons are converted into photons which travel through the light pipe to the photomulitiplier tube (PMT) so that the signal may be amplified to the level required for viewing. The importance of the scintillator in these types of systems cannot be overstated.
Choosing the Right ScintillatorBelow you will find a list of the important characteristics of a scintillator with a brief description of each. It is recommended that you order the list below in a manner that is meaningful to you. Once you have ordered the characteristics, use the chart below to select the scintillator that is right for you and your application. Of course, we will be happy to help at any point during this process; simply call or email us with your questions!
- Peak Emission Wavelength: It is important to match the characterisitics of the equipment that will process the light generated by the scintillator with the characteristics of the scintillator itself. In the case of an SEM, each PMT is designed to operate over a specific band of wavelengths. If your scintillator produces photons that are outside this band, you will not be able to image your sample.
- Relative Light Output: The higher the output of the scintillator, the lower the gain requirements of the system. This is a key characterisitic of any scintillator. It describes how effective the scintillator is at doing it's job: converting incident radiation to light.
- Decay Time: The lower the decay time, the shorter each fluorescent event. This allows for the processing of more incident radiation per unit of time. For example, if you are scanning at TV rates, you will require a scintillator that is relatively fast - or one that has a lower decay time.
- Lifetime: This describes the durability of the scintillator. A longer lifetime means the scintillator can be in use for a longer period of time without the need for replacement. If you use your instrument seldomly or if other characteristics are more important, perhaps lifetime can be sacrificed.
- Cost: The relative price of each replacement scintillator.
- Lead Time: The amount of time required to obtain each scintilltor. Since the single-crystal scintillators are more expensive, we keep very few in stock. As a result, they must be machined, polished and coated to order.
|P47 on BK7||Organic Plastic on BK7||YAG (single crystal)||YAP (single crystal)||LXSR (single crystal)|
|Peak Emission Wavelength||400 nm||408 nm||560 nm||375 nm||420 nm|
|Relative Light Output (%NaI:TI)||~80||~30||15||40||80|
|Decay Time||40 ns||1.8 ns||70 ns||~27 ns||42 ns|
|Lifetime||Medium||Short||Very Long||Very Long||Very Long|
|Lead Time||In Stock||In Stock||4-6 Weeks||4-6 Weeks||2-3 Weeks|
Frequently Asked QuestionsWhy does the scintillator degrade?
All scintillators will degrade over time; some will degrade faster than others. In the past, the radiation doses received by organic scintillators cut their lifetime short. Today, improvements in scintillator design have greatly extended the useful lifetime of these scintillators. If your plastic scintillator is degrading, you may notice that you must adjust the gain higher and higher to obtain an image as bright as one you may have obtained when the scintillator was new. A more common reason for degradation of any scintillator, regardless of composition, is chamber contamination. Oil vapors contaminate the surface of the scintillator, forming a barrier to incoming electrons. Even turbo or ion pumped systems having an oil based roughing pump have this problem in the long run. Outgassing of the sample and adhesives used to attach the samples also produce contamination. Once you replace your scintillator, you will see an improved image since more of your signal is being processed.
Can I change the scintillator myself?
YES, IN MOST SEM'S. Your service manual should include easy to follow directions. If not, call the manufacturer for assistance. Since the scintillator is considered a consumable it is not usually covered under standard service contracts. It has been our experience that if your service engineer is doing routine maintenance and you have one on hand they will gladly replace it. It is a good idea to keep at least one spare in stock so that it is immediately available if needed. Assuming the scintillator is kept in its original container it will last many years. The material on the surface of the disc is fragile so please handle with care. It has been our company policy to replace, at no charge, any scintillator (purchased from us) damaged during installation. There are limitations and conditions to this policy.
How often should the scintillator be changed?
That depends on the type of scintillator you are using and how often your instrument is used. In the case of our most popular scintillator, the P47 type, and assuming average use, the scintillator should be changed once a year. If your system experiences heavy use (i.e. daily operation) you will probably need to change it more often. However, if you have a single crystal scintillator you can operate for much longer with less down time. Each single crystal scintillator is coated with 50 nm of Aluminum. If contaminants build up on the surface of this type of scintillator, the Aluminum coat can be stripped and reapplied to the crystal. This type of refurbishment is inexpensive and can be performed within 1 day of being shipped back here. Under normal use, this type of scintillator may last years before an Aluminum recoat is needed. The crystal itself may outlive your instrument!