Transcript
Physics Accessories
Extension Cables
• Low noise - less than 5fA leakage • Triaxial or coaxial cables • Custom lengths & connector configurations CNMC supplies triaxial and coaxial extension cable assemblies in standard 10-meter or custom lengths, terminated with connectors to fit almost any commerciallyavailable electrometer/dosimeter and ion chamber. The CNMC triaxial cable assemblies are made of flexible, white, 3mm diameter, shielded, low-noise, high-impedance, radiation-resistant cable, designed to carry the extremely small currents generated by ion chambers without signal degradation. Optionally, CNMC can supply the Suhner Swiss grey, 5.34mm diameter triaxial cable found on Nuclear Enterprise ion chambers. Extension cable assemblies can be made to custom lengths in 1 meter increments. A true extension cable will have a male connector on one end to mate with the electrometer input and a female connector on the other end to mate with the ion chamber output connector. Other configurations are possible, such as male/male to accommodate the Memorial parallel plate ion chamber, or female/female bulkhead-type connectors for through-the-wall mounting. Additionally, a wide selection of adapters offers virtually unlimited connector arrangements. The model number for the extension cable assemblies may be assembled according to the example below: 3 B M - F 10 Length in meters (1m = 3.28 ft) Gender: M = male, F = female * Gender: M = male, F = female * Connector: B = BNC , T = TNC Cable: 2 = coaxial, 3 = triaxial
Cable Reels CNMC cable reel assemblies offer a great convenience in winding and storage of extension cables. A cable reel will extend the life of a cable significantly by eliminating kinks and providing protection during storage. Only as much cable as needed has to be reeled out at any time. A cable reel will accommodate a white extension cable up to 50 meters in length or a grey cable up to 20 meters. Either gender may be specified at the hub, depending on whether the cable reel will be kept at the electrometer (male connector at the hub) or inside the treatment room (female connector at the hub).
Additional letters may be inserted immediately after the first number for the following options: R = Reel mounted (specify hub gender) G = Suhner Swiss grey 5.34mm cable (default = white Microdot)
Triaxial BNC (bayonet) connectors are the 2-slot/lug, 1/4turn locking type. The male connector has a pin insert and is always accompanied by a ferrule with locking slots. It is available only for cable mounting. The female connector has a socket insert and locking lugs that mate with the ferrule. It is available for cable mounting or panel (bulkhead) mounting, with either flange or threaded mounting. * Triaxial TNC (threaded) connectors have a body with an outside thread or a knurled ferrule with inside thread. Either may be fitted with a male pin or a female socket insert. The TNC cable model number has two letters designating each connector. The first letter designates the gender of the center conductor, while the second letter designates the gender of the threaded outer housing (example: 3TMF-FF10).
CNMC Company 865 Easthagan Drive, Nashville, TN 37217 USA tel. 615-391-3076 fax 615-885-0285 order desk: 800-635-2662 www.cnmcco.com
Technical Notes
Identifying Cable Connector Types Bayonet-type (BNC) Connectors Triaxial BNC-Male and BNC-Female BNC-M
BNC-F
Threaded-type (TNC) Connectors Triaxial TNC-Male/Female and TNC-Female/Male TNC-M/F
TNC-F/M
Triaxial TNC-Male/Male and TNC-Female/Female TNC-M/M
TNC-F/F
Note: For TNC connectors, the first letter designates the gender of the center conductor. The second letter designates the gender of the threaded outer housing.
CNMC Company
865 Easthagan Drive, Nashville, TN 37217 USA tel. 615-391-3076 fax 615-885-0285 order desk: 800-635-2662 www.cnmcco.com
Technical Notes
About Chamber Cable Lengths and Connectors
Short vs. Long Cable Length
BNC vs TNC Connectors
Ion chamber to electrometer interconnecting cable is an essential part of the dosimetry system, therefore, in any purchase of a dosimetry system the following must be considered: a. Total length b. Type of connectors Some ionization chambers are still being sold with a permanently-attached cable of 10 meter length. The current trend, however, is to have zero to two meters of cable attached to the chamber and to use a separate extension cable. There are several good reasons for this approach: 1. Ten meters is an arbitrary number that is sufficiently long to satisfy the requirements of most treatment facilities. There are many facilities, however, which require a 12-meter, 15-meter, or even as long as a 20-meter cable length. In that case, an ion chamber with a 10-meter integral cable still needs an extension cable. 2. Many treatment facilities use permanently-installed cables in a duct. In such cases the cable necessarily becomes a separate part of the calibration system. 3. At CNMC we have observed a far greater incidence of broken thimbles on ion chambers with long integral cables. The damage invariably occurs during the coiling of the cable with the attached ion chamber. Since the bulk of the cost, including the cost of re-calibration, is in the precision-built and delicate ion chamber, it makes far greater sense to handle the less expensive and more rugged cable separately, avoiding the possibility of damage to the ion chamber thimble. A chamber with a short cable can be handled with greater safety and stored separately in a compact storage case. 4. Most treatment facilities own several ion chambers and some of them may have a short cable. One extension cable can be used with numerous ion chambers, although a spare extension cable provides a redundancy as well. 5. The cable is a vulnerable part of the ion chamber, as it can be subjected to rough treatment. An ion chamber that is separate from the cable can remain in service while the damaged extension cable is being sent for repair. 6. When an ion chamber is sent to a calibration laboratory or for repair, the extension cable can remain behind. Substitution of the laboratory cable or the different cable length does not affect the calibration of an ion chamber. 7. A chamber with a short cable can be stored more conveniently and safely in a compact, padded case. 8. Contrary to some beliefs, additional in-line connectors do not present appreciable signal loss if care is taken to keep the connectors capped when not in use to prevent dust and moisture deposits on the insulators.
Many stories could be told about purchasing an electrometer and an ion chamber from separate vendors only to experience incompatibility of connectors. Fortunately, over the years the connector choices have narrowed down to two main types; BNC (bayonet, sometimes called BNT) and TNC (threaded). In the USA, the BNC system enjoys the overwhelming popularity due to several basic shortcomings of the TNC design: 1. A TNC connector requires several turns of a knurled ring to make a reliable connection; as compared to the one-quarter turn click-on action of the BNC connector. 2. Constant turning causes threads to produce fine metal dust that can cause problems such as noise over the signal. During a period of 20 years of cable and chamber repair experience, we have observed a far greater incidence of TNC connected chambers and cables requiring repairs. 3. Compatibility within a system using TNC connectors is complicated by the fact that four combinations are possible; male or female, either one can be had with an outside threaded or an inside threaded body. It is not generally known that Capintec and Wellhöfer make chambers with a male connector having an outside threaded body, whereas NE makes the chambers with a male connector having an inside threaded ring. This means that the cable made for a Capintec or a Wellhöfer chamber cannot be used with an NE chamber. Wellhöfer recently adopted the NE system, which further complicates the determination of compatibility. Conversely, a BNC system is simpler, as there are only two types of connectors: a male and a female. Although at CNMC we recommend and normally supply all chambers, electrometers and extension cables with triaxial BNC connectors, we will supply TNC connectors on request. Our extensive experience and knowledge of various systems enables us to assure compatibility with existing systems by providing a proper connector the first time around. We also stock various connector adaptors to provide adaptability among systems. Additionally, to effect standardization within a facility, we will modify your electrometer input, the ion chamber and extension cables with BNC connectors, preferably scheduled simultaneously with repair or ADCL calibration. CNMC offers all ionization chambers with a permanently attached short cable, 1 to 1.5 meters in length, terminated with a triaxial BNC connector (TNC is optional), with dust and moisture protective cap secured by a chain, or in few cases with a connector mounted on the chamber itself. Each ionization chamber is supplied in a foam-lined case for protection and convenient storage
CNMC Company 865 Easthagan Drive, Nashville, TN 37217 USA tel. 615-391-3076 fax 615-885-0285 order desk: 800-635-2662 www.cnmcco.com
