From time to time, we get phone calls from end users of LMI instruments who have questions about check sources that are optional accessories with our instruments. Many of you are familiar with check sources, what they are and how they are used; however, for those of you who are not, here is a short explanation.
A check source is "a radioactive source, not necessarily calibrated, which is used to confirm the continuing functionality of an instrument" (ANSI N323AB-2013: American National Standard Radiation Protection Instrumentation Test and Calibration, Portable Survey Instruments). ANSI N323AB-2013 calls this a "source response check" which falls into the category of an "environmental check". For the sake of clarity, we will refer to it herein as a source response check.
The most common check source we supply is a 137Cs source of approximately 1 microcurie (µCi) in activity. The source is currently labeled on one side with the isotope name, the half-life, date of manufacture, a unique serial number, manufacturer name, and a statement about being an exempt quantity. LMI refers to this side as the "label side". The opposite side is blank.
When a check source is purchased with a new instrument, it can be mounted in a holder that is mounted on the side of the instrument or adhered to the side with an adhesive. Attachment of the check source to the instrument prevents misplacement of the check source assigned to the instrument and allows for convenient source response checks. A check source may also be purchased without the holder.
Source Check Tests
The same source to detector geometry routine shall be used when performing the source check tests as was used by the calibration laboratory. It is the policy of LMI to state the geometry used when a source response check reading is referenced on a certificate of calibration. The following example, a Model 2241-2 purchased with a Model 44-7 detector and a check source mounted in a holder, illustrates typical language:
"Cs137 check source s/n 1234 is ≈ 1 µCi reads ≈ 35.5 mR/hr with window end of 44-7 probe placed against check source holder with source holder door in the open position and Model 2241-2 set to Det. (1) and ratemeter position."
In the case of a check source purchased without a source holder, the following example is typical:
"Cs137 check source s/n 1234 is ≈ 1 µCi reads ≈ 200 µR/hr with label side of check source placed against window of the detector."
When a customer receives a new LMI instrument, we recommend that the end user duplicate the check source reading stated on the certificate of calibration and also on the instrument calibration sticker, using the geometry explained on the certificate. That will establish clearly how the source response check should be performed for subsequent checks. If it is not possible to reproduce the reference reading, please notify us.
ANSI N323AB-2013 does not include acceptance criteria for the source response test. However, the older standard, ANSI N-323-1978 stated in part:
“If at any time the instrument response to the check source differs from the original reading by more than + 20%, the instrument shall be returned to the calibration facility for maintenance, repair, and recalibration as required.”
Therefore, Ludlum Measurements recommends using + 20% as the acceptance criteria when performing a source response check.
Ludlum Check Sources & Holders
137Cs check sources supplied to LMI are designed for one side of the source to have a greater emission rate than the other, allowing beta radiation to be detected. Normally, this "hot" side is the label side. Infrequently, however, the opposite side produces a greater source count than the label side. Standard practice at LMI is to mount the check source with the label side facing out of the holder. However, if a customer requests a certain preference; as for example, a gamma-only orientation, the source will be tested to determine which side has the smaller emission rate and mounted in the holder accordingly.
Source holders may be purchased with or without the check source. Check sources other than 137Cs are also available. Visit the Check Sources product page to see the products we currently offer. If you are interested in purchasing any of these products, please contact us for current prices.