General Guidelines for NIST Proficiency Tests Offered for X-Ray and
Gamma-Ray Air-Kerma Traceability
- The calibration facility shall initiate contact with NIST and request a
proficiency test for the reference radiation qualities within their scope
of accreditation.
- NIST will calibrate a NIST reference-class, transfer ionization chamber
using mutually decided upon reference radiation qualities. NIST will then
send the transfer standard to the participating calibration facility, along
with a proficiency-test protocol, describing the details of the NIST
calibration conditions and the required dates and addresses for the proper
shipment of the chamber. The NIST calibration results for the chamber are
not revealed to the test facility until the completion of the test.
- Calibrations of the NIST transfer chamber will be made at the participating
facility using the beam qualities identified in the test protocol. After
the participating facility calibrates the NIST ionization chamber, using
their appropriate equivalent beam quality(s), the chamber is returned to
NIST, or to the next participating facility if the test involves multiple
participants. The participant provides NIST with the calibration
coefficient(s) for the NIST chamber measured at their facility in units of
Gy/C in terms of the HVL of the specified beam quality(s). The participant’s
calibration coefficient(s) should be provided to NIST, normalized to one
standard atmosphere (101.325 kPa) and 22 °C for a direct
comparison with the NIST calibration coefficients.
- Upon receipt of the chamber at NIST, it will be re-calibrated to assure
stability and proper working conditions.
- Each result from a participating facility will be compared to the average of
the NIST calibration coefficients for that beam quality. At the completion
of the test a NIST proficiency report will be issued to the participating
calibration facility. The report will state the level of agreement with
NIST under the conditions of the proficiency test. The proficiency test can
be used to demonstrate that the participating facility can calibrate a NIST
transfer standard, using characterized x-rays or gamma-rays and NIST
traceable instruments of sufficient quality permitting acceptable agreement
with NIST. The results will reveal the degree to which the participating
calibration facility can demonstrate proficiency in transferring a NIST
air-kerma calibration under the conditions of the said facility at the time
of the proficiency test.
- The selection of the reference radiation qualities used for the proficiency
test depends on the scope of the accreditation. In most cases it is not
feasible that an annual or biennial proficiency test cover the entire range
of air-kerma rates. NIST advises that the proficiency test cover a
representative part of the scope of accreditation, eventually testing the
complete scope of the reference radiation qualities, typically using two to
four qualities per test. The accreditation guidelines generally state the
frequency and extent of the required test.
- The proficiency test protocol will contain information concerning the
air-kerma rate, corrections, calibration distance and beam size used for
the NIST calibrations. The protocol will be issued to the participating
calibration facility at the time of receipt of the NIST transfer standard.
However, the calibration facility should use their routine calibration
conditions for proficiency tests. The goal of the proficiency test is to
test that the calibration facility is transferring quality calibrations
under routine conditions.
- NIST will conduct the proficiency test using a NIST-owned
transfer-quality ionization chamber. The participating facility is
responsible for obtaining the necessary electrometer. NIST will not
provide a chamber/electrometer unit.
- NIST will conduct a proficiency test on an individual basis, as well as
part of an accreditation program. For a proficiency test involving multiple
facilities, questions or concerns should be directed to the NIST technical
contact, and no communication should be made amongst the facilities
concerning the test results until the completion of the test.
- A proficiency test for one calibration facility will cost the same as
a NIST calibration plus 15 percent. If multiple calibration facilities
participate in one proficiency test then the cost of the test is twice the
total cost of the NIST calibrations plus 15 percent, to be divided by the
number of participants.
- If a proficiency-test customer does not meet the level of agreement
required by their accrediting guidelines, a retest can be conducted at the
customer’s expense. Before a retest is conducted, NIST encourages open
dialogue with the customer to assist with the identification of the
problem(s) that contribute to the disagreement. Typically NIST will
suggest that the customer review the data for calculation or typographical
errors, verify the transfer of traceability of air-kerma, and investigate
the calibration history of the transfer chamber.
- It is the responsibility of the participant to inform the accrediting body
of the results of the NIST proficiency test. The results are considered
confidential and proprietary, and NIST will not provide copies of the
reports to the accrediting body unless written authorization is provided by
the customer
Inquiries or comments:
s.seltzer@nist.gov
Online: July 2006
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