Ludlum Measurements 319110, Руководство пользователя

Страница: 7 / 13

Model 44-110-4 Windowless Gas Flow Proportional Detector
page 2
before starting count. Purge times can be
adjusted by adjusting the gas flow rate, but
excessive flow could loosen contamination
on the samples and contaminate the interior
of the chamber.
After count is complete, turn off toggle valve and open drawer.
A background count should be taken after
each
source count to check for
contamination on the sample holder or area
within the O-ring.
4. CALIBRATION
CAUTION: Do not tip sample counter
over, which could cause contamination
of the anode wires and damage or
destroy the instrument.
For instruments with separate power
supplies (fixed threshold), the alpha channel
will operate at a lower voltage than the beta
channel.
4.1 Counting Instrument
Calibrated scaler instrument
HV range, nominally 1700-1900 V
Nominal input sensitivity:
-2 mV for a single-channel
instrument or
alpha channel = -75 mV
beta channel = -2 mV (with upper
discriminator set at 50 mV)
4.2 Operating Voltage
1. Set the scaler count time for 60
seconds.
2. Connect the Model 44-110-4
detector and place the source in the
sample cavity. Close drawer, turn on
gas, and allow chamber to purge.
3. Begin the search for the operating
voltage at 1600 V. Observing the
count, increase the HV until the
desired efficiency is achieved. Note
the HV.
4. Using one-minute count times, record count
rates for the desired alpha and/or beta
sources. The starting HV value should be 25
V less than the HV reading obtained in the
above step. Continue in 25-volt steps until
any one of the necessary criteria is not met
(that is, alpha or beta efficiency, acceptable
background, or “cross talk” parameters). The
ending HV value should be at least 25 V
greater than the HV reading obtained in the
above step.
5. Select the optimum operating voltage that
gives the greatest alpha and beta source
efficiency and acceptable background cpm.
4.3 Calculating Efficiency
1.
NIST-traceable sources required.
2.
Set HV as determined above.
3. Record a one-minute background and
one-minute source count. Subtract the
background count from the source count.
Divide the net source count by the dpm
value of the source, times 100 for 4π
efficiency.
If the source value is listed in µCi
(activity):
4. Convert the µCi value to a dpm value by
multiplying the microcurie value by 2.22
x 10
6
. Calculate the 4π efficiency as in
the previous steps.