DURRIDGE NATURAL ROCK SAMPLE User manual

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NATURAL ROCK SAMPLE
Performance Monitoring Accessory for the RAD7
User Manual

TABLE OF CONTENTS
TABLE OF CONTENTS 2
1 INTRODUCTION 3
2 RAD7 RADON SENSITIVITY CHECK 4
2.1 Purge the RAD7 5
2.2 Hook up to the Natural Rock Sample 5
2.3 Start a 3-Hour Test 5
2.3.1 Drying and Resealing the Natural Rock Sample 5
2.4 Radon Data Assessment 6
2.5 Multiple RAD7s 8
2.6 Cumulative Spectrum 9
3 RAD7 THORON SENSITIVITY CHECK 10
3.1 Thoron Measurement 10
3.1.1 Radon Interference 10
3.2 Rough Thoron Check 11
3.3 Better Thoron Check 11
3.3.1 Experimental Protocol 13
3.3.2 Calculation 13
3.3.3 Assessing The Thoron Data 13
Table of Contents 2

1 INTRODUCTION
e Natural Rock Sample is a 21oz (600g) sample of granite gravel, enclosed in a container that may be
sealed using a pair of ball valves and end caps.
Granite generally contains trace amounts of both uranium and thorium that has been in the rock since
it was formed, millions if not billions of years ago. erefore all the progeny of both the uranium and
thorium decay chains are fully supported and in full equilibrium down to 226-Ra and 224-Ra.
When the Natural Rock Sample is kept dry, it constitutes a rock-steady emitter of both radon and
thoron. is manual shows how to take advantage of that property to check the performance of a
RAD7. Please note that to keep the emission constant it is essential that the Natural Rock Sample be
kept fully dry. is is because the radon and thoron emanation efficiency depends upon the humidity at
the rock-air interface on the surface of the rocks. Always use a Small Drying Tube, with blue, active
desiccant in it, in the closed-loop radon calibration circuit, or upstream of the desiccant in the open-
loop thoron calibration configuration. If the desiccant in the Small Drying Tube gets used up it should
be replaced or regenerated. e Natural Rock Sample should be thoroughly dried by purging with dry
air every 30 - 50 days. is can be done as part of the standard radon calibration check protocol (see
Section 2.3.1) or, if no calibration check is required, as a standalone procedure. e Natural Rock
Sample has a desiccant pack and humidity indicator built into it for maintaining and verifying dryness.
For thoron sensitivity measurement, the carrier air, pushing the thoron into the sample path, should be
dried before it enters the rock sample. ese configurations are illustrated on the following pages.
Aer calibration at DURRIDGE Company, and with the optional oron Calibration Check Kit, the
Natural Rock Sample can be used for low-precision checking of both the radon and thoron sensitivities
of any RAD7. e Natural Rock Sample can provide a means for monitoring long-term changes in both
radon and thoron sensitivity.
Figure 1 Natural Rock Sample Components
Sec$on 1 Introduction 3

2 RAD7 RADON SENSITIVITY CHECK
Aer sitting sealed on a shelf for one month, the radon in the Natural Rock Sample container will have
reached within 1% of a steady value, in which the rate of emission of radon into the container is equal
to the rate of loss by decay and leakage. If that quantity of radon is known, and if it is distributed
around an air loop of known volume, then the radon concentration in the air is known. If one of the
devices in the air loop is a RAD7, the RAD7’s reported radon concentration may be compared with the
known concentration, and thus the sensitivity of the RAD7 may be effectively checked. A correction
may be applied to the Natural Rock Sample radon reading to account for the ambient air radon
concentration (see Section 2.4).
Figure 2 Radon Sensitivity Check Setup
In practice, provided the device and tubing configuration is identical to the calibration setup, all that is
required to check the RAD7 sensitivity is to compare the ‘standardized’ average RAD7 reading (see
below) with the standardized average reading on the Natural Rock Sample calibration certificate. e
ratio of the two will give a correction factor that may be applied directly to RAD7 readings.
Sec$on 2 RAD7 Radon Sensitivity Check 4

2.1 Purge the RAD7
First, purge the RAD7 with dry air in an open loop for at least 5 minutes, then switch to 1-day protocol:
select Setup, Protocol, 1-day, and push [ENTER]. Start a reading, still sampling fresh air through a
drying unit. Go to the third status window (push the [Menu] key, then push [ENTER], [ENTER], Rt
Arrow, Rt Arrow, and observe the relative humidity in the top right corner. Keep the reading going
until the RH is below 9%, then switch offthe RAD7.
If a correction is to be made to the Natural Rock Sample radon reading for radon added to the air loop
by the ambient air, aer purging the RAD7, (see section 2.4), the fresh-air measurement should be kept
going until at least one and preferably two or three cycles are completed.
2.2 Hook up to the Natural Rock Sample
Hook the RAD7 up to the Natural Rock Sample in a closed loop, using the tubing and Small Drying
Tube provided with the Natural Rock Sample. Use the configuration shown in Figure 2, with the Small
Drying Tube upstream of the RAD7, and the Natural Rock Sample connection upstream of the Small
Drying Tube. If one end of the Small Drying Tube has started to turn pink, indicating that it is wet, that
end should be positioned closest to the RAD7 outlet.
2.3 Start a 3-Hour Test
Switch on the RAD7 and press the menu key. Select 1-day protocol (Setup, Protocol, 1-day,
[ENTER]). Change the Recycle number to 6 (Setup, Protocol, Recycle, 06, [ENTER]). Put the
RAD7’s infrared printer in place and switch offthe RAD7.
Switch on the RAD7 printer. Switch on the RAD7 and let it print out the header, including a review of
the settings. Check the printed review to make sure the Cycle time is 00:30 (30 minutes) and the
Recycle number is 06. Open the ball valves on the Natural Rock Sample container and start a test
(Test, Start, [ENTER]).
Keep the measurement going until all six cycles are complete and the summary is printed out (3 hours).
Average the last four readings. Record that average with the date. If a thoron test is to be conducted (see
Section 3), the best time to do it would be 1 - 2 hours aer the radon check (see Section 3.1.1 for
details).
2.3.1 Drying and Resealing the Natural Rock Sample
Aer testing is complete, the Natural Rock Sample should be thoroughly dried and prepared for
storage. To do this, set up the Natural Rock Sample in a closed loop with a RAD7 and a laboratory
drying unit containing fresh desiccant. (At this point the Small Drying Tube that was used in the 3-
hour test should be sealed with yellow caps and set aside until the next 3-hour test is conducted.) e
Natural Rock Sample should be positioned downstream of the laboratory drying unit, and the
laboratory drying unit should be connected with the end furthest from the screw cap connected to the
Natural Rock Sample. e screw cap end, which may be pink (indicating that it is wet), should be
connected to the RAD7 outlet as shown in Figure 3, on the following page.
Sec$on 2 RAD7 Radon Sensitivity Check 5

Figure 3 Natural Rock Sample drying loop configuration
Set the RAD7 pump to ‘ON’ (Setup, Pump, On [ENTER]), go to third status window (Setup,
Enter, Enter, Rt. Arrow, Rt. Arrow.) and monitor the RH reading. Keep the air circulating
through the loop until the relative humidity (RH) drops to 4% or below. e drying process may take
many hours.
Once the RH stays at 4% or below, turn offthe pump (Setup, Pump, Off [ENTER]), close the ball
valves, disconnect the tubing, and insert the end caps on the Natural Rock Sample tube. Mark the date
on the label and store the Natural Rock Sample with its tubing and Small Drying Tube on a shelf for
between 30 and 50 days before repeating the process.
If the ball valves are opened for any reason on a later date, remove the old label and replace it with a
new label showing the last date on which the valves were closed, and leclosed. e rock sample will
be ready for another radon check not before one month from that date. e next measurement should
be made within 50 days of the date on the label, to avoid the risk of moisture creeping in and changing
the emanation efficiency, and hence the radon and thoron output, of the Natural Rock Sample.
2.4 Radon Data Assessment
If the Natural Rock Sample was never calibrated, keep the average of the last four readings (3, 4, 5, and
6) in a record for future reference. (Figure 4 on the next page shows a typical data set in DURRIDGE’s
CAPTURE soware, with the last four readings selected.) Every standard RAD7 should produce nearly
the same result with that Natural Rock Sample if its sensitivity is as calibrated and if the rock sample is
dry and has been sealed for between 30 and 50 days. For greater reproducibility, 2/3 of the ambient air
radon concentration can be subtracted from the averaged readings to correct for the residual radon in
the air in the system aer purging. However, if the ambient radon concentration in the air is less than
100 Bq/m3(2.7 pCi/L) the error introduced by ignoring it will be less than 1%.
Sec$on 2 RAD7 Radon Sensitivity Check 6

If the Natural Rock Sample has been calibrated by DURRIDGE Company, the calibration certificate
gives the standardized average of readings 3, 4, 5 and 6, obtained with the standard setup, a well purged
and dry RAD7 and 30-minute cycles. us the ratio of the value stated on the rock sample calibration
certificate to the measured standardized average gives you the correction factor directly. RAD7
readings multiplied by that ratio will give readings consistent with the DURRIDGE calibration system
to within 15%.
You may also, if you wish, calculate the total radon amount in the air loop, and hence originally in the
Natural Rock Sample container. If the radon amount is Rn and the volume of the air loop is V, then the
radon concentration is Rn/V. e air volume in the Natural Rock Sample is about 460ml, a standard
RAD7 is 800ml, and the tubing with small drying tube is 16ml, for a total volume of a standard Natural
Rock Sample system with standard RAD7 of 1.275 L. If Rn is expressed in pCi, then the radon
concentration will be Rn/1.275 pCi/L. If Rn is expressed in Bq, then the radon concentration will be
Rn/1.275 Bq/L, or 1000 * Rn / 1.275 Bq/m3.
In pCi, Rn = 1.275 * radon concentration in pCi/L.
In Bq, Rn = 0.001275 * radon concentration in Bq/m3
If the certified and observed standardized average values differ by more than 15%, we recommend that
the RAD7 be returned to DURRIDGE Company for service. If, aer several monthly sensitivity checks,
there is a consistent ratio between the observed and the certified standardized average radon
concentration, you may consider applying a multiplier to all readings made with that RAD7, to bring
them in line with the Natural Rock Sample standard. If you notice a big jump, up or down, in the
reading, from the previous month, first look to see if the RAD7 was properly purged before the
measurement was started. If there is no obvious reason for the jump, the RAD7 should be returned to
DURRIDGE for service.
Figure 4 Graph of Natural Rock Sample sensitivity test data
Sec$on 2 RAD7 Radon Sensitivity Check 7

2.5 Multiple RAD7s
Before using the Natural Rock Sample to check a RAD7’s radon sensitivity, the Natural Rock Sample
must be lesealed for a month, in order for the radon inside the container to reach equilibrium. at
means a RAD7 can have its sensitivity checked each month on a regular basis. However, there is
nothing to stop us hooking up multiple RAD7s in series in the loop connected to the Natural Rock
Sample. is increases the volume of the loop and hence decreases the radon concentration in the loop,
but by a predictable and reproducible amount. is method is recommended for checking up to six
RAD7s on a monthly basis.
For the sake of argument, using rounded figures, let us assume that the total activity of radon inside the
Natural Rock Sample container, when in equilibrium, is equal to 2 Bq. Let us also assume that the
Natural Rock Sample and tubing has an air volume of 500ml while a RAD7 is 1L. en when the RAD7
is connected up to the Natural Rock Sample and the radon distributed around the loop, the radon
concentration will be 2 / 1.5 = 1.333 Bq/L = 1333 Bq/m3. If every additional RAD7 adds 1L to the loop,
then four RAD7s would total 4.5L, including the Natural Rock Sample and tubing. e radon
concentration in the loop would then become 2 / 4.5 = 0.444 Bq/L = 444 Bq/m3. While this is less than
1333 Bq/m3, it is still enough to make a good reading in two hours and a high precision reading in 12
hours.
Figure 5 Multiple RAD7 Sensitivity Check Configuration
Each month, all four RAD7s can be connected in series with one another and thoroughly purged and
dried out, before being connected to the rock sample to obtain a sensitivity check. Each month the
setup should be identical, with the same number of RAD7s and the same pieces of tubing to connect up
the entire loop. During the measurement all the RAD7 pumps, except one, should be set to OFF
(Setup, Pump, Off, [ENTER].)
If any of the RAD7s had developed a leak, the radon concentration in the loop would fall rapidly. us
in addition to keeping a record of the average readings during the check, for each month, a record of
Sec$on 2 RAD7 Radon Sensitivity Check 8

the slope of the average for all RAD7s should also be maintained, as the radon concentration falls off
over time from decay and any leakage. If during one month the slope is steeper than usual, that would
be an indication of a leak in the loop somewhere.
Although it is possible to combine several RAD7s in a loop for monthly radon sensitivity checks, this
technique will not work for checking thoron sensitivity, due to the decay of thoron in the air as it travels
around the loop (see Section 3.1). However, there is no need to wait between successive thoron checks,
and all the RAD7s can undergo the rough thoron check individually, sequentially, without any delay
between measuring the thoron response of one RAD7 and the next. e monthly protocol may
therefore be:
1. Connect all RAD7s in series, set all protocols to 1-day, set all pumps, except one, to OFF. Put a
Laboratory Drying Unit upstream of the line, with one end open to fresh air.
2. Set the RAD7 with an active pump to Purge, for 15 minutes, to purge all residual radon out of the
RAD7s and start to dry them out.
3. Select Test, Start, [ENTER] on all RAD7s, starting a 1-day test in each of them (with only one
pump operating).
4. Aer at least two cycles (1 hour), or preferably 3 cycles (1.5 hours), stop the tests.
5. Disconnect the Laboratory Drying Unit. Connect the string of RAD7s to the Natural Rock Sample
and Small Drying Tube.
6. Open the valves and select Test, Start, [ENTER] on all RAD7s (all still in 1-day protocol and
with all pumps, except one, set to OFF).
7. Depending on the precision required, aer between 3 and 24 hours, the tests may be stopped and the
RAD7s disconnected.
8. If required, rough thoron checks may then be performed with each RAD7, separately connected to
the Natural Rock Sample. Wait 1-2 hours before doing this, to allow time for the C window radon
counts to decay away (see Section 3.1.1).
9. Careful notes should be kept of all actions and readings, and all data downloaded from each RAD7
to a computer using DURRIDGE’s CAPTURE soware.
10.e Natural Rock Sample should be thoroughly dried and stored for 30 - 50 days, using the
procedure described in Section 2.3.1, before making the next calibration check.
2.6 Cumulative Spectrum
It is important that the cumulative spectrum (based on counts from multiple cycles) be printed and
inspected periodically. It gives excellent diagnostic information on the health of the instrument. Using
the Natural Rock Sample to check the calibration of the RAD7 every month is a convenient
opportunity to inspect the cumulative spectrum. With the Recycle set to 6 cycles, as indicated above, at
the end of the sixth cycle the RAD7 will automatically finish the run and print out a run summary. is
consists of some data, a bar chart of the six readings and then the cumulative spectrum. at spectrum
should show a sharp peak in the A window, about midway between the boundaries. ere may also be
peaks in the other three windows and at 5.3 MeV (210-Po) just to the leof the A window. Please see
the RAD7 manual for a description and image of some possible pathological spectra to watch out for.
Sec$on 2 RAD7 Radon Sensitivity Check 9

3 RAD7 THORON SENSITIVITY CHECK
3.1 Thoron Measurement
e RAD7 is capable of making a direct measurement of thoron gas concentration in air. It does this by
counting 216Po decays inside its measurement chamber. However, oron has a short half life, 55.6s, so
most of the thoron in the sample will be lost during acquisition if the time from sampling to entering
the measurement chamber exceeds even a minute. erefore when measuring thoron the sample
acquisition time must be as short as conveniently possible, and always consistent. Additionally, the
exact same setup should be used for thoron calibration as for thoron measurement. is setup
necessarily involves a specific length of tubing and a Small Drying Tube full of desiccant, as the RAD7
is less sensitive in a humid environment. Just as with radon, the Natural Rock Sample's thoron output is
a function of humidity at the rock-air interface, so it is important to ensure that your Natural Rock
Sample is thoroughly dried out before making a measurement.
e setup assumed in the RAD7 data processing and stated as standard in the manual consists of a
Small Drying Tube, which may be used as a wand for sniffing, along with a standard input tubing of 36”
(91.4cm) length and inner diameter of 3/16” (4.8mm), as shown below.
is typically gives the RAD7 a thoron sensitivity of about half its radon Sniffsensitivity. e 216Po
daughter of thoron has only a 145mS half life so the main component in the response time of the RAD7
to a step change in thoron concentration is the time taken to acquire the sample. e response is
virtually instantaneous.
Figure 6 Thoron Measurement Standard Setup
3.1.1 Radon Interference
e alpha particle from 216Po decay has an energy of 6.78 MeV, which is between the 218Po and 214Po
peaks in the 222Rn decay chain. at places it within the B window of the RAD7 spectrum, while the
214Po peak lies in the C window. About 1% of the 214Po counts are in a low energy tail from the peak,
and spill into window B. e spill factor is measured during calibration and the count in window B is
corrected for that spill by the RAD7 and by DURRIDGE’s CAPTURE soware when calculating the
Sec$on 3 RAD7 Thoron Sensitivity Check 10

thoron concentration. Nevertheless, the presence of a large spill from window C would increase the
uncertainty of the thoron count in window B. It is therefore important that the C window count be low
when performing a thoron measurement or calibration.
If a radon calibration check of a RAD7 has been performed with a Rock Sample, it is good practice to
wait an hour or two for the count rate in window C to drop below 10 cpm, before proceeding with the
thoron calibration check. During this time, the Natural Rock Sample can be lepurging with a RAD7
(set Test > Purge) and a Laboratory Drying Unit, which will ensure that the Natural Rock Sample is
thoroughly dry before the measurement begins.
3.2 Rough Thoron Check
Before closing the ball valves and disassembling the setup, change the protocol to oron by selecting
Setup, Protocol, Thoron, [ENTER]. en turn the RAD7 offand back on, and let the printer
print out the new header for thoron.
Start a measurement. Let it run for at least 15 minutes (3 five-minute cycles), or more if you would like
a higher precision average. Average all the thoron readings. Store the thoron average reading with the
radon average.
is thoron reading cannot be used to check the thoron sensitivity of the RAD7, as the reading is
heavily dependent on the pump flow rate and even the position of items in the setup. For example, if
the desiccant is placed upstream instead of downstream of the Natural Rock Sample it will make no
difference to the average radon reading, but will make a big difference to the thoron readings. Even just
switching the connections to the Natural Rock Sample can affect the thoron readings. However, if the
setup remains identical in every respect from one monthly check to the next, if the RAD7 pump is still
working at close to the same flow rate, and if the inlet filter is not blocked, then thoron readings made
with this setup will be similar from one month to the next. Aer several months of experience the user
will know the typical variation and will recognize an anomalous shi, should it occur.
3.3 Better Thoron Check
A more accurate thoron sensitivity check involves the oron Calibration Check Kit, which is sold
separately. To check the thoron sensitivity of a RAD7 using this kit, the measurement must be made in
precisely the same conditions and using the same configuration as thoron measurements in regular use.
In standard thoron protocol, the RAD7 has 1m of 3/16” ID tubing between the inlet filter and a Small
Drying Tube, which may be used as a wand for sniffing purposes.
e thoron check process therefore requires that the sampling point at the end of the Small Drying
Tube must be open to the air. It is the thoron concentration at that sampling point that we measure.
us we put a T-connector at that location, inject thoron at a known rate, using a carrier with less than
the RAD7 flow rate, and with fresh air from the other side of the T-connector supplying the balance of
air to make up the RAD7 flow rate. is configuration is illustrated in Figure 7.
Sec$on 3 RAD7 Thoron Sensitivity Check 11

Figure 7 Thoron Sensitivity Check configuration
Sec$on 3 RAD7 Thoron Sensitivity Check 12

3.3.1 Experimental Protocol
a) Set up the experiment exactly as in the diagram above, or else with the RAD7 sampling path in
exactly the same setup as will be used for measuring thoron.
b) Adjust the flow rate through the Natural Rock Sample to 0.55 L/min. A flow rate of 0.55 L/min is
chosen as a standard flow rate for injecting thoron into the RAD7 sample path because that may be
relied upon to be less than the RAD7 pump flow rate, which may range from 0.6 L/min to 1 L/min or
higher. With the set up as shown in the diagram above, run the external pump and adjust the needle
valve so that the air flowing through the Natural Rock Sample is 0.55 L/min.
c) Move the flow meter to the RAD7 outlet. Remove the flow meter from its position between the
needle valve and the Natural Rock Sample and reconnect the tubing so that the needle valve is
connected directly to the Natural Rock Sample. Connect RAD7 outlet to the input of the flow meter.
e external pump can be kept running during this maneuver.
d) Change the RAD7 to thoron protocol. Use the default thoron protocol (Setup, Protocol,
Thoron,[ENTER]), or choose the default and then change individual parameters (e.g. cycle time) as
desired. Turn offthe RAD7, turn on the printer, turn on the RAD7 and let it print out a header.
Examine the header to make sure everything is OK.
e) Start a reading: Test, Start, [ENTER].
3.3.2 Calculation
With an identical air flow rate (0.55 L/min) through the Natural Rock Sample and identical tubing
attached to the T connector, for every thoron calibration check, the rate of injection of thoron into the
sample flow will always be the same. Let this be . e units of are pCi per minute or Bq per
minute.
If the RAD7 flow rate, measured at the RAD7 outlet, is V L/min, then the thoron concentration in the
sample air flow at the sampling point (which is very close to the T connection) will be /V. If is
expressed in pCi/min and V is in L/min then /V will be pCi/L. If is expressed in Bq/min and the
flow is in L/min then /V will be Bq/L. To convert to Bq/m3multiply by 1000.
3.3.3 Assessing The Thoron Data
For an uncalibrated Natural Rock Sample, note the average thoron reading and the RAD7 flow rate.
Multiply the thoron reading by the flow rate to get , the rate of injection of thoron into the sample
flow (for units of Bq/m3and L/min, divide the product by 1,000 to get Bq/min). e experiment may
be repeated as oen as desired; there is no need to wait for any ingrowth. However, aer conducting
this experiment, the Natural Rock Sample must be lesealed for a month before a radon check may be
performed. e obvious time to do this experiment is 1 - 2 hours aer a radon check.
As an option, DURRIDGE Company will calibrate a Natural Rock Sample for thoron. With a setup as
above and a flow rate of 0.55 L/min through the Natural Rock Sample, we give you the rate of injection
of thoron into the RAD7 sample path. You may then divide that by the flow rate of the target RAD7 to
obtain the thoron concentration at the sampling point. is may be compared with the thoron reading
in the RAD7 and a correction factor derived.
Records of the radon and thoron readings should be kept safely for later reference and comparison.
Sec$on 3 RAD7 Thoron Sensitivity Check 13

14
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Revision 2021-02-15