DURRIDGE NATURAL ROCK SAMPLE Bedienungsanleitung

NATURAL ROCK SAMPLE

Performance Monitoring Accessory for the RAD7

User Manual

TABLE OF CONTENTS 
TABLE OF CONTENTS 2 
INTRODUCTION 3 
RAD7 RADON SENSITIVITY CHECK 4 
1 Purge the RAD7 5 
2 Hook up to the Natural Rock Sample 5 
3 Start a 3-Hour Test 5 
3.1 Drying and Resealing the Natural Rock Sample 5 
4 Radon Data Assessment 6 
5 Multiple RAD7s 8 
6 Cumulative Spectrum 9 
RAD7 THORON SENSITIVITY CHECK 10 
1 Thoron Measurement 10 
1.1 Radon Interference 10 
2 Rough Thoron Check 11 
3 Better Thoron Check 11 
3.1 Experimental Protocol 13 
3.2 Calculation 13 
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

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