NIST Technical Note 1864

Scaling of Testing Speed for Different 

Source-to-Detector Distances

B. Norman 

A.L. Sallaska

L. Pibida 

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1864
NIST Technical Note 1864

Scaling of Testing Speed for Different 

Source-to-Detector Distances

B. Norman 

A.L. Sallaska

L. Pibida

Radiation Physics Division 

Physical Measurement Laboratory

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1864

February 2015

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Willie May, Acting Under Secretary of Commerce for Standards and Technology and Acting Director 
Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1864

Natl. Inst. Stand. Technol. Tech. Note 1864, 15 pages (February 2015) 

CODEN: NTNOEF

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1864
1

Scaling of Testing Speed for Different Source-to-Detector Distances

B. Norman, A.L. Sallaska, and L. Pibida

National Institute of Standards and Technology, Gaithersburg, MD 20899-8462

Abstract

Several American National Standard Institute/Institute of Electrical and Electronics Engineers

(ANSI/IEEE) and International Electrotechnical Commission (IEC) standards were developed for radiation 

detection systems used for homeland security applications and the detection of illicit trafficking of 

radioactive materials. These standards cover a wide range of tests, some of which require sources to 

pass by an instrument at a certain speed producing a fixed exposure rate at the distance of closest 

approach. Some also allow this distance to vary within a specified range. This work investigates the 

need to apply a scaling factor dependent on distance to the testing speed in order to compensate for 

the time that the instrument is exposed to the radiation field. It was observed that there is an increase 

in the instrument response, with factors ranging from 1.5 to 4, when sources are placed at farther 

distances while maintaining the same testing speed and the same exposure rate at the distance of 

closest approach between the source and the detector. When the speed is scaled, most of these

differences are reduced to values of approximately ±20 %, within the measurement uncertainties. 

Measurements of alarm verification with speeds both scaled and unscaled were also performed. The 

alarm probability was observed to decrease significantly if the speed was not scaled. 

Key words: speed scaling; testing of RIDs; moving source, ANSI N42 standards for homeland security; IEC 

standards

Introduction

Several American National Standard Institute/Institute of Electrical and Electronics Engineers

(ANSI/IEEE) and International Electrotechnical Commission (IEC) standards were developed for radiation

detection systems used for homeland security applications and the detection of illicit trafficking of 

radioactive materials. Several of these standards are currently being revised or developed. These 

standards cover a wide range of radiation detection systems from small personal radiation detectors 

(PRDs), to large radiation portal monitors (RPMs). The main function of these detectors is to measure 

the magnitude of the radiation field (often expressed as exposure rate or ambient dose equivalent rate),

and notify the user by means of an alarm of the presence of radiation and possible identification of the 

radionuclides present. These systems are not designed to provide a dose record to the user. In fact, 
2

some of the instruments only report the radiation field in units of counts per second or some unit-less 

type display.

These standards cover a wide range of requirements that are verified using well defined test methods. 

Some of these test methods require sources to pass by an instrument at a fixed speed. The sources are 

required to produce a fixed radiation field at the distance of closest approach between the instrument 

and the source (e.g., 50 μR/h, 10 μR/h, 5 μR/h)1, while allowing for the source-to-detector distance to 

change within a certain range (e.g., 0.8 m to 1.2 m or 1 m to 3 m). Allowing for such variation in the 

testing distance creates differences in the radiation fields seen by the instruments. If the instrument is 

tested with a larger source at a far-away distance, the probability of the instrument to detect such 

source is larger than testing the same instrument with a smaller source at a closer distance, due to the 

increased time the source spends in the instrument’s field of view. This can cause different results 

depending on how the test was performed. In order to eliminate such dependence, the speed needs to 

be adjusted (or scaled), when testing at different distances. This will require the standards to set the 

requirements not only for a fixed speed, but also for a fixed testing distance. If the standard allows the 

testing distance to change within a certain range, then the speed needs to be scaled in order to maintain 

the same radiation field over the time that the instrument is exposed to the source. 

During the revision of the ANSI/IEEE N42.34 [1] and IEC 62327 [2] standards for radionuclide 

identification devices (RIDs), there was concern regarding the necessity to scale the speed when testing 

the instruments using different sources placed at different distances. Therefore, this paper will 

summarize the calculations and measurements performed in order to clarify how the scaling of the 

speed as a function of the testing distance applies to instruments. In particular, the RIDs response is 

emphasized, as it will provide the information needed for the revision of the standards. Measurements 

of the effect of scaled and unscaled speeds on alarm response are also discussed. 

Scaling calculation for testing speed

Most of the standards test the different types of radiation detection systems by passing a source in front 

of the instrument at a certain speed and verifying how many times the system alarms for a fixed number 

of trials. In addition, some standards require that the alarms are produced within a certain time period. 

This test is usually performed by requiring a radiation field of a certain exposure rate or ambient dose 

equivalent rate value. Setting up the radiation field based on this rate allows testing laboratories to use 

existing sources for a long period of time by changing the source-to-detector distance, minimizing the

need to replace or purchase sources to perform the tests. In addition, some of the standards permit a 

range of testing distances, such as 1 m to 3 m or 0.8 m to 1.2 m. Restricting the source-to-detector 

distances limits the source activities that can be used for the tests. 

 

1 NIST does not endorse the use of non-SI units. This paper uses non-SI units because it addresses the 

requirements listed in the ANSI/IEEE published standards. The SI unit of exposure rate is C/kg/h where 1 mR/h = 

2.58 × 10-7 C/kg/h.
3

If the testing speed is kept constant and the source-to-detector distances is changed within the 

suggested range in the standards, while the radiation field is kept unchanged at the reference point of 

the instrument, then the total exposure or ambient dose equivalent value measured by the instrument 

will be lower if the source is at the closer distance, see Figure 1. Therefore, the instrument is exposed to 

a stronger radiation field, above the instrument alarm threshold, for a longer period of time when the 

source is at a faraway distance. This can lead to differences in test results if testing is performed at 

different distances at different testing laboratories, where the instruments tested at the largest sourceto-detector distances have the advantage.

-8 -6 -4 -2 0 2 4 6 8

0

10

20

30

40

50 Exposure rate (R/h)

Time (s)

 0.8 m

 1.0 m

 1.2 m

 1.5 m

 2.0 m

 3.0 m

Alarm Threshold

Figure 1: Calculated exposure rate profile as a function of time for a source moving at a speed of 1.2 m/s at different sourceto-detector distances

As mentioned in the introduction, in order to address this issue, the standards need to specify the 

instrument performance requirement at a fixed distance and speed, allowing the source-to-detector 

distance to change within a range only if the source speed is scaled to cancel this effect. If the standard 

specifies the requirements at a speed, v0, and a source-to-detector distance, d0, and the test is 

performed at the source-to-detector distance, d, then testing speed shall be adjusted or scaled to a 

value, v, given by:
4

v = v0 × d/d0 . (1)

Therefore, the exposure rate, Ẋ, at the reference point of the instrument, at a distance r, is equal to 

Ẋ = 50μR/h

d

2

r

2 = 50μR/h

d

2

v

2 t2+d2

 , (2)

where v is the source speed, t is the time, d is the distance of closest approach, and 50 μR/h is the 

exposure rate at the reference point of the instrument when the source is at the distance of closest 

approach, as shown in Figure 2.

Figure 2: Diagram of source movement with an instrument mounted on a phantom

The total exposure, X, measured by the instrument is given by

X = ∫ 50μR/h

d

2

v

2 t2+d2

T

2

⁄

−T

2

⁄

dt = 2 × 50μR/h ×

d

v

[atan (

Tv

2d

)], (3)

where T is the integration time. The integration time is determined based on the instruments’

requirements; it can be either given by the time it takes the instrument to alarm or the integration time 

of the instrument.

The following example illustrates the differences in the total radiation field measured by an instrument 

when exposed to a source that is moving at a speed of 1.2 m/s placed a different distances from the 

instrument. In general, the standards require the instrument alarms within 2 s from the time of 

maximum exposure. It is assumed that the source is moving in a direction that is perpendicular to the 

instrument, so the instrument will see the source approaching and the total radiation field measured 

will depend on the instrument integration time. For illustration purposes, an integration time is 4 s and 

an exposure rate at the distance of closest approach is 50 μR/h are assumed. If the standard allows the 
5

testing distance to vary between 1 m and 3 m, then the total radiation field measured by the instrument 

at 3 m will be 72 % larger than at a 1 m. If the standard allows the testing distance to vary between 

0.8 m and 1.2 m, then the total integrated exposure measured by the instrument at 1.2 m will be 33 % 

larger than at a 0.8 m. Clearly, not scaling the speed has a significant effect on the test results.

Experimental setup

In order to illustrate this effect, measurements were performed with a multitude of sources (including 

the radionuclides,

241Am, 133Ba, 137Cs, and 60Co), at varying distances and speeds. For a given 

radionuclide and set of distances, d (see Figure 2), different source activities were chosen for a pair of 

sources in order to obtain the same radiation field at the reference point of the instruments at the 

distance of closest approach. 

For all measurements, the figure-of-merit was the ratio of the instrument readings for a given 

radionuclide for sources placed at a pair of different distances (tested separately) and does not depend 

on the absolute value of the exposure rates. This quantity was used only to determine which source 

pairs yielded the same exposure rate at differing distances. Some of these exposure rate values

(expressed in units of R/h), X,

̇

 were calculated using the point source approximation for a known source 

activity and encapsulation using [3, 4]:































   air

air

air i

j

j j

j

j i

i air

en i

i i

r ρ

ρ

μ E

z ρ

ρ

μ E

ρ

μ E

P E

πd

A

X

( )

exp

( )

exp

( )

4

114.1

 2

 , (4)

where μen(Ei) is the mass energy-absorption in air, μj/ρj the mass attenuation coefficient for the 

encapsulating-layer material of thickness zj and a density ρj, and μair/ρair is that for air, A is the source 

activity, Pi is the probability per disintegration that a photon of energy Ei is emitted, and d is the sourceto detector distance at the point of closest approach. The 114.1 value is expressed in units of R·s2

/m2.

The cut-off energy, δ, used for the exposure rate calculations was 40 keV.

Some of the exposure rate values were measured using a pressurised ionization chamber. The 

pressurized ionization chamber is a 25.4 cm diameter hollow stainless steel sphere with a 0.32 cm wall 

thickness containing pressurized argon at 25 atmospheres. This chamber is designed to measure 

exposure rates from background levels up to 100 R/h, with a known energy response for photons 

between 50 keV and 1.33 MeV. 

Two different linear motion systems were used to cover a large range of speeds. For one system, the 

speed was set from a computer controlled motor. The computer control specifies the revolutions per 

minute of the motor, and the speed was determined using the measured calibration curve shown in 

Figure 3. The other linear motion system produces fixed speeds of 1.2 m/s and 2.2 m/s.
6

0 500 1000 1500 2000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 Speed (m/s)

Revolutions per minute (rpm)

Figure 3: Speed calibration of the linear motion system

Scaling measurements

Several instruments were used to evaluate the response to scaling. The instruments included: one 

spectroscopic radiation portal monitor (SRPM), one backpack-type radiation detector (BRD), five 

radionuclide identification devices (RIDs), and one personal radiation detector (PRD). While maintaining 

the same exposure rate at the distance of closest approach between the source and instrument, several 

combinations of sources, speeds, and distances were used to assess instrument response when testing

at different distances, while for two Cases:

1. Maintaining the same speed at each distance,

2. Scaling the speed for one source at a given distance, using Equation 1. 

Different exposure rates values, produced by each of the four radionuclides (241Am, 133Ba, 137Cs, 60Co), 

were used in order to determine if there were any effects in the instrument response with the radiation 

field strength.

Table 1 lists the different test configurations used to measure the instrument response for both Case 1 

and 2. The ratio of the maximum value of the instruments’ readings for the two testing distances is 

listed and is plotted in Figure 4. Some short distances were chosen as the geometrical effect could be 

larger, potentially increasing the differences in the instrument response when scaling is applied to the 

testing speed. From Figure 4, it can be observed that regardless of the type of instrument under test, the 

measured exposure rate is larger when the source is placed at a faraway distance while maintaining the 
7

same exposure rate at the distance of closest approach. These differences varied between 

approximately a factor 1.5 and 4.

Table 1: List of test configurations used for the measurements and the ratio of exposure rates measured by the instruments 

at a distance d relative to distance do. The exposure rate, Ẋ

δ, produced by each source is determined at the distance of 

closest approach.

Instrument Source

Exposure 

rate,

Ẋ

δ (μR/h)

d0

(m)

d

(m)

v

(m/s)

Case 1: 

Ratio without scaling

SRPM 133Ba 35 0.25 0.65 2.2 3.37 ± 0.99

SRPM 133Ba 35 0.25 0.65 1.2 2.73 ± 0.71

SRPM 137Cs 17 0.50 1.20 2.2 2.14 ± 0.57

SRPM 137Cs 17 0.50 1.20 1.2 1.66 ± 0.27

SRPM 60Co (1) 42 0.25 0.95 2.2 3.85 ± 0.95

SRPM 60Co (1) 42 0.25 0.95 1.2 3.59 ± 0.65

SRPM 60Co (2) 38 0.50 1.00 2.2 2.01 ± 0.46

SRPM 60Co (2) 38 0.50 1.00 1.2 1.57 ± 0.29

BRD 133Ba 35 0.25 0.65 2.2 3.53 ± 0.60

BRD 133Ba 35 0.25 0.65 1.2 3.26 ± 0.38

BRD 137Cs 17 0.50 1.20 2.2 2.53 ± 0.28

BRD 137Cs 17 0.50 1.20 1.2 2.41 ± 0.25

BRD 60Co (1) 42 0.25 0.95 2.2 4.27 ± 0.60

BRD 60Co (1) 42 0.25 0.95 1.2 4.19 ± 0.56

BRD 60Co (2) 38 0.50 1.00 2.2 2.24 ± 0.23

BRD 60Co (2) 38 0.50 1.00 1.2 2.19 ± 0.21

RID 1 133Ba 35 0.25 0.65 2.2 1.68 ± 0.40

RID 1 133Ba 35 0.25 0.65 1.2 1.50 ± 0.33

PRD 133Ba 35 0.25 0.65 1.2 2.09 ± 0.74

Instrument Source

Exposure 

rate,

Ẋ

δ (μR/h))

d0

(m)

d

(m)

V0

(m/s)

v

(m/s)

Case 2: 

Ratio with 

scaling

SRPM 60Co (1) 45 0.25 0.46 1.2 2.2 1.42 ± 0.33

SRPM 60Co (2) 42 0.50 0.92 1.2 2.2 1.26 ± 0.24

BRD 60Co (1) 45 0.25 0.46 1.2 2.2 1.29 ± 0.19

BRD 60Co (2) 42 0.50 0.92 1.2 2.2 1.40 ± 0.14

 
8

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

137Cs 60Co (2) 60Co (1) Exposure rated / Exposure rated0

SRPM: 1.2 m/s SRPM: 2.2 m/s BRD: 2.2 m/s BRD: 1.2 m/s RID: 1.2 m/s RID: 2.2 m/s

 PRD: 1.2 m/s 

133Ba

Figure 4: Exposure rate ratio measurements for different sources measured at two speeds and two distances for different 

types of instruments. Speed values are not scaled for the different testing distances. The exposure rate, Ẋ

δ, at the distance 

of closest approach produced by each source is kept constant (within ±20 %).

Additional measurements were performed using the RIDs in order to assess the effects on this particular 

type of instruments. The test configurations used for these measurements are listed in Table 2. For 

most cases the determination of the exposure rate was performed by calculation (using Equation 4), for 

some it was measured using the pressurized ionization chamber (these cases are marked in Table 2). 

When the speed is scaled using Equation 1, most of the ratios between the measurements at the 

different distances are approximately within ±20 %, see Figure 5.

Table 2: List of test configurations used for the RIDs measurements and the ratio of the exposure rates measured by the RIDs 

at a distance d relative to distance do. The exposure rate, Ẋ

δ, produced by each source is determined at the distance of 

closest approach.

Instrument Source

Exposure 

rate,

Ẋ

δ (μR/h)

d0

(m)

d

(m)

v

(m/s)

Case 1:

Ratio without scaling

RID 2 133Ba (1) 35 0.25 0.65 0.5 2.01 ± 0.23

RID 2 137Cs (2) 17 0.50 1.20 0.5 1.94 ± 0.38

RID 2 60Co (1) 40 0.50 1.00 0.5 1.80 ± 0.23

RID 2 60Co (1) 40 0.25 0.95 0.5 2.24 ± 0.48

Instrument Source

Exposure 

rate,

Ẋ

δ (μR/h)

d0

(m)

d

(m)

V0

(m/s)

v

(m/s)

Case 2:

Ratio with 

scaling

RID 1* 241Am 55 0.30 0.71 0.21 0.50 0.99 ± 0.11
9

RID 1 133Ba (1) 35 0.25 0.65 0.50 1.3 1.15 ± 0.16

RID 1* 133Ba (2) 61 0.50 0.92 1.20 2.20 0.92 ± 0.17

RID 1 137Cs (1) 17 0.50 1.20 0.50 1.20 0.73 ± 0.16

RID 1 137Cs (2) 17 0.50 1.20 0.21 0.50 0.92 ± 0.19

RID 1†

137Cs (3) 48 0.80 3.00 0.13 0.50 0.82 ± 0.11

RID 1 60Co (1) 40 0.50 1.00 0.50 1.00 0.96 ± 0.21

RID 1 60Co (2) 40 0.50 1.00 0.25 0.50 0.92 ± 0.16

RID 1 60Co (3) 40 0.25 0.95 0.13 0.50 0.71 ± 0.11

RID 1† 60Co (4) 58 0.80 2.66 0.15 0.50 0.94 ± 0.11

RID 2 133Ba (1) 35 0.25 0.65 0.50 1.3 1.11 ± 0.15

RID 2 137Cs (1) 17 0.50 1.20 0.50 1.20 1.38 ± 0.32

RID 2 137Cs (2) 17 0.50 1.20 0.21 0.50 1.13 ± 0.20

RID 2 60Co (1) 40 0.50 1.00 0.50 1.00 1.10 ± 0.19

RID 2 60Co (2) 40 0.50 1.00 0.25 0.50 1.14 ± 0.14

RID 2 60Co (3) 40 0.25 0.95 0.13 0.50 1.15 ± 0.16

RID 3* 241Am 55 0.30 0.71 0.21 0.50 1.40 ± 0.17

RID 3 133Ba (1) 35 0.25 0.65 0.50 1.3 1.08 ± 0.20

RID 3* 133Ba (2) 61 0.50 0.92 1.20 2.20 1.14 ± 0.27

RID 3 137Cs (1) 17 0.50 1.20 0.50 1.20 0.76 ± 0.15

RID 3 137Cs (2) 17 0.50 1.20 0.21 0.50 1.11 ± 0.15

RID 3† 137Cs (3) 48 0.80 3.00 0.13 0.50 1.00 ± 0.13

RID 3 60Co (1) 40 0.50 1.00 0.50 1.00 1.07 ± 0.20

RID 3 60Co (2) 40 0.50 1.00 0.25 0.50 1.10 ± 0.14

RID 3 60Co (3) 40 0.25 0.95 0.13 0.50 1.12 ± 0.13

RID 3† 60Co (4) 58 0.80 2.66 0.15 0.50 1.01 ± 0.13

RID 4* 241Am 55 0.30 0.71 0.21 0.50 1.31 ± 0.14

RID 4 133Ba (1) 35 0.25 0.65 0.50 1.3 1.15 ±0.11

RID 4* 133Ba (2) 61 0.50 0.92 1.20 2.20 1.11 ± 0.12

RID 4 137Cs (1) 17 0.50 1.20 0.50 1.20 1.43 ± 0.17

RID 4 137Cs (2) 17 0.50 1.20 0.21 0.50 1.09 ± 0.12

RID 4† 137Cs (3) 48 0.80 3.00 0.13 0.50 0.97 ± 0.10

RID 4 60Co (1) 40 0.50 1.00 0.50 1.00 1.05 ± 0.11

RID 4 60Co (2) 40 0.50 1.00 0.25 0.50 1.10 ± 0.12

RID 4 60Co (3) 40 0.25 0.95 0.13 0.50 1.23 ± 0.13

RID 4† 60Co (4) 58 0.80 2.66 0.15 0.50 0.99 ± 0.09

RID 5 133Ba (1) 35 0.25 0.65 0.50 1.3 1.02 ± 0.18

RID 5 137Cs (1) 17 0.50 1.20 0.50 1.20 1.41 ± 0.57

RID 5 137Cs (2) 17 0.50 1.20 0.21 0.50 0.94 ± 0.42

RID 5 60Co (1) 40 0.50 1.00 0.50 1.00 1.06 ± 0.33

RID 5 60Co (2) 40 0.50 1.00 0.25 0.50 1.18 ± 0.25

RID 5 60Co (3) 40 0.25 0.95 0.13 0.50 1.13 ± 0.24

* exposure rates measured at both distances

† exposure rate calculated for close distances and measured for farther distances
10

-- (1) (2) (1) (2) (3) (4) (1) (2) (3) 0.5

1.0

1.5

2.0

2.5

241Am 137Cs 60Co 133Ba Exposure rated / Exposure rated0

+20 %

 RID 1 RID 2

 RID 3 RID 4

 RID 5 RID 2 (unscaled)

+30 %

-30 %

-20 %

 

Figure 5: Ratio of exposure rate measurements for scaled and unscaled testing speeds for the different RIDs. Uncertainty 

bars represent combined uncertainties, as described in the text.

For each source, the background was subtracted, and the statistical uncertainty is the standard 

deviation of 10 readings for each instrument. Each reading was normalized to its calculated or 

measured exposure rate value in order to account for small differences in expected exposure rates due 

to the sources that were available. The absolute value of the exposure rate is eliminated when 

calculating the ratio. The maximum difference in exposure rates for source pairs was less than 8 % in all 

cases, with the exception of 137Cs pair number (3) (see Table 2 and Figure 5) in which the difference was 

17 %. Uncertainties were added in quadrature to include a ±5 % uncertainty on each source activity (for 

calculated exposure rates), the exposure rate measurement (if measurements were used), and the 

uncertainty in the positioning of the instruments. The uncertainty for the exposure rate measurements 

vary between 3 % and 9 %, depending on the calibration uncertainty for the different photon energies.

For the SRPM and BRD the uncertainty in the distance, d, was determined to be 1 cm, as these 

instruments were quite large and difficult to accurately position. For PRD and the RIDs the uncertainty 
11

in the distance was estimated to be 0.5 cm, as the positioning of the smaller instruments can be more 

precise. These values were used to combine the uncertainty in quadrature for each source and testing 

distance. 

From Figure 4 and Figure 5 it can be observed that the ratio between the instrument readings at two 

different testing distances can vary between 1.5 and 4.3. These differences were observed for different 

types of instruments, including PRDs, RIDs, BRDs, and SRPMs. When scaling is applied to the speed, 

using Equation 1, the ratio between the instrument readings at two different testing distances varies 

between 0.7 and 1.4 (while maintaining the same radiation field at the distance of closest approach in 

both cases). These differences suggest the need to use a scaling factor in order to ensure that testing 

with different source configurations should achieve similar results. Most of the scaled values show ratios 

that are within ±20 % from the expected value. These variations are expected due to the different 

uncertainty components that are involved in the setup and measurements using these different types of 

instruments. There does not seem to be a correlation between the ratios and testing distances. Further

testing distances do not reduce the value of the ratios for the instrument readings when the speed is 

scaled (e.g., for 133Ba measurements were performed between 25 cm and 92 cm and for 137Cs between 

50 cm and 300 cm). At closer distances, the ratios of the instruments’ readings seem to be larger when 

the speed is not scaled. Smaller radiation fields lead to larger uncertainty values and larger differences in 

the scaled ratios. 

Alarm response verification

The RID 1 instrument was used to verify the alarm response for the scaled and unscaled speeds, as the 

exposure rate measurements showed the same type of response for all RIDs. For these measurements 

the RID dose rate measurements were used to setup the testing fields of 133Ba and 137Cs sources. The 

test configurations used for these measurements are listed in Table 3. For each source, the measured 

dose rates for both testing distances are the same within the measured uncertainty. The dose rate was 

measured by placing (statically) each of the sources in front of the RID 1 at the distance of closest 

approach, d. The source speeds, v, used were unscaled and scaled (using Equation 1). 

The RID 1 alarm threshold was adjusted based on the maximum dose rate readings measured when the 

source was moving in front of the RID; values are shown in Table 3. The alarm threshold was adjusted 

for these measurements because the ANSI standards’ testing is performed at radiation fields close to the 

value of the instruments’ threshold. For each test configuration, a total of 50 trials were performed to 

reduce the uncertainty of the measurements in determining of the probability of alarm, with the 

associated 95 % lower and upper confidence interval. Results are shown in Table 4. From this table, for 

the 133Ba source, it can be observed if the dose rate and speed were kept the same and the testing 

distance was changed (2 different source activities were used), the probability of alarm was reduced 

from 100 % at the faraway distance to 10 % at the closest distance. If the source speed was scaled using 

Equation 1 for the closer distance, then the probability of alarm increased back to 100 % as that 

obtained for the faraway distance. Similar behavior was observed for the 137Cs source. In this case, the 
12

alarm probability was reduced from 88 % to 50 % when the speed was not scaled. When the speed was 

scaled, the alarm probability was equal to 82 %, which is statistically indistinguishable from the original 

88 % based on the measured 95 % lower and upper confidence bounds.

Table 3: List of test configurations used for the RIDs alarm verification measurements

Instrument Source Dose rate

(mrem/h)

d

(m)

v

(m/s)

Alarm 

threshold

(mrem/h)

Dose rate moving 

source 

(mrem/h)

RID 1 133Ba 0.0795 ± 0.0008 0.65 0.5 0.65 0.073 ± 0.004

RID 1 133Ba 0.0798 ± 0.0007 0.288 0.5 0.65 0.057 ± 0.009

RID 1* 133Ba 0.0798 ± 0.0007 0.288 0.22 0.65 0.071 ± 0.004

RID 1 137Cs 0.0263 ± 0.0007 1.20 0.5 0.20 0.022 ± 0.002

RID 1 137Cs 0.0260 ± 0.0006 0.561 0.5 0.20 0.020 ± 0.003

RID 1* 137Cs 0.0260 ± 0.0006 0.561 0.23 0.20 0.021 ± 0.002

* Scaled speed

Table 4: List of test configurations used for the RIDs alarm verification measurements and test results

Instrument Source d

(m)

v

(m/s)

Alarm 

threshold

(mrem/h)

Probability 

of alarm

(%)

95 % Confidence 

Interval

Lower Upper

RID 1 133Ba 0.65 0.5 0.65 100.00 92.87 100.00

RID 1 133Ba 0.288 0.5 0.65 10.00 4.35 21.36

RID 1* 133Ba 0.288 0.22 0.65 100.00 92.87 100.00

RID 1 137Cs 1.20 0.5 0.20 88.00 76.20 94.38

RID 1 137Cs 0.561 0.5 0.20 50.00 36.64 63.36

RID 1* 137Cs 0.561 0.23 0.20 82.00 69.20 90.23

* Scaled speed

Conclusions

The measurements performed with the different types of instruments, in particular with the RIDs, show 

that there is a significant increase in the instrument response (factors ranging from 1.5 to 4) when 

sources are placed at farther distances while maintaining the same testing speed and the same exposure 

rate at the distance of closest approach between the source and the detector. When scaling is applied to 

the speed, most of these differences are reduced to values of approximately ±20 %. From the alarm 

response measurements, it is clear that the probability of alarm is reduced if different testing distances 

are used while maintaining the same radiation field and source speed. When the speed is scaled, the 

probability of alarm at the closer distance matches that of the faraway distance. Per these results, it is 

recommended to define the requirements in the standards at a fixed distance, speed, and exposure rate 
13

at the distance of closest approach, reducing the uncertainty in the measurements and having better 

reproducibility of the radiation fields between different testing laboratories. The standard can also allow 

the testing laboratories to use a different testing distance if they do not possess the required source as 

specified by the standard’s requirement and if they have the ability to scale the speed in order to 

account for the differences in the testing geometry. If the speed is scaled, there seems to be no need to 

specify a range for the testing distances, as the ratio of the exposure rates seems to be independent of 

the testing distances for distances between 25 cm and 300 cm. 

Acknowledgments

The authors would like to thank the Department of Homeland Security (DHS) Domestic Nuclear 

Detection Office (DNDO) for funding this work. 

References 

[1] ANSI/IEEE N42.34-2006, “American National Standard Performance Criteria for Hand-Held 

Instruments for the Detection and Identification of Radionuclides.”

[2] IEC-62327-2006, “Radiation protection instrumentation - Hand-held instruments for the detection 

and identification of radionuclides and for the indication of ambient dose equivalent rate from photon 

radiation.”

[3] Seltzer S. Air-kerma rate coefficients for selected photon-emitting radionuclide sources. 

Gaithersburg, MD: National Institute of Standards and Technology; Report No. NISTIR 7092A; 2004.

[4] Johns HE, Cunningham JR. The physics of radiology. Springfield, IL: Charles C. Thomas Publisher; 

1983.