Vietnam Conference on Nuclear Science and Technology
VINANST-13
SECTION C: RADIATION MEASUREMENT, RADIATION SAFETY
AND ENVIRONMENTAL MONITORINGESTABLISHMENT OF AN EXPERIMENTAL SYSTEM FOR
RADIATION MANAGEMENT IN TEACHING OF SCIENCE
USING CROOKES TUBESDo Duy KHIEM1),2), Kenji YAMAMOTO1), Masaya YASHIKI1),
Hiroto MATSUURA1), Masafumi AKIYOSHI1)
1)Osaka Prefecture University2)Center for Nuclear Techniques in Ho Chi Minh City – VINATOM

Email: doduy_khiem@yahoo.com

August 7 – 9, 2019, Ha Long City, Quang Ninh, Vietnam

1

WHAT IS A CROOKES TUBE?
New Crookes tubes

 1875, William Crookes invented the Crookes tube.
 1895, Wilhelm Conrad Röntgen discovered X-rays.
 1901, W. C. Röntgen got the first Nobel Prize in Physics.

Conventional Crookes tube

William Crookes

Induction coil

X-ray of Kölliker's
hand, made by
Röntgen (1896)

Wilhelm Conrad
RöntgenMechanism of a Crookes tube
1
2

Tube vacuum:
0.005 - 0.1 Pa

3X-rays1

Cations in the evacuated tube areaccelerated and impact the cathode,which knock out secondary electrons.

2

These electrons are accelerated under theapplied HV.

3

Accelerated electrons collide the glass
wall to radiate bremsstrahlung X-rays.

HV
Several tens of kV

2

EXPOSURE TO X-RAY RADIATION FROM A CROOKES TUBE
Crookes tube has been used inteaching of science at juniorhigh schools in Japan.

Energy was approximately 20 keV.
Hp(10) was up to 143 mSv/h at 5 cm.

X-rays The primary purpose is to teach the characteristics of electrons and current.
 To date, the leakage dose, properties of X-rays, and their relevance with operating
settings have not been investigated systematically and sufficiently yet in Japan.
 From 2017, the project of “Establishment of radiation safety management system for low
energy X-rays irradiated from Crookes tubes at education sites” has been started inJapan. Its scopes aim to establish a guideline on radiation safety management at theeducational sites.
3

PROBLEMS ON ESTIMATE OF A CROOKES TUBE
 Low energy X-rays (approximately 20 keV): hardly measure by conventional meters (evenused HPGe, NaI, Ge detectors).
 The pulse-shaped voltage creates the heterogeneous radiation: produces pile-up effect,broadened energy spectrum.
 Instability of induction coil and applied voltage: affected by temperature, humidity.
 Difficult in effective dose estimate:o dose distribution in horizontal plane is inhomogenous,
o cannot assume an aligned and expanded radiation field,o cannot use Hp(10) as an approximation of effective dose.

SCOPES OF THE RESEARCH
 Investigating the characteristics and properties of X-rays:o X-ray energy spectrum
o Applied voltage distribution.
 Optimizing operating conditions to reduce the dose:o Applied voltage (output power)
o Currento As low as reasonably achievable (ALARA principle): time, distance, and shielding Relevance between electric settings and leakage dose.
 Stimulating X-ray energy spectrum using cloud chamber.
 Submitting the results as the recommendation and guideline for radiation protection atjunior-high school science class.
4

CHARACTERISTICS AND PROPERTIES OF X-RAYS
EMITTED BY A CROOKES TUBE
 X-ray energy spectrum
 Applied voltage distribution

5

X-RAY SPECTRUM ACQUISITION AND VOLTAGE MESUREMENT
X-ray Spectrometer

Crookes tube (3CB, Kenis Ltd., Japan)

CZT Detector (XR100T-CZT, Amptek
Inc., USA)

Cylindrical Pb collimators of 3 mm
pinhole, and thin plate Pb collimatorsof 0.5 mm pinhole were used.

Power supply andAmplifier (PX2T,
Amptek Inc., USA)

Pocket Multichannel(MCA8000D, Amptek
Inc., USA)

X-ray spectrum setting
0.5 mmCollimator

3 mm

Fixed distance(90 cm)PC USB Oscilloscope (6000BD,
Hantek Ltd.)DPPMCA Software (Amptek Inc., USA)

HV distribution

CZT detector

Voltage divider circuit: Reducing the magnitude of voltage
during measurement against damage tooscilloscope voltage probes due to highpulse.
 The circuit consists of two resistors inseries, one of 500 MΩ glass resistor,
and another of 100 kΩ.
6HIGH VOLTAGE APPLIED BY THE INDUCTION COIL
(1) Electrode distance

50

1mm  1kV(2) Output power

Induction coil

Impressed voltage (kV)

Impressed voltage

40

35 kV

30
20
10
0

-10
-20
0

20

40

60

80

100

120

140

160

Time (ms)The induction coil produces an applied voltage in pulsed-shape.
 The desired output voltage can be obtained by regulating (1) electrode distance or (2) output
power on the induction coil.
(1) The electrode distance ranges from 10 to 100 mm. Each of the distances limits a maximum
voltage to the Crookes tube.
(2) The output power controller (PW) has twenty-one scales (from 0 to 20). By increasing theoutput power, the applied voltage will continually increase until a spark occurs to reach the
maximum voltage.
 The nominal dielectric breakdown voltage in the air is approximately 1 kV at 1 mm. For instance,
the electrode distance of 35 mm produces a maximum voltage approximately of 35 kV.
7EFFECT OF CHANGES IN APPLIED VOLTAGE
6PW0 (16 kV)
PW2 (17 kV)
PW4 (18 kV)
Gauss Fitting of PW0
Gauss Fitting of PW2
Gauss Fitting of PW4

10

12

14

16

18

20

22

24

26

28

30

400

PW0 (15.8 keV)
PW2 (17.0 keV)
PW4 (18.1 keV)
Gauss Fitting

35
30

Intensity

500

Frequency

8

40

600

300

Shifted energy

25
20

Shifted energy

15

200

10
100

5
0

0
0

5

10

15

20

25

30

35

40

High voltage (kV)

6

8

10

12

14

16

18

20

22

24

26

28

30

Energy (keV) Electrode distance was fixed at 40 mm, and output power controlled from 0 to 20.
 The applied voltage in pulsed-shape created pulsed radiation.
 X-ray energy spectrum had a broadened distribution due to heterogeneous radiation andBremsstrahlung photons.
 X-ray energy was shifted to higher region in the spectrum with increasing HV.

 The results indicated a good correlation in the distribution behavior between the X-ray
energy spectrum and the applied voltage.
8

CORRELATION BETWEEN APPLIED VOLTAGE AND X-RAY ENERGY
45

45

40

40
X-ray Energy
Maximum Voltage
Peak Voltage

30

35
30

Voltage (kV)

X-ray energy (keV)35

 X-ray energy and applied voltage kept
stability when a spark occurred.
o Effective energy was approximately 20 keV.
o The maximum operating voltage was

25

25

20

20

15

15

10

10

 There was a good agreement between the55peak energy of X-rays and the peak voltage.

0
0

2

4

6

8

10

12

14

16

18

20

roughly 40 kV that matched the nominal
voltage at electrode distance of 40 mm.

022

Output PowerThe relevant graph of the applied
voltage and the spectral distribution
corresponding to the output voltage
distribution from PW0 to PW20.

9OPTIMIZATION OF OPERATING CONDITIONSTO REDUCE THE DOSE Distance Shielding ALARA principle
 Time Current Applied voltage

10EFFECT OF DISTANCE ON DOSE
X Axis Title

4

Experimental settings:a)Discharge electrode distance 30mm PW4Discharge electrode distance 20mm PW2Discharge electrode distance 15mm PW0

 Electrode distance at 15 mm, 20 mm, and 30 mm.

 Glass badge dosimeters (type-FX, Chiyoda
Technol) were put at a distance from 15 to 50 cm.

3
1
2
Intensity=Distance The Hp(0.07) equivalent dose measured in 10 min,then it was converted to dose rate per hour.

Hp(0.07) Dose (mSv/h))

2

1

0

Inverse Square Distance (m-2)
30 mm

4

 The output power was set at which spark
just occurred.
 Dose decreased with distance as theinverse square law. At a distance of 1 m,
dose was declined to 1/100 from that of
10 cm.
 Dose decreased significantly with thevoltage limitation (electrode gap). It
should be fixed the gap shorter than 20
mm during a demonstration.

b)

3

20 mm

2

1

15 mm
0

0

5

10

15

20

25

30

35

40

45

50

55

Distance (cm)11

EFFECT OF SHIELDING ON DOSE
Transmission measurement
 For teaching science, shielding
materials should ensure radiation
safetyanddemonstration
observation for students.
 Transparent shielding materials:
Acrylic (9.3 mm)
Lead acrylic (8 mm)
Aluminum (1 mm)

o Lead acrylic: 0.3 mm leadequivalence, Kyowaglas-XA H-8,
KURARAY Co., LTD.

o Acrylic: 9.9 mm, Amersham
Pharmicia Biotech LTD.
o Pure aluminum: 99.5%, A1050
The Lambert-Beer law:
𝐼=𝐼0 𝑒 -µxµ: Linear attenuation coefficient
I0: Initial intensity
I: Transmitted intensity
x: Thickness of an attenuator The transmission of X-rays wasestimated by the ambient dose
equivalent H*(0.07) using an
ionization chamber (ICS-1323,
Hitachi Ltd., Japan).

12

EFFECT OF SHIELDING ON DOSE
Experimental settings:

 For each electrode distance, the output
power was set at the position that the
spark just occurred.
 H*(0.07) dose measured for 3 min at a
distance of 50 cm.

Transmission (%)

 Electrode distance was fixed at 20 mm, 25
mm, 30 mm, 40 mm, and 50 mm.

60

Acrylic
Aluminum
Lead acrylic

a)

50
40
30
20
10

1.65 %

The percentage of transmission was
calculated as a ratio of initial dose totransmitted dose.

The transmitted dose of acrylic is higher
than aluminum. It caused a higher
transmission for acrylic.
Lead acrylic predominated in radiation
absorption with the transmission of lessthan 2%.

H*(0.07) dose rate (mSv/h)

0
1.4
1.2

Initial doseTransmitted dose by Acrylic
Transmitted dose by AluminumTransmitted dose by Lead acrylic

b)

1.0
0.8
0.6
0.4
0.2

0.02 mSv/h
0.0
15

20

25

30

35

40

Discharge distance (mm)

45

50
13

EFFECT OF OUTPUT POWER ON DOSE
Initial Dose
Transmitted dose by Acrylic
Transmitted dose by Lead acrylic

Experimental settings:

 Electrode distance at 30 mm.
 Changing output power.
 H*(0.07) dose measured for 3
min at a distance of 50 cm.
 The
outputpower
increased energy and HV.

H*(0.07) dose rate (mSv/h)

1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0

 The dose was proportional
to the output power which
control applied voltage.
 Therefore, output power
must be kept as small as
possible.
 Dose and energy kept
stability when the spark
occurred on the induction
coil.

0

2

4

6

8

10

12

Output Power

sparking
OutputPower

Peak energy of
X-ray (keV)

Initial dose
(µSv)

0
2
4
6
8
10
12

16.4
17.5
18.1
18.0
18.6
18.6
18.4

3.31
10.1
30.4
43.2
70.6
69.2
68.7

Transmitted dose (µSv)
Lead acrylic
Acrylic
0.04
0.11
0.34
0.45
1.09
0.89
0.92

1.10
3.65
13.8
19.0
36.5
34.9
37.3
14

EFFECT OF CURRENT ON DOSE
0.45
Discharge electrode distance 20mm
Discharge electrode distance 25mm
Discharge electrode distance 30mm

3.5

0.35

3.0
2.5
2.0
1.5

0.25
0.20
0.15
0.10

0.5

0.05

0.0

0.00
2.5

40

50

60

70

80

90

100

Average Current (mA) Increasing current caused increasing dose
exponentially.
 At the electrode distance of 20 mm, thespark becomes intense at PW4 then it
keeps stability.
 Whether the transmissivity of glass(forming the Crookes tube itself) changed
the dose exponentially.

Conductance

0.30

1.0

30

Crookes tube VT-7030
Crookes tube VT-7010

0.40

Current (mA)

H*(0.07) Dose at 30 cm (mSv/h)

4.0

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Applied voltage (kV) Some new Crookes tubes releasing alarge current at a low voltage expose
very low leakage dose or complete
elimination.
 Current releases from 0.1 to 0.45 mA,
and the voltage applied from 3.1 to
6.5 kV.

15

ESTIMATE OF X-RAY ENERGY USING
TRANSMISSION MEASUREMENT Lead acrylic attenuators

16

Effective energy of X-rays (keV)

TRANSMISSION IN ENERGY ESTIMATION
25

15

 The acrylic shielding caused hardeningof X-rays owing to absorbing photons withlower energy. As a result, it produced adifference in the energy of X-rays, 7.4%.

10

 The calculation of transmitted dose
was based on Lambert-Beer equation.

a)Shielded X-rays
Unshielded X-rays

20

 The linear attenuation coefficient µ ofacrylic for each energy was interpolatedusing data from the National Institute ofStandards and Technology (NIST, USA).

5

H*(0.07) dose rate (mSv/h)

00.8

Transmitted dose by Acrylic
ExperimentCalculation (shielded X-rays)
Calculation (unshielded X-rays)b)

 For shielded X-rays, calculation
matched well experiment with a differenceof 5%.

0.6

 For unshielded X-rays, calculation was
smaller than the experiment roughly 6.6%.

0.4

 Transmission might be considered as arelatively precise approach to measure Xray energy substituting for othercomplicated methods.

0.2

0.0
0

2

4

6

8

Output Power

10

12

14
17

SIMULATION OF X-RAY ENERGY SPECTRUM USING
A CLOUD CHAMBER Radiation tracks observation
 Simulation of X-ray energy spectrum

18WHAT IS A CLOUD CHAMBER?
 A
high-performance“fog box” used a
Peltier-cooler to drop
the temperature.

 To support students tovisualize
radiationinteraction
inradiological education.

Ethanol vapor

Supersaturated vapor-20℃

Cloud chamberHigh vapor pressure athigh temperature

A saturation vapor pressure
decreases at low temperature

Liquid ethanol drops At room temperature, vapor pressure of ethanol isrelatively high.

 Ethanol vapor gets cold at bottom of the chamber, andthen a saturated vapor pressure decreases.
 The vapor comes to supersaturated condition.
Cooled by a Peltier device
to less than -20℃

 At less than -20℃, small stimulation makes thesupersaturated vapor to tiny liquid drops.
19

MECHANISM OF A CLOUD CHAMBER
 Charged particle travels
through the air and repels
many electrons from atoms
to form positive and negative
ion pairs (ionization).
 These ions are formed insupersaturated
ethanol
vapor as a core of smallliquid particles.

e-

e+

+

+
e-

Ionization

e-

+

+

e-

e+

Charged particle

 Many of the particles of thisliquid are formed after theradiation passed, so theradiation trace is observed asa white line.

 The cloud chamber can observe not only
alpha particles but also beta particles, and
X-rays irradiated from a Crookes tube.
 Basing on the correlation between X-ray
track length and X-ray energy, the cloud
chamber was applied to simulate the X-ray
energy spectrum from the Crookes tube.
20

X-RAY ENERGY SPECTRUM SIMULATION BY A CLOUD CHAMBER
25

X-ray energy spectrum of 19.5 keV
Gauss fitting

Intensity

20

A camera attached outside
the cloud chamber.

15

10

1

5

0
15

10

5

35

30

25

20

X-ray energy (keV)The
correlation
between the X-ray
spectrumandsimulating spectrum.

A low energy electron of 20
keV travels a mean freepath approximately 8.1 mm
in the air (density of air at200C) (ICRU 37 ).

4

2

Image analysis of
the length ofradiation track.

40
Length of radiation tracks
Gauss fitting

35

Frequency

30

3

25
20
15
10
5
0

~1

1~2

2~3

3~4

4~5

5~6

6~7

Length of track (mm)

7~8

8~9

9~10

Counting lengthofradiation
tracks to build
the spectrum.

The mean free path of X-ray energy of 20 keV
is approximately 3.6 mm in cloud chamber.
21

SUMMARY

Properties and characteristics of X-rays emitted by Crookes tube:o X-rays emitted with soft energy of approximately 20 keV.
o The exposure and X-ray energy changed with electrical settings such as output power,
and electrode distance.



Optimizing in operations:o Set output power as low as possible.
o Never remove discharge electrodes.
o Set the distance shorter than 20 mm.

o Take distance as far as possible. For student, more than 1 m is recommended.
o Keep the demonstration time shorter than 10 min.


Cloud chamber is practical in radiation tracks observation for student.



X-ray energy spectrum might be stimulated by a cloud chamber.



According to ICRP 36, the dose limit recommended 0.05 mSv per each teaching
exercise. The lead acrylic shield was well-suited to radiation safety for the Crookes tube.

2223