2019/05/30
3rd International Conference on Dosimetry
and its Applications (ICDA-3) at Lisbon, Portugal
Radiation Shielding and Dosimetry at Accelerators II, O2.4

Measurements of low energy X-rays radiated from
Crookes tubes in education field by using
radiophotoluminescence dosimeter
Osaka Prefecture University,
Radiation Research Center,
Masafumi Akiyoshi, Do Duy Khiem,
Ryoichi Taniguchi, Hiroto Matsuura.
Special thanks: Crookes Project Member in Japanakiyoshi@riast.osakafu-u.ac.jp
http://bigbird.riast.osakafu-u.ac.jp/~akiyoshi/Works/index.htm

What is Crookes tube?
Wilhelm Konrad Rontgen
1895, Found the X-ray during the experiment of discharge tube
1901, Got the first Nobel prize in physics

William Crookes

①

－ －

②

－

－

＋

Cathode③－－

Accelerationof electron
Vacuum of
0.005 - 0.1 Pa① At the cold cathode, cation in
air is accelerated and knockout secondary electrons.

＋

electrons are accele② These
rated as the applied HV.

Anodeelectrons hit③ Accelerated
glass wall and radiate

10-20kV HV from an induction coilbremsstrahlung X-rays.

How to Establish Safty Management
for Crookes tube?
Crookes tube has been used in junior-high science classes in Japan, and the primary purpose is toteach the characteristics of electrons and current, not for radiological education. Therefore, someteachers are not recognizing the radiation of X-ray from Crookes tube, and most of them have no
information of the dose. However, it is possible to expose high dose of X-ray to students using a
Crookes tube, where Hp(0.07) reaches 200mSv/h at a distance of 15cm.
Some discharge tube that use hot cathode isoperated with only several 100V, and even withcold cathode, some equipment can be operated
at about 5kV. With this low voltage, radiated
X-ray is shielded completely by the glass wall.

Basic Plan
By using low voltage type equipment, teachers
never required to consider the radiation andstudents can observe electron beam very safely.
The problem is resolved completely!

Advanced Plan
A Crookes tube operated by 5kV(Horizon Co. VT-7010)

5kV CW high voltage unit
driven by 9V battery

Junior-high school in Japan
have quite limited budget!

1) Cannot replace legacy devices with economicalreason2) Advenced education program that utilize X-rays
radiated from Crookes tube
Anyway, radiation safty guide line to limit X-rays
dose is required.

ICRP Basic Radiation Safety Criteria
Justification:
No practice shall be adopted unless its introduction produces a net positive benefit.
・Is the experiment required? Is it accepted to show with photo or video?
→The experiment using Crookes tube gives quite strong impact to students and have excellent
educational effects.

Optimization:
All exposure shall be kept as low as reasonably achievable (ALARA concept), economic
and social factors being taken into account.
・Low voltage operated equipments can make the exposure to zero and it is firstly recommended.
However, economic factor make it difficult to replace all equipments in all schools. Therefore,
optimization on operation conditions (applied voltage, current, distance, time and shielding) is required.

Dose limitation:
The dose equivalent to individuals shall not exceed the limits recommended for the appropriate
circumstances by the Commition.
・In Japan, dose limit for general public proposed by ICRP Pub1990/2007 is not taken into the domestic
low. Now we survey international regulations and the actual control conditions to settle the reasonable
management target dose.

Problems on management of X-rays from Crookes tube
Dose Constraint for Education Field is not Discussed well
Dose limit for general public have been proposed by ICRP Pub1990/2007 as 1mSv/y, but it
is for all additional exposure. ICRP Pub 36 is the only instance.

Definition of X-ray Device is not Clear
In Japanese domestic low, `X-ray device' is not defined strictly. There is no exemption levelthat gives a confusion to safety management of X-rays in many fields.

Difficult Estimation of Effective DoseFor 20keV X-rays, the half value layer is about 1cm in human body. The absorbancedose to tissues in body depends on its depth from the surface. The transmittance ischanged with energy of X-ray drastically around this energy range and the X-rays
have broad spectrum.
Further more, dose distribution in horizontal plane is not uniform. Therefore wecannot assume aligned and expanded radiation field, and we cannot use 1cm dose
equivalent as approximation of effective dose.

Problems on Estimation of X-rays from Crookes Tube

Low Energy X-Rays ( arround 20keV )Not only conventional survey meters for public use, but also reliable NaI survey meters forgeneral radiological management are useless for low energy X-rays from Crookes tube.
(below 50keV pulse is ignored by conventioal NaI surver meters )Radiated in Sharp Pulse
Even using detector with Be window for low energy X-rays measurement, radiation in sharp pulse
give pile up and dose is estimated as very small value. Only ionization chamber that measure
averaged current or solid detector that integrate the absorbed dose is useful.

Instability of Induction Coil and Applied Voltage
Induction coil generate high voltage pulse mechanically, that is affected by conditions such astemperature, and firing voltage between discharge electrodes is also affected by temperature or
humidity. For systematic measurement, some physical parameter is required to compare results.

Dose Measurement by Various Survey Meters
Crookes tube: Kenis 3C-B with Maltese cross plate, Induction coil: Kenis ID-6
Distance of discharge electrodes: 25 mm、Output power 6、Averaged current 40 μA
Measured in front direction of the tube without Maltese cross plate.

Ionization chamber
Hitachi ICS-1323
distance
cm
15
30
50

Hp(0.07)
mSv/ h7 .3
1 .7
0 .5 8

Hp(10)
mSv/ h
3.0
0.73
0.27

NaI scintillator
Fuji Electric NHC6

distance
cm
15
30
50

Be window
μSv/h
1.3
10
13

Aloca TCS-172
general purpose
μSv/h
0.17
0.17
0.15

radiophotoluminescence dosimeter

GM tubeS.E. International
Chiyoda Tech. Glass Badge type FX
RangerHp(0.07)
Hp(10)
1min scaler
mSv/ hmSv/ h
kc pm
1.6
34
4 .6
0.48
32
1 .3
0
27
0 .4 8
Plastic scintillator
JSF Kind-miniwithout cover
μSv/h
118
64
24

CsI
scintillator

Si semiconductor

Aircounter EX

Aircounter S

without cover
μSv/h
13
13
8

μSv/h
<9.99
0.05 blink<9.99

High Voltage Applied by a Induction CoilDistance ofDischarge Electrode

DDE

PW

Output power dial changing the voltage
impressed to the primary side of atransformer, and that control output
voltage continuously.
Since the dielectric breakdown voltage
in the air is about 1 mm for 1 kV, thedistance of discharge electrodes canlimit the maximum voltage. If the
distance is settled to 20mm, themaximum voltage impressed to a
Crookes tube was limitted to 20keV,
therefore it work as a safety device.

Impressed Voltage / kV

30

Output power

20
10
0

-10
10

11

12
Time / ms

13

Distance of discharge electrodes 20mm,
Output power 4, Averaged current 80μA

14

To measure high voltage pulse, voltage
divider that use a large registor( < 100MΩ) and a small resister (about
100kΩ) is used to protect a oscilloscope.
In the most case, an induction coil is notconnected to the ground. In such case,the electric potential is floating and must
be used 2 divider and 2 probe to takedifference between anode and cathode.

Dose Control
Distance of discharge electrodes: 30, 20, 15mm
Output power was set to the just firing voltage.
4.50

Reduced

DDE 30mm PW 4
DDE 20mm PW 2
DDE 15mm PW 0

3.50
3.00
2.50
2.00

Reduced70μm dose equivalent /mSv/h

4.00

1.50
1.00
0.50
0.00
0

10

20

30

40

50

60

Distance from surface of Crookes tube/ cm

・Dose is reduced drastically with thevoltage limitation
Distance of discharge electrodes must be shorter than 20mm

・Dose is changed with distance as theinverse square law
At a distance of 1m, dose is reduced to 1/100 from that of 10cm.

70μm dose equivalent＠30cm / mSv/h

Averaged current was changed with output power.
The averaged current was measured by simple analog current meter.

4
3

DDE 20mm
DDE 25mm
DDE 30mm

21
00Maximum doseat DDE 20mm

20

40

60

80

100

Averaged current / μA

・Increasing current rising dose
exponentially
The output power increase current and also voltage. The
voltage changes the energy of X-rays that changes
transmittance drastically. Therefore, output power must be kept
as small as possible. Furthermore, discharge electrode act as asafety valve to limit the voltage.

Spectrum Measurement by a CZT Detector
Amptek XR-100T-CZTCZT(Cd0.9Zn0.1Te)
detector with Be window,
cooled by Peltier device

200

Intensity

by CZT Detector
M easurement Distance 80cm
DDE PW Peak Energy
15mm 2 14.0keV
20mm 4 16.4keV
25mm 6 17.3keV
30mm 8 18.3keV

100
at DDE 15mm: φ3 lead collimater only
the other: + φ0.5 lead thin plate collimater
Measured time: 10min

0
0

10

20
30
40
X-ray Energy / keV

50

φ0.5mm lead collimater can reduce count rate to several cps
that avoid pile up in the spectrum

Collimaters
φ0.5 lead thin plate
φ1.0 lead thin plate
φ2 co-axial brass
φ3 co-axial lead

Drastic Dose Change with Slight Voltage Change
The radiated X-rays are shielded by the grass tube that forms
Crookes tube it self. Slight energy change around 20keV
gives drastic change in transmittance.
1.0
1.E-04

TransmittanceTransmittance

0.9
0.8
0.7
0.6

1.E-101.E-16
1.E-22
1.E-280.5

1.E-34

0.4
0

200

400

600

800

X-ray Energy / keV

Above 100keV, transmittance changes slightly

0

10

20

30

40

X-ray Energy / keVTransmittance at 30keV is higher 100 times
than that of 15keV

Transmittance of 2mm glass for X/γ-rays

50

Efficiency of Shielding
70μm dose equivalent / mSv （10min）

0.7

In calculation, 20keV X-rays are shielded to
1/2 with 1cm acrylic and 1/50 with 5mm glass.

DDE 20mm (Peak energy 17keV)

0.6
Original
Acrylic 3mm
Acrylic 10mm

0.5
0.4
0.3
0.2
0.1
0
0

20

40

60

Distance from Crookes tube / cm

The cost of aqualium is only 12.3US

80

Actually, energy of X-rays from Crooks tube
is a little low, therefore acrylic showedshielding of 1/2 with 3mm and 1/3 with 1cm.
A very light (1.5kg) aqualium with 1.9mm glass
showed 1/20 - 1/50 shielding. It was too lightand feel like plastic, so it is not incredible toshield with glass aqualium at the real education
field.
Hp(0.07) (μSv/h) Transmittance(%)
PW Original Shielded
0
600
11
1.8
1
620
12
1.9
2
1300
60
4.6
3
3000
160
5.3
Measured distance from Crookes tube 15cm, DDE 20mm
Shielded with 1.9mm thick glass aqualium
https://www.amazon.co.jp/gp/product/B00W5DSU0C

Measurement at Real Education Field
Measurements of leaked X-rays from37 Crookes tubes at junior-highschool in Japan were performed
using radiophotoluminescence
dosimeters by science teachers.

Setting of induction coil were same as their usually.

In the 37 Crookes tubes, Hp(0.07) with 10min exposure was;
25 tubes < 50μSv @ 1m (extrapolated)
18 tubes < 50μSv @ 15cm (detection limit)

The dosimeter were sticked on 2L PET
bottle and put from distances of 15, 30,
50cm and irradiated during 10min for each.
The radiophotoluminescence dosimeter was
Glass Badge type FX (Chiyoda Technol)that can estimate effective energy and canestimate back ground radiation with Sn
shielded element.

One tube radiate 600μSv in 10min at 1m with minimum output power.
The tube showed intermittent beam and lookes current was small.

Minimum power、at a distance of 30cm

DDE 30mm: 2mSv/h
DDE 50mm: 30mSv/h

DDE 20mm: 40μSv/h
at 1m、10min radiation gives only 0.6μSv exposure

Charactor of High Dose Crookes Tube
Inside of the Crookes tube, vacuum is not so high andsome gas molecules are enclosed. These molecules are
ionized by natural radiation and anions are accelerated tocathode to knock out secondary electrons (cold cathode).
There is some Crookes tube that lack of gas. In oldequipment, gas molecules are absorbed to glass wall
gently or some equipment lack gas on the manufacturing.
With such Crookes tube, electrons are difficult to flow
and more high voltage is required to emit beam. With
induction coil, electromagnetic energy is charged to thecoil until enough voltage is obtained that applies higher
voltage than aimed with the output power . That increasetransmittance of X-ray to glass wall.

At the age of Roentgen,necessity of gas-pressure
control is well known andsome "softener" devices
were incorporated that
released a small amount ofgas.

How To Reduce the Dose from Crookes tube
First of all

Replace to low voltage equipment
10,000 Junior-High School × 400USD / ea. = 4Million USD is required in Japan
But Crookes tube is used at just a 1 lecture in 3 year.

Inherent Safety
No concern is required

With Economic Factor, Safety Guideline for Conventional Crookes tube is Required
1) Applied Voltage
2) Beam Current
3) Diatance4) Shielding5) TimeReduce Radiated
X-raysRadiation Protection
Three Principles

Reduce applied voltage gives low transmission of
X-rays to the glass wall drastically.
When we shielding, acrylic is not effective but
thin glass is enough effective.
To take distane is most easy way to reduce dose.

・ Set output power as low as possible

Provisional Guideline
with our study
Is it enough?
Substantiate study isrequired.

・ Never remove discharge electrodes and set the
distance smaller than 20mm
・ Take distance as far as possible. For student, morethan 1m is recommended.
・ Keep the display time shorter than 10min.

To Estimate Effective DoseFor Eye Lens, Equivalent
Dose is also must be
estimated.

EVEN

At the center of front,
Hp(0.07)=200mSv/h at 15cm

Dosimeter ① At a distance of 1m;

Reduced to 1/44

③ In Holizontally, Peripheral Dose issmaller than that measured at the cente（50cm shift gives 2/3 reduction）

It is assumed that in 10min
demo, Effective Dose isresulted within 50μSv.

② 20keV X-rays are reduced to
one half every 1cm in body.
Hp(10) does not represent
Effective Dose.

Calculation with PHITS code using:・Hp(0.07) or Air Kerma
・Energy spectrum
・Horizontal Distribution

ICRP Pub116 Fig.5.2

E(AP) ≒ 1/5 Hp(10)≒ 1/10 Hp(0.07)
at 20keVDifference Between
Protection and Operational Quantity

ICRP Pub116
Fig. 5.2
Ratios of photon effective dose (present report) to ambient dose equivalent.
Students observe a Crookes tube, then orientation is fix to AP.
At 20keV, H*(10) gives 5 times overestimation than Effective Dose.

Horizontal Distribution
At a distance of 1m, holizontal
distribution is measured by
Glass Badge at once.
At 60cm shift from the center,the dose was gently reduced
to 70%.

x cm

100cm
Fitting curve showed
a little higher value thanthe experimental for
Hp(0.07).
It may represent the angle
to the glass of tube.

0.2

0.1

0

Hp(0.07)
Hp(10)

-40
-20
0
20
40
X Dsitance from the Center / cm

60

Dose Equivalent (1m, 30min) / mSv

Distance from the center of
the Crookes tube

Dose Equivalent (1m, 30min) / mSv

Fitting curve was obtaind asa/(1002 + (x+b)2)
0.2

0.1

0

Hp(0.07)
Hp(10)

-40
-20
0
20
40
Y Dsitance from the Center / cm

60

Conclusion
・Character of X-rays from Crookes tube was investigated.
・Conventional survey meters shows quite low dose that gives misunderstandfor protection.
・Solid state dosimeters like radiophotoluminescence badge is good solution
to investigate the dose distribution in education field.
・Some equipments which hold few gas inside radiate quite strong X-rayseven with the minimum power setting. Then, Discharge Electrode works assafety device. The distance must be kept smaller than 20mm.
・For low energy X-rays, Hp(10) does not represent effective dose. Protection
quantity is now investigated with experimental values and PHITS code.
・Radiation protection for general public people is not adopted to Japanese
domestic law. Furthermore, dose constraint for education field is notdiscussed well. Protection rule and regulation situation in the world is
required to establish the Comon Guideline for Radiation Safty in
Education field.

NaI scintillation survey meters for low energy X-rays

Fuji Electric NHC6
φ12.7×12.7mm NaI scintillator
Range X-ray 8～300keV（～60μSv/h）,
γ-ray 50～1500keV（～600μSv/h）

Rigaku Get Smart XU
NaI scintillator
Range 5～300keV（～10μSv/h）

Not only general purpose NaI scintillator covered by SS housing, but also special
equipment for low energy X-ray cannot measuree X-rays from Crookes tube.

Arrised from the pulse shape radiation