U.S. patent application number 12/259111 was filed with the patent office on 2010-01-07 for stabilization of a scintillation detector.
Invention is credited to Guntram Pausch, Jurgen Stein.
Application Number | 20100001201 12/259111 |
Document ID | / |
Family ID | 34957865 |
Filed Date | 2010-01-07 |
United States Patent
Application |
20100001201 |
Kind Code |
A1 |
Stein; Jurgen ; et
al. |
January 7, 2010 |
STABILIZATION OF A SCINTILLATION DETECTOR
Abstract
A detector for the measurement of radiation, preferably ionizing
radiation, includes a medium, means for the conversion of the
radiation energy absorbed by the medium into electrical charge,
means for digital sampling of the charge signals, means for the
determination of a calibration factor K, and means for the
stabilization of the output signals of the detector. The medium at
least partly absorbs the radiation to be measured. The electric
charge is at least partially proportional to the energy of the
radiation. The sampling is done preferably with a sampling rate
between 1 and 1000 MHz. Further signal processing is digital. The
calibration factor K has a fixed relation with respect to the decay
time .tau. of the medium. The output signals of the detector are
mainly proportional to the radiation energy, and are stabilized
with the help of the calibration factor K.
Inventors: |
Stein; Jurgen; (Wuppertal,
DE) ; Pausch; Guntram; (Dresden, DE) |
Correspondence
Address: |
IP STRATEGIES
12 1/2 WALL STREET, SUITE E
ASHEVILLE
NC
28801
US
|
Family ID: |
34957865 |
Appl. No.: |
12/259111 |
Filed: |
October 27, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11128129 |
May 10, 2005 |
7485868 |
|
|
12259111 |
|
|
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Current U.S.
Class: |
250/395 ;
250/370.11; 250/389 |
Current CPC
Class: |
G01T 1/208 20130101;
G01T 1/40 20130101 |
Class at
Publication: |
250/395 ;
250/389; 250/370.11 |
International
Class: |
G01T 1/20 20060101
G01T001/20; G01T 1/40 20060101 G01T001/40 |
Foreign Application Data
Date |
Code |
Application Number |
May 10, 2004 |
EP |
PCT/EP2004/050754 |
Claims
1-20. (canceled)
21. A method of stabilizing signals generated by a scintillation
detector that measures radiation, wherein the signals are dependent
on an operating temperature of the detector, comprising: absorbing
the radiation at least in part within the detector; determining, at
least in part within the detector, a temperature-dependent
calibration factor based on a shape of signals generated by the
absorbed radiation; generating an output signal corresponding to an
energy level of the absorbed radiation by receiving light at the
detector, measuring a decay time of the signals generated by the
absorbed radiation corresponding to the received light, and
providing a signal corresponding to the measured decay time, all at
least in part within the detector; determining a pulse form
parameter by evaluating a shape of the output signal, at least in
part within the detector; and applying the calibration factor to
the output signal according to a ratio with respect to the pulse
form parameter in order to stabilize the output signal, at least in
part within the detector.
22. The method of claim 21, wherein the radiation is ionizing
radiation.
23. The method of claim 21, where the pulse form parameter is
determined by evaluating at least one of a length of a
unipolar-formed output signal of the detector, a rise time of the
unipolar-formed output signal of the detector, and a time between a
beginning of the output signal and a zero crossing of a
bipolar-formed output signal of the detector.
24. The method of claim 21, wherein determining the calibration
factor includes generating a charge signal from the signals
generated by the absorbed radiation, determining a rise time of the
charge signal, wherein the rise time of the charge signal is
substantially proportional to the decay time of the signals
generated by the absorbed radiation, and determining the
calibration factor based on the rise time of the charge signal.
25. The method of claim 24, further comprising digitally sampling
the charge signal.
26. The method of claim 25, wherein the charge signal is sampled at
a sampling rate that is between 1 MHz and 1000 MHz.
27. The method of claim 26, wherein the charge signal is sampled at
a sampling rate that is between 5 MHz and 100 MHz.
28. The method of claim 27, wherein the charge signal is sampled at
a sampling rate that is between 5 MHz and 25 MHz.
29. The method of claim 21, wherein determining the calibration
factor includes generating a current signal such that a length and
a decay time of the current signal are substantially proportional
to the decay time of the signals generated by the absorbed
radiation, and determining the calibration factor based on one of
the length and the decay time of the current signal.
30. The method of claim 29, further comprising digitally sampling
the current signal.
31. The method of claim 30, wherein the current signal is sampled
at a sampling rate that is between 1 MHz and 1000 MHz.
32. The method of claim 31, wherein the current signal is sampled
at a sampling rate that is between 5 MHz and 100 MHz.
33. The method of claim 32, wherein the current signal is sampled
at a sampling rate that is between 5 MHz to 25 MHz.
34. The method of claim 21, where the pulse form parameter is
determined in a numerical manner.
35. The method of claim 21, where the calibration factor is
determined using a predetermined mathematical function based on the
pulse form parameter.
36. The method of claim 35, wherein the predetermined function is
mainly linear.
37. The method of claim 35, wherein the predetermined function is
mainly polynomic.
38. The method of claim 36, further comprising storing the
predetermined function in the detector in a readable manner.
39. The method of claim 21, wherein applying the calibration factor
to the output signal according to a ratio with respect to the pulse
form parameter includes correlating the calibration factor with a
predetermined calibration table that includes the pulse form
parameter.
40. The method of claim 39, further comprising storing the
calibration table in the detector in a readable manner.
41. The method of claim 21, wherein the application of the
calibration factor to the output signal according to a ratio with
respect to the pulse form parameter is performed during radiation
measurement by the scintillation detector in real time.
Description
[0001] The invention relates to a method for stabilizing signals,
generated by a scintillation detector for the measurement of
radiation, preferably ionizing radiation, after it has absorbed the
radiation at least in part within the detector and whereas those
signals are dependent from the operation temperature of the
detector. The invention also relates to a detector for the
measurement of radiation, preferably ionizing radiation.
[0002] Respective methods and detectors are known in the prior art.
A scintillator in a scintillation detector absorbs the radiation to
be measured, thereby generating excited states within the
scintillator. Those excited states decay with a decay time .tau.
under the emission of light, whereas the amount of light is a
measure for the absorbed energy of the incoming radiation. The
light is directed to a photocathode, emitting electrons in
dependence of the amount of light, being absorbed there, being
usually amplified by photomultiplier. The output signal of the
photomultiplier therefore is a measure for the total energy of the
absorbed radiation.
[0003] It is known that the light output of a scintillator is
dependent from its temperature, so that the output signal, being
proportional to the measured energy, is also dependent from the
temperature of the scintillator. As it is often not possible to
operate the scintillation detector at a constant known temperature,
the detector's accuracy of measurement is substantially impaired by
the temperature changes.
[0004] According to the known prior art, this is achieved by a
calibration, being applied before or after the measurement, whereas
a so called calibration source, that is a radiation source with a
known energy of radiation, is used for calibration. As an
alternative or in addition, the calibration may be effected on the
basis of known lines with known energy, being present in the
measured spectrum.
[0005] This has the disadvantage that temperature changes,
occurring between the time of calibration and the time of
measurement, lead to an additional uncertainty of the measurement.
Especially with detectors, being used under changing external
operation conditions, especially outside of a laboratory, this
disadvantage is of importance. Furthermore, it has often to be
assumed, especially in security engineering--contrary to classical
research applications--that they are not enough lines of previously
known energy present within the spectrum, so that the measured
spectrum has to be evaluated in advance in order to be able to
allocate specific energies to single measured lines. Because of
possible incorrect allocations, this is subject to errors. As the
security personal usually has no nuclear physics knowledge, the
allocation of single lines of the measured spectrum to specific
known energies is a difficulty in addition.
[0006] Applicant therefore developed a scintillation detector and a
method for operation of such a detector, in which the known energy
of a calibration source can be measured continuously, or, as the
case may be, in defined, comparably short time gaps, by the
detector so that the detector could be calibrated during the
measurement with the known energy of the radiation of the
calibration source. Therewith it is possible also for persons
without physics knowledge to collect a spectrum of ionizing
radiation with high accuracy.
[0007] The radiation within the energy range of the radiation,
being emitted from the calibration source, is nevertheless
superposed by exactly this radiation of the radiation source and
therefore not measured in an optimal manner. In case one does not
calibrate the detector continuously, but, alternatively in larger
time gaps, the radiation within the energy range of the calibration
source could be measured also, nevertheless at the same time the
energy resolution becomes worse by temperature changes not being
picked up. Therefore, it is for principle reasons very difficult to
achieve a high energy resolution by a continuous calibration with
at the same time high sensitivity in the complete energy area, that
is also in the area of the radiation, being emitted by the
calibration source.
[0008] An additional problem is that, in order to calibrate for the
measurement of an ionizing radiation, usually a radioactive
calibration source is necessary, which usually is part of the
detector if it is used for security engineering. This requires
substantial efforts during the production of respective detectors.
Because of continuously rising safety measures and the desire to
avoid radioactive material as far as possible, there therefore is a
need to calibrate scintillation detectors without the use of
radioactive material.
[0009] It is therefore the object of the present invention to
provide a method for the calibration of a scintillation detector as
well as a scintillation detector, avoiding the described
disadvantages of the known prior art and especially allowing for a
calibration during the current measurement across the complete
energy spectrum with at the same time high calibration accuracy. A
further object of the invention is to allow the calibration of a
scintillation detector for the measurement of ionizing radiation
without having to use a radioactive calibration source.
[0010] This problem is, according to the invention, solved by a
detector according to the characterizing part of the independent
claims.
[0011] According to this, a temperature dependent calibration
factor K is determined directly from the shape of the signals,
being generated by the radiation to be measured itself In a
specific embodiment of this method, the calibration factor K is
chosen in a predetermined ratio with respect to a pulse form
parameter P, whereby the pulse form parameter P is obtained by
evaluating the shape of the registered detector signals and whereby
the evaluated detector signals are dependent from the time decay
constant .tau. of the scintillation light, being generated within
the scintillation detector. It has been proven an advantage to
determine the pulse form parameter P from at least one of the
following characteristics: peaking time of the unipolar formed
output signal of the detector, rise time of the unipolar formed
output signal of the detector and/or the time between the begin of
the signal and the zero crossing of the bipolar formed output
signal of the detector.
[0012] It has been proven as an advantage to determine the
calibration factor K with the following method steps: generating a
charge signal L from the excited states, being generated by the at
least partly absorption of the radiation within the detector and
decaying with a decay time constant .tau., determining the rise
time of the charge signal L, being substantially proportional to
the decay time constant .tau., and determining the calibration
factor K from the rise time of the charge signal L. Alternatively,
a current signal S could be generated from the initially generated
signal, so that the length and the decay time of the current signal
S is substantially proportional to the decay time constant .tau..
The calibration factor K is then determined from the length or the
parameters of the decay time of the current signal S.
[0013] The pulse form parameter P may be determined electronically
by signal processing. It is, nevertheless, especially advantageous
if the signal processing is done digitally, whereas it is an
advantage to digitally sample the electric charge signal L and/or
the current signal S, whereas the sampling preferably occurs with a
sampling rate between one and 1000 MHz, especially preferred with a
sampling rate between 5 and 100 MHz and even more preferred with 5
to 25 MHz. Best results have been achieved by using 7 MHz as a
sampling rate. Specifically advantageous is to generate the pulse
form parameter P in a numerical manner.
[0014] In addition it is especially advantageous if the calibration
factor K for the stabilization of the measured signals is
determined with the help of a predetermined mathematical function
from the pulse form parameter P, whereas the predetermined function
is preferably mainly linear or polymeric. The predetermined
function may be stored in the detector in a readable manner. It is
also possible to correlate the calibration factor K for the
stabilization of the measured signals with a predetermined
calibration table, containing the pulse form parameter P. It is an
advantage if this calibration table is stored in a readable manner
in the detector. The determination of the calibration factor K from
the pulse form parameter P does occur in a specifically preferred
embodiment of the method during the measurement in real time.
[0015] Furthermore, a detector for the measurement of radiation,
preferably ionizing radiation, is disclosed, in which the output
signals, being mainly proportional to the energy of the radiation,
are stabilized by a method described above.
[0016] Object of this invention is also a detector for measurement
of radiation, preferably ionizing radiation, comprising at least a
medium, at least partly absorbing the radiation to be measured, as
well as means for conversion of the radiation energy, absorbed by
this medium, into electrical charge, whereas the electric charge is
at least partially proportional to the energy of the radiation or
correlates at least in a predetermined ratio to the absorbed
radiation energy. In addition, the detector comprises means for the
determination of a calibration factor K, having a fixed relation
with respect to the decay time constant .tau. of the medium, and
means for the stabilization of the output signals of the detector,
being mainly proportional to the radiation energy, with the help of
the calibration factor K.
[0017] It is advantageous, if the detector comprises the following
means in order to determine the calibration factor K: means for
determining the rise time of the charge signal L, preferably by
conversion of the charge signal L into a current signal S,
preferably by differentiating and determining the decay time of the
current signal S, being proportional to the rise time of the
current signal L whereby the rise time of the current signal L is a
measure for the decay time constant .tau. of the excited states in
the medium, absorbing the radiation, and means for the
determination of the calibration factor K, having a mainly fixed
relation with respect to the decay time constant .tau. of the
medium, from the rise time of the charge signal L.
[0018] As medium, absorbing the radiation, preferably a
scintillation crystal is used, preferably sodium iodide (NaI),
cadmium wolframate (CWO), caesium iodide (CsI), bismuth germanate
(BGO) or, especially preferred, lanthanum chloride (LaCI.sub.3) or
lanthanum bromide (LaBr.sub.3).
[0019] The means for converting energy, absorbed by the medium,
being at least in part proportional to the radiation energy, into
electric charge, preferably comprise a light detector, for example,
a photomultiplier, comprising a photocathode with a photomultiplier
coupled to it, or a photodiode, especially preferable with a charge
sensitive pre-amplifier coupled thereto, or a hybrid
photomultiplier or an avalanche photodiode, whereas the electric
charge is converted into a current signal S by using a pulseforming
and amplifier circuit. Thereby a time t between two defined points
in the bipolar modified signal is measured, preferably between that
point of the rising flank of the signal, at which it has achieved a
third of its full maximum, and the point, at which the signal
height is 0 Volt, whereby the so measured time t is a measure for
the decay time constant .tau.. The time of the zero crossing of the
bipolar signal is also denominated as t.sub.zc.
[0020] In another embodiment, the signal processing occurs
digitally, whereas the digital signal sampling is preferably done
with a sampling rate between 1 and 1000 MHz, especially preferable
with a sampling rate between 5 and 100 MHz and explicitly preferred
with a sampling rate between 5 and 25 MHz, whereas a sampling rate
of 7 MHz proved to be very good.
[0021] After such a sampling has been done, the parameters of the
exponential decaying flank of the current signal S, being a direct
measure for the decay time constant .tau., can be determined within
the detector by digital signal processing. It has proven an
advantage to scale the parameter, being proportional to the decay
time constant .tau., with a calibration table and to correct the
measured signals with the resulting value in order to calibrate the
measured values and to use them to stabilize the detector.
[0022] The charge q measured at the electric output of the
detector, which usually is the output of the photomultiplier,
thereby is dependent on following other dimensions:
q=E*w.sub.scnt(T)*.epsilon..sub.opt*S(.lamda.,T)*V.sub.PMT(U.sub.A,
T, N, Hist)
[0023] Therein, E stands for the energy of the particle to be
measured, w.sub.scnt for the light output of the scintillator,
.epsilon..sub.opt for the optical efficiency of the light
collection at the photocathode of the scintillator, S.sub.phk for
the sensitivity of the photocathode coupled directly to the
scintillator and V.sub.PMT for the own amplification of the
photomultiplier. It turns out that the light output depends on the
temperature T.sub.S of the scintillator and the sensitivity of the
photo cathode depends on the temperature T.sub.F of the
photocathode, whereas the photocathode is usually thermally
directly connected to the scintillator, so that only the common
temperature T=T.sub.F=T.sub.S is to be viewed, whereas the
sensitivity of the photocathode is, in addition, a function of the
wave length .lamda..
[0024] The own amplification of the photo multiplier V.sub.PMT
depends from the operation voltage U.sub.A, the temperature T, the
counting rate N and non-linear effects from the history Hist of the
detector. In the following those dependencies will be ignored.
[0025] It is known from J. S. Schweitzer and W. Ziehl, IEEE Trans.
Nucl. Sci. NS-30(1), 380 (1983), that the decay time constant .tau.
of the excited states depends from the temperature of the
crystal.
[0026] From JP 06-258 446 A, a detector system is known, where the
optical scintillator and the electrical equipment, including the
light detector, are separated in space. Disclosed is a
photoconductive wave guide scintillator, comprising a scintillator,
a wave length shifter and a wave guide. In addition, this patent
application discloses a temperature correction for this
photoconductive wave guide scintillator when it is used as part of
a system in combination with a preamplifier.
[0027] A disadvantage of JP 06-258 446 A is that the proposed
advantage, namely the separation of scintillator and
photomultiplier, leads to specific negative effects, as temperature
changes of the photomultiplier cannot be dealt with. In addition,
the disclosure requires complicated electronic equipment, as the
output signal of the photomultiplier has to be split in order to
allow for a separate and distinguished evaluation electronics--one
handling the measurement, the other one handling the temperature
evaluation. This leads not only to a complicated, large and
expensive setup, but the separate individual handling of the
various signals also leads to additional sources of error.
Furthermore, the temperature correction is based on a determination
of the damping time of the light in the scintillator, that is on a
parameter which is difficult to determine directly, depending on
the used electronics.
[0028] The present invention is now, for the first time,
identifying further parameters, showing a fixed relationship to the
decay time constant .tau.. With the present method, the crystal
temperature T and/or calibration factors K are determined during
the running measurement from those parameters, and are used to
stabilize the detector.
[0029] The very specific here is that this stabilization cannot
only be done online during the running measurement, but that the
calibration factors K could be derived directly from the form of
the signals to be measured themselves, so that no radioactive
calibration source is needed for stabilization and that there is no
need for a signal split, that is for a separate handling of signals
with additional hardware, resulting in a much simpler, more
efficient, reliable and cheaper electronics. More specifically, the
use of a preamplifier can be avoided.
[0030] Therefore, this method is, already because of the continuous
opportunities for stabilization, more exact than the known methods
using a calibration source for principal reasons. At the same time
no calibration source is necessary so that the use of radioactive
material can be omitted and, in addition, the complete resolution
of the energy spectrum of the radiation to be measured is
available, including where otherwise the signals of the calibration
source occur and hinder the measurement of low doses.
[0031] In the following, a specific embodiment is discussed along
FIGS. 1 to 5. They show:
[0032] FIG. 1: a schematic setup of a scintillation detector with a
photomultiplier,
[0033] FIG. 2a: light emission across the time, shown for two
incidences with varying decay time constants, but with the same
amount of light (energy),
[0034] FIG. 2b: charge signal L(t) for the two incidences of FIG.
2a,
[0035] FIG. 2c: current signal S(t) for the two incidences of FIG.
2a,
[0036] FIG. 2d: bipolar signal B(t) for the two incidences of FIG.
2a,
[0037] FIG. 2e: unipolar signal U(t) for the two incidences of FIG.
2a,
[0038] FIG. 2f: charge signal L(t) with and without RC discharging
for a row of incidences,
[0039] FIG. 3: energy spectrum of a .sup.137Cs source, taken by
various temperatures,
[0040] FIG. 4: zero crossing time spectra, being taken along with
the energy spectra according to FIG. 3,
[0041] FIG. 5a: average zero crossing time <t.sub.ZC> as a
function of the temperature of the detector system,
[0042] FIG. 5b: position of the 662-keV peak within the energy
spectrum according to FIG. 3 as a function of the temperature of
the detector system,
[0043] FIG. 5c: position of the 662-keV peak in the energy spectrum
as a function of the average zero crossing time
<t.sub.ZC>,
[0044] FIG. 5d: correction factor K as a function of the zero
crossing time <t.sub.ZC>.
[0045] FIG. 1 exemplarily shows the schematic setup of a
scintillation detector 100. Shown are the scintillation crystal
110, the photocathode 120 and the photomultiplier 130. The
radiation is absorbed at least in part by the scintillation crystal
and is generating excited states in there, decaying again under the
emission of photons. Those hit the photocathode 120, emitting
electrons in dependence of the amount of induced light, which again
are multiplied by a photomultiplier 130. The output signal of the
photomultiplier 130 then is further processed in order to finally
provide an output signal related to the absorbed energy of the
radiation, which is further processed in the evaluation electronics
140.
[0046] The light emission occurs together with the decay of the
excited states and therefore mainly exponential with a decay time
constant .tau.. FIGS. 2a to 2e show the calculated simulated light
emission for two incidences with different decay time constants
(.tau..sub.A=100 ns, .tau..sub.B=150 ns) and the following signal
processing.
[0047] The distribution of the light emission over the time is
shown in FIG. 2a. The charge q, collected completely up to the time
t, can be seen in the current signal L(t) (FIG. 2b). The height of
the charge signal L(t) after the light pulses have been decayed (in
FIG. 2b: roughly after 1000 ns) is a measure for the amount of the
totally emitted photons and therefore for the energy E to be
measured, whereas the steepness of the rising flank of the charge
signal L reflects the decay time constant of the scintillation
material.
[0048] As can be seen from FIG. 2f, several consecutive signals
would lead to the result that the charge signal would rise steadily
(diagram "without RC-discharge"). For technical reasons, the charge
signal therefore is discharged usually via an RC-element with an
electronic time constant .THETA., being large compared to .tau.
(diagram "with RC-discharge"). Nevertheless, this discharge is of
no importance for the following principle discussion, so that FIG.
2b does not show this detail for the purpose of clearness.
[0049] The time-wise distribution of the light emission can be
reconstructed nearly in an electronic manner by differentiating the
charge signal L(t), preferably electronically with usual pulse
forming and amplifier circuits, therefore resulting in a current
signal S(t) (FIG. 2c). The information concerning the decay time
constant .tau. can be extracted from the form and length of the
current signal S(t).
[0050] By consecutive integration and differentiation steps,
preferably done electronically with usual impulse forming and
amplification circuits, the initial charge or current signal can be
further formed into a bipolar signal B(t) (FIG. 2d) or into a
unipolar signal U(t) (FIG. 2e).
[0051] From FIG. 2d it becomes clear that the time of the zero
crossing in the bipolar signal B(t) depends on the decay time
constant .tau.. The zero crossing time can be measured very exactly
with a zero crossing detector. It is independent of the amplitude
of the signal and therefore independent of the energy of the
detected particle as well as of shifts of the signal
amplification.
[0052] FIG. 2e demonstrates that the decay time constant .tau. is
determined by both, the rising as well as the peaking time of the
unipolar signal U(t), that is the time in which the maximum of the
signal has been reached. Again, these parameters can be measured
with electronic circuits according to the prior art.
[0053] All methods as described above could be used for the
determination of the decay time constant .tau. or of the
temperature of the scintillation crystal correlated therewith or of
a correction factor K, depending on the temperature which could be
used to stabilize the detector, from the form of the detector
signal. The embodiment as described in the following is
demonstrating this principle in a specific case:
[0054] A scintillation detector, consisting of a NaI(T1)-crystal
with a photomultiplier coupled thereto, is exposed to radioactive
radiation. The signals of the photo multiplier are formed in a
bipolar manner according to FIG. 2d and then sampled digitally with
a sampling rate of f.sub.sampl=25 MHz. The zero crossing time
t.sub.ZC is determined for every detected signal by [0055] the time
t.sub.1, at which the signal front is reaching the third part of
the signal maximum, [0056] the time t.sub.2, at which the signal is
crossing the zero line, and finally [0057] the difference
t.sub.ZC=t.sub.2-t.sub.1 numerically calculated from the single
sampling points.
[0058] By using suitable numerical methods, in the presented case
by linear interpolation between the single sampling points, the
zero crossing time t.sub.ZC can be determined with an uncertainty
.DELTA.t, being substantially smaller than the sampling interval
(.DELTA.t<<1/f.sub.sampl).
[0059] The maximum of the bipolar signal is used as a measure for
the energy E.
[0060] During the measurement, a zero crossing time spectrum,
generated by a pulse height analysis of the parameter t.sub.ZC, is
generated in addition to the energy spectrum, generated by pulse
height analysis of the parameter E.
[0061] FIG. 3 shows energy spectra, which have been measured with
the detector after radiating it with a .sup.137Cs source at
different temperatures. FIG. 4 shows the zero crossing spectra
being measured with this system at the same time.
[0062] In FIG. 5a the average zero crossing time <t.sub.ZC>
is determined by calculating the center of gravity of the single
zero crossing time spectra as a function of the related temperature
of the detector system. <t.sub.ZC> is distinctly correlated
with the temperature of the detector system. As expected and known
already, the position of the 662-keV peak within the energy
spectrum depends on the temperature (FIG. 5b). This effect has to
be compensated by a correction factor K. In order to do so, one can
at first show the position of the 662-keV peak as a function of the
parameter <t.sub.ZC>. FIG. 5c demonstrates that this function
is of such a condition that the position X.sub.662 of the 662-keV
peak can be predicted clearly from <t.sub.ZC>.
[0063] With the help of the correction factor
K(<t.sub.ZC>)=X.sub.662(350
ns)/X.sub.662(<t.sub.ZC>)
[0064] the actual position of the peak can be corrected in such a
way that
K(<t.sub.ZC>)*X.sub.662(<t.sub.ZC>)=X.sub.662(350
ns)=const.
[0065] is true--that is the corrected peak position is independent
from <t.sub.ZC> and therefore independent from the
temperature of the detector system. For the specific detector
system, a correction factor K is calculated, being shown in FIG.
5d. This function has to be determined individually for every
detector.
[0066] The calibration factor K, being determined during the
measurement from the shape of the measured signals themselves,
being mostly independent from the energy E of the measured
radiation, can either be stored in a table within the detector, so
that the stabilization can occur on the basis of the data already
stored in that table. It is also possible, as shown exemplarily
above, to catch the dependency in a functional manner and to store
the function in the detector and to stabilize it by using the
function.
[0067] In a further embodiment the rise time t.sub.r of the signal
according to FIG. 2c can be determined as a pulse form parameter P.
Suitable for the method according to the invention are,
nevertheless, all other parameters, which do show a predetermined
correlation to the decay time .tau..
[0068] Multiple possibilities are available in case the signal
processing does not occur analog but digital. In this case, the
output signal of the photomultiplier or the charge sensitive
preamplifier is sampled digitally with a sampling rate of 25 MHz in
one embodiment of this invention. Measurements confirm that this
sampling rate is already sufficient in order to achieve
sufficiently exact results.
[0069] The digital signals are technically easier and cheaper to
evaluate than the comparable analog signals. For example, the
conversion in a bipolar signal, which zero crossing time t.sub.ZC
can easily be measured with analog devices, can be omitted as a
equally suitable pulse form parameter can be derived directly from
the digitized output signal of the photomultiplier also. In
addition, the processing of the measured signals and their
evaluation, for example with a multi channel analyzer VKA, is
preferred in case the measured signals are available digitally
already.
[0070] This embodiment, especially FIG. 5a, shows that the
dependence of the pulse form parameter t.sub.ZC is sufficiently
distinctive especially for the interesting area of temperature of
-15.degree. C. to +55.degree. C.
[0071] The method according to the invention, nevertheless, cannot
only be used for detectors. As the pulse form parameter P is always
a measure for the crystal temperature T, this method could also be
used to operate a thermometer. As radiation source usually the
background radiation, being present anyway, is sufficient. As the
measurement is not depending on the radiation energy, a
stabilization of the measured energy is not necessary.
[0072] The advantage of such a thermometer is that at the place of
measurement, no supply of energy, in addition to the background
radiation being present anyway, is necessary in order to operate
the thermometer, so that the temperature of the object to be
measured is not influenced by the measurement itself.
[0073] Such temperature measurements can be used within the
detector itself for calibration of other components also, but they
also could be used outside the detector.
[0074] This is especially true as no mechanical but only an optical
coupling between the scintillator and the light detector is
necessary. This suggests exploiting small scintillator samples,
excited by background radiation or radioactive seeds, as
contact-free, passive (remote) thermometers which are distinguished
by zero power consumption. Such probes are very useful in thermally
well insulated environments such as a vacuum.
[0075] Another application of the invention is a position sensitive
detector. If a scintillator crystal is provided with a static
temperature field gradient, for example by keeping crystal faces at
different but fixed temperatures, the decay time of the individual
signals does provide additional position information.
[0076] The method of this invention can also be used to investigate
the proverties of various scintillation materials. For example, the
dependence of the scintillation decay time on parameters like
temperature, chemical composition, radiation type and the like can
easily and in a very inexpensive manner be measured with the
inventive method.
[0077] Last but not least, it is possible to adapt the filter
parameters, especially used in digital signal processing, to the
actual conditions during the measurement, therefore allowing for an
adaptive filtering.
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