1.  Field of the Invention
The invention relates to the field of electronic instrumentation in which a random signal is produced or in particular a random number generator.  The invention relates to the apparatus and methods for generation of random numbers or random signals. 
2.  Description of the Prior Art
Long sequences of random numbers are essential in mathematical statistics, data protection, communication security, mathematical simulation of natural phenomena and technological processes, etc.  A random number generator (hereinafter, RNG) is at the heart of any information security technology where it is used for encoding key generation.  For some applications the xe2x80x9cquality of the random numbersxe2x80x9d is absolutely crucial.  For instance, if the random numbers used in the data protection applications are not xe2x80x9crandom enoughxe2x80x9d, it will make the encryption code breakable and may pose serious information security problems, no matter how advanced and sophisticated the encoding procedure is. 
Random numbers are produced by random number generators (RNG""s), which for the most part are computer programs based on sophisticated mathematical algorithms.  Most standard PC software packages include one or several such algorithmic RNG""s.  It is commonly recognized that any algorithmically generated digital sequence must develop apparent or hidden correlations and, hence, cannot be truly random.  There are several standard random distributions, such as, Poisson, Bernoulli, etc.,  each of which can be converted to another.  These standard distributions relate to truly random processes, meaning the absence of a statistical correlation between different events or numbers no matter how close or distant from one other they are.  Such distributions correspond to the maximal output entropy.  Thus, the quality of a random number generator is defined by the proximity of its output to one of the standard truly random distributions. 
As long as the inevitable faultiness of algorithmically generated random sequences is not critical for an application, there is no need to look for something else.  But there certainly exist a variety of important applications for which hidden long-range correlation in the RNG output is unacceptable.  For instance, if the RNG is anything but perfect, the encryption code can be broken, and it does happen from time to time.  In other words, the vulnerability of the encrypted information directly relates to the defectiveness of the RNG used.  The only way to ensure the data protection, no matter how resourceful and well equipped the code-breakers are, is to use a perfect RNG for encoding key generation.  In the case of RNG applications in mathematical statistics or computer simulation, the presence of a hidden correlation in the RNG output can and sometimes does make the results of statistical calculations unreliable or even worthless. 
The only viable alternative to the inherently faulty algorithmic RNG""s is a natural, or physical random number generator.  A physical RNG is based on naturally occurring random phenomena, such as thermodynamic or quantum fluctuations, radioactive decay, etc. 
Most of the existing physical RNG""s are based on low energy random phenomena, particularly, thermal fluctuations (Johnson noise), or electronic quantum fluctuations in solids.  All such devices have two major problems.  Firstly, they inevitably display some autocorrelations and instability due to the physical nature of the underlying physical processes.  Secondly, the low energy fluctuation can be affected by ubiquitous external and internal electromagnetic interference, the noise associated with the device electronic circuitry, acoustic noise, etc.  These unwanted signals are never truly random and may well contribute to the deviation of the digital output of the physical RNG from the standard random distribution. 
A radioactive decay is a natural process ideally suited as a source of randomness.  The energy associated with a single event of spontaneous nuclear decay is by 5-7 orders of magnitude higher compared to other physical processes.  Therefore, each and every event of a spontaneous radioactive decay does not depend on any external conditions, such as, the quantum state of atomic electrons, presence of other atoms or electromagnetic fields, ambient chemistry, temperature, etc.  In this respect, spontaneous radioactive decay is unique.  Several physical random number generators based on radioactive decay are known in the art.  However, there is room for improvement. 
Generally, the existing physical random number generators based on natural radioactive decay are superior compared to those based on low energy random phenomena.  Still, there are several problems remaining. 
The first one relates to the physical source of randomness itself.  The standard Poisson time distribution of the events only applies to those ideal sources which display neither secondary radioactive decay, nor any kind of induced radiation which could be later mistaken for a primary radioactive decay.  The induced radiation may include the X-ray quanta, the electrons knocked out of the atoms by the primary radiation, etc.  If anything but the prime events is registered by the detector, then the digital output of RNG will inevitably display some autocorrelations.  The reason is that different events, such as the primary and the secondary radioactive decays, or the primary events and the induced radiation, are related to one other and, hence, correlated in time.  An additional complication may arise from the fact that the total number of unstable nuclei in a radioactive source gradually decreases in time and so does the mean radiation event frequency. 
The second problem is associated with the signal registration method.  For instance, if the energy of a single radioactive particle is first converted in electric or acoustic noise and only after that is digitized (as is shown in Mike Rosing and Patrick Emin, Ionization from Alpha Decay for Random Bit Generation.  University of New Brunswick.),  then one will face all the problems associated with physical RNG""s based on low energy fluctuations. 
One possible way to overcome the above problems is to utilize a directional randomness of a natural radioactive decay, rather than the temporal randomness.  See Edelkind, et al.,  U.S. Pat.  No.  5,987,483 (Nov. 16, 1999).  The directional randomness implies that the direction of propagation of emitted radiation produced by individual events is a perfectly random characteristic of the process.  However, utilization of the directional randomness requires a plurality of independent detectors surrounding a single source of radiation.  Every detector should be supplied with independent electric circuitry.  The mutual arrangement of the source and the plurality of detectors must exclude the possibility of detecting a single event of radioactive decay by more than one detector. 
In present invention we propose the alternative solution that is thought to be less costly and much easier to implement.  The proposed device requires a single detector of emitted radiation and utilizes the temporal randomness of spontaneous decay.  At the same time the proposed device solves the problem of producing a standard, correlation free random sequence resistant to any kind of internal and external interference (electromagnetic, acoustic, etc.).  Finally, consider a comparative analysis of spontaneous alpha decay versus beta and gamma decay.  The whole variety of radioactive isotopes differs by the type of emitting particles. 
Alpha decay produces helium nuclei.  They have the largest mass and electric charge.  Therefore, they get absorbed by the matter within a very short range.  In the air alpha particles can travel just a few centimeters.  Even a thin sheet of paper will totally absorb them.  Typical energy of an alpha particle is around 5-6 MeV (compare to less than 1.5 MeV of the beta radiation and 0.5-1.5 MeV of the gamma radiation).  The higher the particle energy is, the stronger signal it produces in the detector.  More importantly, the energy of emitted alpha particles lies within a very narrow band.  So, one can easily and reliably separate the signals produced by a particular type of alpha decay from any other sources of ionizing radiation, including high energy electrons, X-rays, as well as the alpha particles produced by radioactive isotopes different from those of the specified one.  This latest feature is absolutely crucial for the creation of a flawless RNG.  Indeed, as it was stated above, the clear separation of the signal produced by a particular radioactive event from all other sources of ionizing radiation is the necessary precondition for the device output to be a known standard random distribution.  One of the most convenient and reliable sources of the alpha radiation is Am-241 (this isotope is widely used in household smoke detectors). 
Beta decay emits electrons.  Light-weighted beta particles can travel much longer distance through the medium, compared to the alpha radiation.  They have substantially lower energy and therefore produce weaker electrical pulses in the detector.  The main disadvantage of beta particles is that their energy is rather unpredictable and spread over the broad spectrum (they share specific energy of the decay with neutrinos).  As a result, it is impossible to positively identify the signal produced by a beta particle emitted from the prime source of radiation and separate it from the background signals created by other kinds of ionizing radiation.  Due to this background contribution, the temporal distribution of the registered events will inevitably deviate from the standard Poisson distribution p(n).  That means that the digital output of the RNG device based on the beta decay will not be flawless. 
Finally, the gamma decay radiates electromagnetic gamma quanta.  The gamma radiation, having neither mass nor electrical charge, is highly penetrating.  Although the spectrum of gamma quanta can be relatively narrow, their detection always imposes additional and substantial broadening of the spectrum that eventually creates the same problem as in the case with the beta particles.  Besides, gamma particle absorption in the detector may occur in three different ways: photoelectric effect, Compton scattering, and creation of an electronxe2x80x94positron pair.  Therefore, a single gamma quantum can produce several different signals.  All this contributes to the complexity of the analogue output of the detector and makes it difficult to extract from it the standard random distribution.  Finally, gamma radiation is not safe.  Their use would require at least a 5 cmxe2x80x94thick lead shield around the scintillator. 
The proposed natural RNG utilizing spontaneous nuclear decay of the alpha-type is designed to be free of the flaws which are unavoidable in all other known physical RNGs.  Below, we will show how the utilization of spontaneous alpha decay allows separating the signals produced by the events of the primary radioactive decay from those of a different origin, and thereby obtain virtually perfect random digital output.  Besides, the proposed device is portable, durable, and absolutely safe.  In particular, it can be installed in virtually any standard PC. 
The invention is an apparatus for generation of random numbers comprising a source of alpha particles, and a detector of the alpha particles.  The detector is disposed in a position relative to the source to detect the alpha particles coming from the source, and to generate a standard electric pulse in response to a detection of every single alpha particle.  Necessary precautions should be taken to ensure that the energy spectrum of the alpha particles reached the detector is not substantially broadened compared to the original spectrum of the emitted alpha particles. 
The detector comprises a detection device and an amplifier coupled to the detection device, and in particular comprises a silicon semiconductor detector and a detector bias supply circuit.  The amplifier comprises a charge preamplifier and a linear amplifier coupled to the charge preamplifier. 
A signal amplifier is followed by a selective discriminator to identify detection signals corresponding to an actual detection of the alpha particles emitted by the source.  The transmission bandwidth of the discriminator must be broad enough to account for virtually all the alpha particles, reached the detector.  At the same time the transmission bandwidth of the discriminator must be narrow enough to filter out improper pulses originated from the ionizing radiation other than the alpha particles coming from the source.  A logic unit converts a sequence of the randomly distributed in time identical electrical pulses into a binary sequence. 
The source of the alpha radiation has a half-life of one hundred years or more to ensure a stable performance of the device for at least several years.  The decay products must be stable or, at least, must not produce nuclear radiation with the energy equal or higher than that of the original alpha decay.  In the illustrated embodiment, the source is Am 241. 
The selective discriminator comprises a differential discriminator to determine if the detected signal has an amplitude characteristic of the detection of the alpha particles originated from the source.  The selective discriminator also comprises a logical selector to determine if the detection signal has a pulse shape characteristic of the detection of the alpha decay particles. 
The simplest design of a binary generator is a clocked trigger circuit which toggles with each positively identified detection signal corresponding to an actual detection of the alpha decay particles from the source.  The binary generator is alternatively a clocked time comparator circuit which measures successive time intervals between successive positively identified detection signals corresponding to actual detections of the alpha decay particles from the source and which assigns a binary value to the successive time intervals depending on comparative time lengths of the successive time intervals.  The means by which detections are converted into binary signals can be varied widely to include any means now known or later devised, and it is to be expressly understood that the illustrated means are not to be read as limiting the nature of the binary generator.  Any device capable of converting a randomly arriving sequence of nearly identical short pulses into a binary sequence can be employed with equal ease. 
The apparatus further comprises a host device and an interface circuit coupled to the host device.  The interface circuit is also coupled to the binary generator.  The host device is a data system, which uses random numbers.  The binary generator provides to the host device a signal that is represented by random binary numbers. 
The invention is also defined as a method for generation of random numbers comprising the steps of producing alpha decay particles, detecting the alpha decay particles, generating a detection signal in response to detection of the alpha decay particles, identifying detection signals corresponding to an actual detection of the alpha decay particles from the source.  And generating a binary signal in response to positive identification by the discriminator of the detection signals corresponding to an actual detection of the alpha decay particles from the source. 
The invention having been briefly summarized, the invention may be better visualized by turning to the following drawings wherein like elements are referenced by like numerals.