The Perspective-Zephyst Editorial

Aditya Guha, Chief Editor

The Perspective-Zephyst is out with its final edition. As in our previous edition, we have focussed mainly on research articles by undergraduates. In fact, in our endeavour to encourage authentic original research, we’ve had to reject a few articles for the first time in our publishing history. Nevertheless, we present a plethora of articles upholding the highest quality standards ranging from Quantum Mechanics and Relativity to Personal Opinions of a graduate student studying in Germany; thus presenting a unique blend of just what you would want to read. We continue to encourage original research reports with due references and experience/opinion articles from students and researchers across the academic spectrum.

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Happy New Year and Happy Birthday to Us!

-Aditya Guha, Editor
A Very Happy New Year from The Perspective-Zephyst team! This day is also extra special for us as it marks the day of our inception as we mark our first anniversary! We are proud to let you know, that The Perspective-Zephyst remains perhaps the only active student operated online science forum and second only to Gizmodo India as an active science platform in India. Gaining traction has not been easy and our entire team has spent many sleepless nights pleading for articles, editing them, and of course running a live site. And we hope your best wishes will keep us strong in 2016 as well!
As a reflection of the past year, we present to you 4 of our most viewed articles on our site:
What is encouraging about our statistics is that even significantly technical articles have managed to provide stiff competition in viewership to the opinion pieces.
We will be having our next edition on the 3rd week of January and we encourage you to contribute in the form of articles and the like.
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My College made me what I am

Parichay Mazumdar, currently a student at Bonn-Cologne Graduate School of Physics and Astronomy (BCGS), Germany narrates how his alma mater St. Stephen’s College nurtured the Physicist and the amazing personality in him.

Hi, I am a masters student of Physics in the Bonn-Cologne Graduate School of Physics and Astronomy (BCGS), Germany and an ex-student of the Physics department of the St. Stephen’s college, Delhi. I was asked to share how the experience of the life at St. Stephen’s was for me. I am someone who doesn’t know how to wrap memories into beautiful expressions. Still I will try as much as I possibly can in expressing the unforgettable three years of my life. I belong to an unknown small town of Orissa called Bargarh. It all started when I finished my class 12th exams and was battling with the social pressure of joining a medical/ engineering college. I always wanted to do pure sciences. I was looking for reputed institutes in India for a bachelors in physics. And Google told me that St. Stephen’s was the best ranked college in Natural Sciences. I called up my sister in Delhi and asked about it. Her over-enthusiastic reply made me think “I could give it a try!” (I never though such a casual decision would change my life). So I filled up its application online along with a couple of more colleges.

Then came the cutoffs and I made it into the interview list (although there is a whole new story of how I narrowly missed the interviews!). That was the first day I stepped into the college campus, a random guy from a random place aspiring to get a seat in the college of his first choice. Amidst all the suspense of making it to the list, the campus managed to get into my head. Well maintained grass “courts” and abundant greenery decorating an equally beautiful structure of bricks and stones. I had never seen green and red look so beautiful together before! But I knew it was only half the job done. I still had to face an interview panel about which I had zero idea and I had no contact with anyone from the college to get an overview of what it was gonna be like. I still believed that if I knew enough physics and keep myself relaxed through the interview I could easily get through. On the day of the interview I was in a group of about 20 odd students. It was a mixed feeling of excitement and nervousness. Excitement of the fact that I would be interacting with some really good professors and gain a nice experience but also nervousness of screwing up the interview. But the former overwhelmed the latter somehow. I was one of the last students. People kept going in and coming out. Some smiling, some totally shaken in tears. Everyone around me was having a nervous breakdown and it was getting into my nerves too. Finally came my turn and I went in.There were five professors. They started straightaway with simple physics questions and slowly went on to ask some others which a high school pass-out might find difficult to answer. Fortunately, I could answer most of them easily. Then it became much more comfortable, they asked me about my family and other non-physics stuff. I guess it all took about thirty minutes. I opened the door and as I was about to leave, I heard one of the panelists, whom I went on to admire a lot after I joined college, say “Khub bhalo korechho tumi”, which translates to “you have done well”. I knew then, there was no turning back and I was finally gonna be a Stephanian!

The rest was a fabulous journey. The lack of my vocabulary skills, although I know what snobbish means, might make it sound ordinary, but trust me, it was an extraordinary experience. My classmates were all really smart motivated people trying to make a mark in the world of physics. Suddenly my surroundings changed from a bunch of old people trying to shove me into the engineering fiasco (Yes, you heard me right! That’s what I think it is.) to a group of enthusiastic students and teachers encouraging each other to excel in whatever they found interesting and motivating. There were so many societies in college that I could not decide which ones to join and what to leave. But I had to be careful since I had to make a balance between societies, which would help me build a personality by improving my social skills, and studies which was gonna shape my future. I joined the Music Society, the Social Service League and obviously, the Physics Society. Although music was a greater passion, I always enjoyed doing social work. After a couple of months I was totally integrated into this new community that was young and dynamic and full of positive competition.

Then, there was a whole department of great professors who were nothing short of inspirations for everyone. They always inspired (sometimes scared) students to work hard and enjoy it at the same time. It was in college where I saw for the first time that, the teachers were enjoying what they were teaching and that made learning much more fun. We were encouraged by them and other seniors to apply for summer projects (I had no idea what they were before I stepped onto college!). Thanks to a nice recommendation letter I got to do a very interesting summer project in the National Center for Radio Astrophysics (NCRA-TIFR) Pune. I also worked one winter in the Bose Institute, Kolkata. Interacting with students from other places made me realize that I was indeed very lucky to have such a exceptional bunch of teachers all at one single place while other places hardly even had one or two. While I was busy having a good experience in the world of physics, there were others around me, each with an equally interesting story, if not more, of how they were living their own version of St. Stephen’s. We motivated each other and helped each other to work harder. It might sound like we were just studying and not having fun but the truth is, the amount of fun I had in college is much more than what I had in my whole life. I just don’t wanna explain that because, obviously that’s what college is all about!

I could not stress it enough, but St. Stephen’s changed me from an extremely shy boy from a small town to a really matured and smart person who could take on the world on his own. And its not just me, all my friends have made an equally successful life after college so far. Three students from my class including me got call letters from Cambridge University. Two of them went on and joined Cambridge. Two of my other classmates and I chose BCGS to do our masters degree. Another friend of mine went to Amsterdam via an Erasmus Scholarship. Some joined IISc and IITs after getting enviable ranks in IIT-JAM. Others chose careers totally different from Physics, like fashion or law. A few got hired by MNCs with six digit salaries! St. Stephen’s did a remarkable job in shaping the lives of about a thousand students and it continues doing so every single year. I wish there were at least a hundred more colleges like this in this country of billions so that more people like me could get a good education and a fair shot at life.

A Compilation of Science News over the last Month

Tuhina Sinha, St. Stephen’s College, Delhi

1st October, 2015

Researchers develop technology to produce low-cost heart valves: Researchers at Colorado State University (CSU) and Ohio State University (OSU) say they have developed a technology to produce low-cost heart valves. The new heart valves, which will be made of flexible plastic, which also contains hyaluronan, will cost half the price of an imported metal valve and not require anti-coagulation therapy. However, clinical trials and approval of the new technology is likely to take another two years. Doctors from all the three institutes announced that they have been working on a project to develop low cost replacement heart valves that are better than the existing metal and tissue-based valves and at the same reduce a patient’s need for medication to prevent formation of blood clots.

Asteroids are Moon’s main ‘water supply’: Water reserves found on the Moon are the result of asteroids acting as “delivery vehicles” and not of falling ice comets as was previously thought, a new study using computer simulation has found. Scientists have discovered that a large asteroid can deliver more water to the lunar surface than the cumulative fall of comets over a billion year period. Vladimir Svettsov from Institute for Dynamics of Geospheres and Valery Shuvalov from Moscow Institute of Physics and Technology in Russia developed the most probable mechanism of water delivery to the Moon and an approximate “supply” volume, using computerised modeling of the fall of cosmic bodies onto the surface of the Moon.

moon

NZ plans a France-sized marine sanctuary: New Zealand has plans to create a South Pacific marine sanctuary, about the size of France, to protect one of the world’s most pristine ocean environments. Prime Minister John Key said the Kermadec Ocean Sanctuary would cover an area of 620,000 square kilometres (240,000 square miles) about 1,000 kilometres (620 miles) off New Zealand’s northeast coast. Announcing the plans at the United Nations in New York, Key said the Kermadec area was home to thousands of important species, including whales, dolphins, seabirds and endangered turtles. “It contains the world’s longest underwater volcanic arc and the second deepest ocean trench at 10 kilometers deep,” he said.

30th September, 2015

Plastic eating worms could save the world: A tiny worm, which is actually the larva of a beetle, eats Styrofoam and other forms of polystyrene, a Stanford University researcher has found. Microorganisms in the worms’ guts biodegrade the plastic in the process. This first ever finding holds out hope for a world that is being swamped by plastic. These findings, published studies in Environmental Science and Technology, are co-authored by Wei-Min Wu, a senior research engineer in the Department of Civil and Environmental Engineering at Stanford. “Our findings have opened a new door to solve the global plastic pollution problem,” Wu said.

UAE space agency seeks cooperation with ISRO:  A delegation from the UAE Space Agency has undertaken a visit to the Indian Space Research Organisation (ISRO) to discuss possibilities of cooperation between the two space organizations. The delegation was led by Khalifa Al Rumaithi, chairman of the UAE Space Agency, and Mohammed Nasser Al Ahbabi, it’s director-general, according to statement released by official WAM news agency.
This is part of the agency’s efforts to build strategic partnerships and achieve goals of regional and international cooperation within the industry, it said. The UAE delegation learnt about the Indian space sector and the different programmes that ISRO has led within space exploration, it said.

29th September, 2015

Alzheimer’s agony: Fading memory: “I forgot where I kept the car keys.” “I can’t recall my nephew’s name.” “I don’t remember the road that leads to my home.” It’s not uncommon to hear these statements from elderly people in our families. And their statements aren’t taken seriously as forgetfulness is considered normal fallout of old age in our society. Family members don’t realize the serious implication behind such forgetfulness, which can be indicative of something as dangerous as dementia or more precisely Alzheimer’s Disease. Alzheimer’s Disease, one of the commonest forms of dementia (progressive memory loss), is a disease without cure but at the same time, the progress of the disease can be slowed down with early detection and timely treatment. But sadly, people associate memory loss with old age and don’t bother about early screening or cutting down on the risk factors, which is the only way to delay the progression of dementia.

dementia

How NASA discovered flowing water on Mars and what it means: Streaks of salty water flow down the mountains and canyons of Mars during summer seasons, raising the odds that life may one day be found on the Red Planet, NASA scientists have announced. An analysis of the dark patches that ebb and flow on the slopes of Mars shows that they contain “hydrated” salt crystals containing molecules of water — the first time scientists have directly detected liquid water on Mars. “The detection of hydrated salts on these slopes means that water plays a vital role in the formation of these streaks,” said Lujendra Ojha, of the Georgia Institute of Technology in Atlanta. Liquid water is considered an essential ingredient for life. However, there is as yet no suggestion that NASA is even close to finding signs of simple microbial life forms on Mars, where the surface is constantly bombarded by life-destroying cosmic radiation, and experiences wide temperature extremes.

Cancer treatment during pregnancy not unsafe: It is among the most delicate and difficult dilemmas in medicine: Should a pregnant woman who has received a cancer diagnosis begin treatment before her child is born? Some hesitant doctors counsel women to deliver preterm or even terminate the pregnancy first. But a new study of more than 100 children who were exposed to cancer treatment during the last two trimesters of their mother’s pregnancy showed they had normal cognitive and cardiac function, researchers said on Monday.”The main message of this study is that termination of pregnancy is not necessarily warrant ed, and that early preterm delivery to be able to do cancer treatment isn’t warranted, either,” said Dr Elyce H Cardonick, a maternal-fetal specialist at Cooper Medica School of Rowan University in Camden, NJ, who was not involved in the new research.

27th September, 2015

Rare supermoon lunar eclipse on Sunday night: A total lunar eclipse will share the stage with a so-called supermoon on Sunday night or early Monday, depending where you are. That combination hasn’t been seen since 1982 and won’t happen again until 2033.When a full or new moon makes its closest approach to Earth, that’s a supermoon. Although, still about 220,000 miles away, this full moon will look bigger and brighter than usual. In fact, it will be the closest full moon of the year, about 30,000 miles closer than the average distance. NASA planetary scientist Noah Petro is hoping the celestial event will ignite more interest in the moon. He is deputy project scientist for the Lunar Reconnaissance Orbiter, or LRO, which has been studying Moon from lunar orbit since 2009.

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Countdown for launch of ISRO’s Astrosat progressing normally: The 50-hour countdown for the launch of Astrosat has been progressing normally after it began at 8am on Saturday, according to an update on ISRO’s website. A Polar Satellite Launch Vehicle (PSLV-C30) carrying Astrosat, India’s first dedicated multi-wavelength space observatory, and six small satellites will fly from Sriharikota spaceport at 10am on Monday. “Everything is progressing well. No issues,” said K Suryanarayana Sharma, project director, Astrosat. The countdown began following clearance from the Mission Readiness Review (MRR) committee and Launch Authorization Board (LAB).

NASA’s Mars rover ‘smarter’ than Curiosity: NASA’s next nuclear-powered Mars rover, slated to launch in 2020, has started taking shape. The new vehicle will be “smarter” than the previous one on many counts, according to the engineers on the project. For one, the overall system is becoming heavier. The rover’s wheels are getting heavier, with its body becoming a little longer. Engineers said that would change the rover’s mobility system and how it interacts with the ground. “We are really looking at ways to make this largely heritage, built-to-print rover drive faster and do more science on the surface of Mars,” JPL’s mission manager for Mars 2020 and surface-phase lead Jennifer Trosper was quoted as saying.

mars_rover_space_nasa

24th September, 2015

ISRO celebrates one year of Mars Orbiter Mission: Mars Orbiter spacecraft marks one year of its life around the red planet on Thursday. After successfully completing one year of the mission life around Mars, a large data set has been acquired by all five payloads of Mars Orbiter Mission. ISRO to mark the first anniversary of Mars Orbit Insertion released an atlas containing photos taken by the colour camera on board the spacecraft and results obtained by other payload results in a form of scientific atlas.

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‘Neglecting arthritis can lead to stress fracture’: Leelaben Raval, 65, from Visnagar had been experiencing severe joint pain, stiffness and weakness in muscles for the past two years. Raval kept neglecting osteoarthritis symptoms and eventually had stress fracture in both her knees. Naranpura resident Rama Patel, 72, also suffered wear and tear of muscles with age. It started with a pain localised to the knee but worsened when she climbed the stairs and stood up from a sitting position. As the disease progressed, the pain made even walking difficult. Both patients recently underwent a customized implant for total knee replacement that could have been avoided had they acted promptly.

21st September, 2015

In this village, girls ‘turn into boys’ at 12: In an isolated village in the Domini can Republic, an estimated one in fifty children are born appearing to be girls but grow male genitalia during puberty. The children are known as Guevedoces, roughly translating as “penis-at-12”, referring to the age where their appearance often starts to change. A BBC documentary called `Countdown to Life: The Extraordinary Making of You’ explores the rare condition as part of a series on development. Johnny, once known as Felicita, told the programme he fought bullies who targeted him when the change started. “They used to say I was a devil, nasty things, bad words and I had no choice but to fight them because they were crossing the line,” he said. “I’d like to get married and have children, a partner who will stand by me through good and bad.”

Dental diseases cost the world $442 billion annually-Report: Improvement in oral health alone can offer the world substantial economic benefit as researchers have estimated that the yearly global economic impact of dental diseases amount to $442 billion. Reporting the economic burden of oral diseases is important to evaluate the societal relevance of preventing and addressing oral diseases. The research by Stefan Listl from Heidelberg University in Germany, and colleagues estimated that the direct treatment costs due to dental diseases worldwide were at $298 billion yearly, corresponding to an average of 4.6 per cent of global health expenditure.

18th September, 2015

In US, 3-yr-old diagnosed with type-2 diabetes: A three-and-a-half-year-old obese Hispanic girl in the United States has become one of the youngest ever to be diagnosed with type 2 diabetes, a disease linked to poor diet and obesity. The girl had symptoms of excessive urination and thirst. A review of her diet showed poor nutritional habits with uncontrolled counting of calories and fat. The child’s weight was 35 kg (in the top 5% of all children her age). The girl was started on a liquid version of the diabetes drug metformin and her calorie intake was controlled, besides increasing her physical activity. Six months after diagnosis, her blood glucose levels improved.

30,000-yr-old frozen virus still infectious: A 30,000-year-old giant virus discovered deep in the Siberian permafrost is still functional and capable of infecting its host, researchers have found. However, the new virus is not a threat to humans; it infected single-celled amoebas during the Upper Paleolithic, or late Stone Age. Dubbed Mollivirus sibericum, the virus was found in a soil sample about 98 feet below the surface and is member of a new viral family, the fourth such family ever found. M sibericum is wider in diameter than the other giant viruses, at 600 nanometers versus 500. It has a genome of 600,000 base pairs which hold the genetic instructions to create 500 proteins.

Shelters on Mars to be 3D-printed: A French firm has designed a conceptual shelter for future astronauts on Mars that would be 3D-printed on the red planet using local materials. Resembling an igloo from the surface, the shelter, dubbed Sfero, would be partially buried beneath the ground. Access to it would be gained by its one long corridor, which contains an airlock. The interior comprises three floors. The uppermost floor measures just 3 sq m, and food can be grown here, while the next floor measures 29 sq m, comprising a work area and bathroom. The lowermost floor measures 40 sq m and contains sleeping quarters. The occupants would navigate between each floor by a spiral staircase.

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12th September, 2015

Found: Galaxy that creates 800 stars/yr: In a rare find, astronomers have discovered a gargantuan galaxy cluster, 9.8 billion light years away , in which the brightest galaxy is rapidly creating about 800 stars every year. The discovery , made with the help of NASA’s Hubble Space telescope, is the first to show that gigantic galaxies at the centre of massive clusters, which are usually made of stellar fossils-old, red or dead stars can grow significantly by feeding off gas stolen from other galaxies. However, the new galaxy, at the heart of a cluster named SpARCS1049+56, seems to be bucking the trend by forming new stars at an incredible rate. “The galaxy is furiously making new stars after merging with a smaller galaxy, which is lending its gas,” said lead author Tracy Webb of McGill University in Canada.

China publishes first report written by ‘robot journalist’: A first business report written by a robot has been published in China this week, stoking fears among local journalists that it could make forays into the country’s state-controlled media, threatening their jobs. The article, was written in Chinese and completed in just one minute by Dreamwriter, a robot journalist designed by Chinese social and gaming giant, Tencent that apparently has few problems covering basic financial news. Tencent released its flawless 916-word article via the company’s instant messaging service. “The piece is very readable. I can’t even tell it wasn’t written by a person,” Li Wei, a reporter was quoted as saying by South China Morning Post.

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11th September, 2015

‘Misplaced’ volcanoes on Jupiter’s moon due to magma oceans: Tides flowing in a subsurface ocean of molten rock, or magma, could explain why Jupiter’s moon Io appears to have its volcanoes in the ‘wrong’ place and could also have implications for the search for alien life, a NASA study has found. The research implies that oceans beneath the crusts of tidally stressed moons may be more common and last longer than expected. “This is the first time the amount and distribution of heat produced by fluid tides in a subterranean magma ocean on Io has been studied in detail,” said lead author Robert Tyler of the University of Maryland and NASA’s Goddard Space Flight Centre in US.

REFERENCES

  1. The Times of India
  2. Hindustan Times
  3. BBC

Your focus is guided by your Perspective.

Kevin S. Varghese, St. Stephen’s College, Delhi

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This picture was taken in Western Ghat region of Kerala during the peak of the monsoon season (under a tall coconut tree which was life threatening). This picture tries to capture a flower with tendrils radiating vibrance at the onset of the rain. The scorching heat which the north India bears is the main reason why monsoon occurs. The heat creates a low pressure region in the northern part of India which the moisture-laden clouds from both Arabian sea and The bay of bengal tries to stabilize causing heavy rainfall throughout the country. But these days effect of global warming is hampering with the whole natural process causing droughts and losses. We as young people should make sure that our nature is in focus all the time. #Spread the word. #Save the Earth

15 Oct 1931 – 27 July 2015 : SPECTRUM OF VERSATILITY

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Pragya Rastogi, St. Stephen’s College, Delhi

Tribute to the persona with unparalleled potential, unalloyed dedication to serve the country and adept to work in science and politics …

Tribute to DR. APJ ABDUL KALAM : THE MISSILE MAN OF INDIA .

The Makings of a Nobel Prize Experiment

Avleen K. Sahni, St. Stephen’s College, Delhi

1Georges Charpak, the last person till date to individually receive a Nobel prize for his revolutionary design in the field of charged particle detectors in particular, the Multi Wire Proportional Counter (or MWPC).

What did he create, and what was the importance of his idea to Science?

Let us consider a typical experiment in nuclear Physics, which aims to understand the structural features of a nucleus, or study different reaction processes involved in heavy ion collisions.

The experiment requires the bombardment of a beam of high energy particles on heavy target nuclei, and the various products of nuclear reactions are detected.

2

Bombarding the projectile on heavy targets and detecting the various products of the nuclear reactions.

Unlike the chemical reactions we all study about in chemistry, nuclear reactions can have different pathways, depending on the reaction conditions, and each set of reaction products has its own cross section (analogous to the probability of each set of products).

A particular experiment can aim to explore any particular set of these products, and thus we would require detectors that are sensitive to our required products, and transparent to all others.

So now we need to know the reaction product positions, numbers and energies. That is, now we need a charged particle detector.

particle detector or a radiation detector is a device used to detect, track or identify high-energy particle products. Depending on the requirement, detectors may be used to measure other attributes such as energy, charge etc. of the particles.

The De Broglie wavelength of any particle is given by: L = h/v

The incoming particles are accelerated to energies of a few MeV to reduce the De Broglie wavelength to the nuclear dimension (10-15 m). Collision of the energetic projectile with target nuclei could trigger a number of reaction processes like scattering, nucleon exchange, fusion etc. Using appropriate detectors, we can characterise the products.

So the question arises: How is Radiation detected?

Nuclear radiation is detected only through interaction with matter. The fundamental principle of detection is based on the transfer of energy from the incident radiation to the detector which is then converted to a form in which it can be processed electronically. This enables accurate and faster measurement of timing, energy, and position etc. of the incident particles.

Detectors are of different types depending on the radiation being detected. For charged particle detectors, the radiation loses energy through Coulomb interaction in the detector medium resulting in the excitation and ionisation of the detector atoms. While the ionisation produces free charges, the de-excitation of the excited atoms results in the emission of photons.

Thus detectors are of two kinds based on which of the two mechanisms they work on:

  1. Ionisation based detectors: Those detecting the free charge carriers
  2. Scintillation detectors: Detectors sensing the luminescence photons

For neutral radiation such as gamma rays or neutrons, however, the process should be two-fold: the liberation of an energetic charged particle, and the subsequent detection of this particle.

While earlier designs like the Geiger counter, the proportional chamber were being used to determine the number of charged particles detected, the MWPC made it possible to know the position of the charged particle being known.

To understand the working of the MWPC, you need to know the simple formula:

E(r) = CVo/ 2rε∏

A cylinder with a conducting wire along its axis to which a positive voltage, + Vo, relative to the walls is applied. A radial electric field with r: radial distance from axis; C: capacitance per unit length of this configuration; ε is the dielectric constant of the medium inside the cylinder.

3

Schematic for MWPC

  1. An array of closely spaced anode wires between 2 plates of cathodes, enclosed in a chamber.
  2. A gas is filled and flows continuously through this chamber.
  3. The beam of charged particles enters through a thin entrance window.
  4. The incoming charged radiation ionise the gaseous molecules, it loses energy and electron – ion pairs are generated.

4 Electric field is uniform, except near the wires  

 5

The motion of the charges is like a drop: with the electrons drifting faster than heavy positive ions.

  1. Electrons formed by ionisation of the gas drift inward towards the plane of anode wires, initially in a nearly uniform field. Near the wire, because of the 1/r dependence, the electric field experienced by the electrons is strong. These electrons acquire sufficient energy to cause secondary collisions, thus creating more electron- ion pairs, leading to an avalanche.
  2. A large signal is induced on the anode wire, closest to the electrons striking. The pulse signal on the electrodes of ionization devices is formed by induction due to the movement of the ions and electrons as they drift towards the cathode and anode, rather than by the actual collection of the charges itself.

This signal helps to identify the position of the striking radiation, though information about the energy is mostly lost. Modifications to the basic design can improve timing resolutions or allow for energy measurement. If 2 plates of anode wires are used instead, we can orient them perpendicular to each other to allow for x and y coordinates of position. The resolution in position is proportional to the spacing between the wires. It is fast because avalanche formation near the wire takes place within <1ns.

Thus, the MWPC offers distinct advantages, including very good time resolution and position accuracy, fast response (electrons are used, instead of ions). They can be fabricated with ease in various sizes and geometries based on the experiment with very little cost.

This important invention not only revolutionised data-taking for experiments, ultimately leading to the discovery of the W and Z particles, but found immense applications in the medical physics.

A Review of Frenkel Poole Emission

Aditya Guha

Department of Physics, St. Stephen’s College, Delhi University, Delhi 110007, India

(Dated: September 19, 2015)

The p-n junction for semiconductors has various mathematical models which try to explain the transport phenomena across the junctions for different semiconductor materials and at different temperatures. Here, we review Frenkel Poole Effect and concentrate in the domain of large Electric Fields where the additional increase in conductivity is independent of the Electric Field.

1. INTRODUCTION

In the domain of very large electric fields E for both conductors and insulators, electrical conductivity exponentially tries to catch up till breakdown occurs. The limiting value of such an E is about 106V/cm and a few thousands of volts/cm for an insulator and conductor respectively. Experimentally, it had been noticed that in such fields, illumination of the semi-conductor surface lead to additional increases in conductivity, which could only be explained by their increase in number density, since their mobility wouldn’t increase further. This led Frenkel to publish an article explaining the same in a note to Phy. Rev. Letters, 1938.

2. FRENKEL’S MATHEMATICAL MODEL

In the exorbitantly high electric field domain, the electrons can be considered to be acted upon by a field screened by the the positive ions. Of course the medium still contains some neutral polarizable atoms. Considering the screening due to polarization, the ionization potential reduces by a factor given by the permittivity of the medium,\epsilon

p1

FIG. 1: Potential Energy as a function of distance from the positive ion. The dotted line is in the presence of the field while the bold one is in absence of external fields.[1]

In the figure above, we note that on increasing the external field E, following a mechanism similar to Schottky Effect inspired by Richardson’s eponymous equation in 1901, decreases the energy. The height of the potential barrier in the field is thus lowered by:

1                                                 (1)

Here, r0 is given by:

2

This thus gives us. Replacing this back in the equation finally gives us:

3                          (2)

Now, the number of free electrons due to thermal ionization is proportional to e(−U0+∆U)/2kT. However, the ionization potential energy had been reduced by a factor,  This thus gives us conductivity differing by:

4                   (3)

This as we see differs from the original Poole’s Law, which was given by

σ = σ0eαE                                                              (4)

Now, the overall conductivity is thus given by:

5                                      (5)

Making use of the modified form of Ohm’s Law,

J = σE                                            (6)

we obtain the standard form of Frenkel-Poole Emission Formula as follows:[3]

6                        (7)

where:

  1. J is the current density
  2. E is the applied Electric Field
  3. e is the electronic charge
  4. U0 is the voltage barrier in the absence of E that an electron must cross to move from one atom to another in the crystal
  5. is the permittivity of the medium
  6. kB is the Boltzmann’s constant
  7. T is the absolute temperature

3. EXPERIMENTAL VERIFICATION 

Frenkel’s mathematical model was found to be in excellent agreement with P. Granofskaja and Joffe’s experiments on pre-breakdown phenomena in electronic semi-conductors performed upto field strengths of about 50,000 volts/cm. [1]

What was unique about the above phenomena was that the effect of the applied field was reduced despite increasing temperature in the given semi-conductor. Electrons can move (slowly) through an insulator by the following method. The electrons are generally trapped in localized states (loosely speaking, they are ”stuck” to a single atom, and not free to move around the crystal). Occasionally, random thermal fluctuations will give that electron enough energy to get out of its localized state, and move to the conduction band. Once there, the electron can move through the crystal, for a brief amount of time, before relaxing into another localized state (in other words, ”sticking” to a different atom).[3]

The Frenkel Poole effect describes how, in a large electric field, the electron doesn’t need as much thermal energy to get into the conduction band (because part of this energy comes from being pulled by the electric field), so it does not need as large a thermal fluctuation and will be able to move more frequently.

4. APPLICATIONS AND SUMMARY

It remains the question about the transport mechanism which is responsible for transport in the case of overoxidized or underoxidized junctions. Several candidates have been checked. In a certain range of voltages and temperatures, Poole-Frenkel emission thermally stimulated emission of carriers from donor like trap sites seems to be the most probable.[2] Its voltage and temperature dependence is described by:

I ∝ √V exp(2a V/kT eU0/kT) (8)

where

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FIG. 2: A plot of ln(I/V ) versus 1/T results in a straight line for the Poole-Frenkel effect, which is found here above 200 K, with an applied voltage of 30 mV.[2]

lines if Poole-Frenkel conduction is indeed the main conduction mechanism. These straight lines have been found for temperatures above 200 K at 30 mV bias voltage Fig. 2. According to the derivation by Frenkel, the dielectric constant enters the formula as the ability of the material surrounding a trap site to screen its positive charge.[2]

Frenkel states a radius of influence of the trap on the order of 30 angstroms. For our thin barriers, however, metallic material of the electrodes is met within this radius. This gives rise to a much more effective screening due to the high mobility of metal electrons, which in turn explains the much higher dielectric constant. Recently, these concepts have been used of in the fabrication of epitaxial layers on the semi-conductors especially benzotriazole and benzothiadiazole based organic devices.[4]

Acknowledgement:

The author thanks Dr Harish Yadav for suggesting the aforementioned topic as an undergraduate curriculum project.

References                                 

  • [1] Frenkel, ”On the Pre-Breakdown Phenomena in Elec-tronic Semi-Conductors”, 1938.
  • [2] Rottlander, M. Hehn, A. Schul ”Determining the Interfacial Barrier Height and its Relation to Magnetoresistance”, 2002.
  • [3] Wikipedia, ”Poole-Frenkel Emission”, last modified 2009.
  • [4] E. Yildiz, ”Leakage current by Frenkel Poole emission on benzotriazole and benzothiadiazole based organic devices”, 2014.